Still lower noise radio astronomy (was: low-noise amplifiers for radio astronomy )

"John (Liberty) Bell" wrote in message
...
George Dishman wrote:
"John (Liberty) Bell" wrote in message
...
wrote:
"John (Liberty) Bell" wrote in message
...
wrote:

snip

The DC input resistance of the HEMT is high but I'm not
sure of the resistive component of the reactance for a
microwave signal. Is there a resistance in series with
the capacitance that is unmeasurable at DC?

Good point. The capacitance is presumably from the gate to the source,
and from the gate to the drain. So that resistance/impedance should be
that of any source resistor/impedance and drain resistor/impedance in
parallel.

It's more complex but you need to look at the equivalent
circuit I mentioned below for an example. However, you
have the idea.

This is starting to sound more like home territory to me, where a low
impedance source is simply plugged direct into a high input impedance
amplifier, with no resultant significant losses to worry about.

Not quite. If you do that at RF, the impedance change
acts like a mirror and reflects all the input power back
to the source. I work in HF and the key is that we must
_always_ match the impedance.

OK. It seems that this is usually achieved by placing an additional
load from the input to ground, before the L and C we were discussing. I
don't like the idea of "wasting" that extra potential power, but if it
is necessary, then it has to be done.

That's one way. I'm more used to bipolars where the
input impedance is intrinsically defined by the junction
and operating point.

However, this does bring me to an additional possible question on
impedance matching which is probably easiest discussed abstractly in
the context of operational amplifiers.
Here the gain is defined as the feedback impedance divided by the
source impedance (which is the whole point of op-amps) and the
inverting input acts as a 'virtual earth'.

Do you know exactly how this would affect the already discussed input
impedance matching, when the source is fed into that inverting input
via said compensating inductor?

For an op-amp, the input is fed to the virtual earth
via a resistor. That would define the input impedance.
Any stray capacitance could be nulled by an input
inductor leaving a pure resistance at the frequency of
interest.

(I am ignoring for the moment that an op-amp has both inverting and non
inverting input transistors which will obviously increase the
transistor noise)

Sure but note that noise can be reduced by using
two transistors in parallel. More components isn't
always bad.

After the LNA where the
signal has been boosted by the gain, it is of less
concern but the key to low noise is matching at the
input.

Yes, I did note, much earlier, that unless I could achieve adequate
impedance matching at the input, there would be a problem with the
design. However, it does seem from Steve Willner's comments that we
should now be discussing direct impedance matching between the detector
and the amp, as opposed to via a 50 ohm transmission line.

Sure, but what is the source impedance of the pickup
in the guide/horn/diplexer/whatever? If it is a
standard component, it will have been designed to
have a common impedance (50 or 75 most likely) but
whatever it is the LNA will be matched to it.

Again I suggest you look at this:

http://www.submm.caltech.edu/cso/rec...ers/ballna.pdf

Unfortunately, the computer I have borrowed for this response does not
have a pdf reader, so the above will have to suffice until I get back
to my own office.

OK, my earlier comments will make more sense when
you see this. However, are you getting distracted?
Impedance matching is always considered in RF
design and as you can see from that paper a lot of
work goes into optimising the input network for
the desired compromise (badwidth, noise, cost etc.)
while your idea was aimed at reducing noise below
what can be achieved by conventional methods.
Wouldn't it be simpler to assume that the LNA
designers have achieved the best matching for the
lowest noise and then see what improvement your
method can achieve treating the LNA as a black box?

George

George Dishman wrote:

Sure, but what is the source impedance of the pickup
in the guide/horn/diplexer/whatever? If it is a
standard component, it will have been designed to
have a common impedance (50 or 75 most likely) but
whatever it is the LNA will be matched to it.

Given the stated estimated budget for upgrading the DSN, I doubt that
those engineers would have necessarily been constrained to use standard
components. Similarly, by combining your comments on the simplicity of
the required sensor, and Steve Willner's comments on state-of-the-ast
radio telescopes, and the relative insignificance of cryogenic costs
for the preamp, I doubt that we are thus constrained either.

This leads me to ask: What is so great about a 50 ohm 'standard',
anyway?

I suspect that this could just be historical. It was a convenient
figure for driving and being driven by pre-WW2 valve amplifiers.

Consequently I still cannot understand why a wire sensor at the focus
of a modern telescope, or a knob in a waveguide, needs to be designed
with a 50 ohm resistance. For that matter, I don't currently see why a
TV arial needs to be 50 ohms either, if boosted by a preamp before
being fed into coax.
The title under current discussion is, after all "Still lower noise
radio astronomy" not "the best we can manage using standard
off-the-shelf components"

Again I suggest you look at this:
http://www.submm.caltech.edu/cso/rec...ers/ballna.pdf

This is certainly a highly pertinent paper, for LNAs in the cm to mm
range, and the transistor schematic is also helpful. (I am still
waiting for my copy of Merian Pospieszalski's paper, which I hope will
be more revealing on magnitudes for the various elements of that
schematic).

On first reading, however, I don't really understand the purpose of the
Lange couplers, why we want to split the signal into 2 components with
a 90 degree phase shift anyway, or why there are 4 of these couplers to
the 2 transistors between input & output in the 1991 design.

I note from this first reading, however:

1) that this amp does indeed appear to use standard components for I/O
(and 50 ohm
coax between transistors)

2) the inductors are also conventional and of sufficiently few turns
that they can:
a) be wound by hand more accurately and cheaply than buying 'off
the shelf'
components
b) utilise wire of whatever thickness you want, to vary their DC
resistance (at
the trade-off of varying their capacitance).

This leads me to ask: at what frequency is it desirable/necessary
to switch from
such an inductor to a 'squiggle' on the pcb?

3) The discrete capacitors too look completely conventional.

However, are you getting distracted?
Impedance matching is always considered in RF
design and as you can see from that paper a lot of
work goes into optimising the input network for
the desired compromise (badwidth, noise, cost etc.)
while your idea was aimed at reducing noise below
what can be achieved by conventional methods.
Wouldn't it be simpler to assume that the LNA
designers have achieved the best matching for the
lowest noise and then see what improvement your
method can achieve treating the LNA as a black box?

That is precisely what I did at the beginning. This led me to conclude
that achieving a 20dB improvement in s/n ratio for the amp was easier
than falling off a log, and significantly larger improvements were
possible with more care. However, as discussed before, that too is a
question of diminishing returns. Why spend ages building and tweaking a
couple of amplifiers to noise performances levels that nobody actually
needs, when you can more easily and consistently produce a more
economical device that still 'blows the socks off' the competition.

Since then I have, I admit, been 'looking for problems' on several
fronts. (Forewarned is forearmed.)

However, I don't believe that line of enquiry is completely fruitless,
if it holds out a possibility of also reducing external noise, and
making the resultant LNA more easily and consistently manufacturable.

Differences may arise because I am approaching this subject as a
physicist, whereas you are approaching it as an RF engineer.
Nevertheless, I believe a fusion of both approaches is required, to
achieve the optimum system solution.


John Bell
(Change John to Liberty to bypass anti-spam email filter)

John (Liberty) Bell wrote:
George Dishman wrote:

Sure, but what is the source impedance of the pickup
in the guide/horn/diplexer/whatever? If it is a
standard component, it will have been designed to
have a common impedance (50 or 75 most likely) but
whatever it is the LNA will be matched to it.

Given the stated estimated budget for upgrading the DSN, I doubt that
those engineers would have necessarily been constrained to use standard
components.

I was not suggesting they were, read my comment again.

As an aside, getting the budget means cutting costs to the
minimum, however if that can be achieved by non-standard
parts then certainly that would be considered. Usually
"specials" put the price up significantly though.

Similarly, by combining your comments on the simplicity of
the required sensor, and Steve Willner's comments on state-of-the-ast
radio telescopes, and the relative insignificance of cryogenic costs
for the preamp, I doubt that we are thus constrained either.

This leads me to ask: What is so great about a 50 ohm 'standard',
anyway?

I suspect that this could just be historical. It was a convenient
figure for driving and being driven by pre-WW2 valve amplifiers.

Consequently I still cannot understand why a wire sensor at the focus
of a modern telescope, or a knob in a waveguide, needs to be designed
with a 50 ohm resistance. For that matter, I don't currently see why a
TV arial needs to be 50 ohms either, if boosted by a preamp before
being fed into coax.

TV was usually 75 ohm again for historical reasons, but that
is not important, what matters is that the equipment and
cables are matched. If you put a 50 ohm cable between
a 75 ohm antenna and a 75 ohm input TV or vice versa then
each mismatch causes a reflection and you see a ghost in
the background of the picture due to the signal reflected first
from the TV and then back down from the antenna. That is
annoying for the viewer but would be disasterous for accurate
measurements at the DSN.

The title under current discussion is, after all "Still lower noise
radio astronomy" not "the best we can manage using standard
off-the-shelf components"

Which is why I said "IF it is a standard component, ...
but WHATEVER it is ..."

Again I suggest you look at this:
http://www.submm.caltech.edu/cso/rec...ers/ballna.pdf

This is certainly a highly pertinent paper, for LNAs in the cm to mm
range, and the transistor schematic is also helpful. (I am still
waiting for my copy of Merian Pospieszalski's paper, which I hope will
be more revealing on magnitudes for the various elements of that
schematic).

On first reading, however, I don't really understand the purpose of the
Lange couplers, why we want to split the signal into 2 components with
a 90 degree phase shift anyway, or why there are 4 of these couplers to
the 2 transistors between input & output in the 1991 design.

I am not familiar with the configuration but the article
appears to say the benefit is they give low return loss
over an octave of bandwidth. Consider that in the context
of the S-band we were discussing where the bandwidth
is less than 1.5% of the center.

I note from this first reading, however:

1) that this amp does indeed appear to use standard components for I/O
(and 50 ohm coax between transistors)

The alternative is to get a manufacturer to set up
a production line to make special connectors. That
would be _very_ expensive, and the users would
ahve to buy special matching connectors. A bit of
different width track on the PCB matches the
preferred standard connector. It would even be easy
to have sevearl PCB designs with slightly different
track width matching but the same layout for the
circuitry to allow various connectors as customer
options.

2) the inductors are also conventional and of sufficiently few turns
that they can:
a) be wound by hand more accurately and cheaply than buying 'off
the shelf' components

Hand winding is less accurate and much more
expensive than off-the-shelf, perhaps $100 per
hour for a skilled coil winder compared to ~$0.2
for off-the-shelf to a guaranteed specification.
However, some parameters may require an
air-spaced coil but you would need to look in
more detail to find why they went that way. We
use a lot of such coils but only in transmitters
where voltages of many kV can appear if the
output isn't properly matched and PCB parts
would fry.

b) utilise wire of whatever thickness you want, to vary their DC
resistance (at the trade-off of varying their capacitance).

This leads me to ask: at what frequency is it desirable/necessary
to switch from
such an inductor to a 'squiggle' on the pcb?

It depends on the space, reliability and cost tradeoffs.

3) The discrete capacitors too look completely conventional.

Off course. Setting up a production line for a single
design would be vastly expensive and I can't see
what possible benefit special capacitors would provide.
The PCB substrate would also be a standard material,
you just select the most suitable.

However, are you getting distracted?
Impedance matching is always considered in RF
design and as you can see from that paper a lot of
work goes into optimising the input network for
the desired compromise (badwidth, noise, cost etc.)
while your idea was aimed at reducing noise below
what can be achieved by conventional methods.
Wouldn't it be simpler to assume that the LNA
designers have achieved the best matching for the
lowest noise and then see what improvement your
method can achieve treating the LNA as a black box?

That is precisely what I did at the beginning. This led me to conclude
that achieving a 20dB improvement in s/n ratio for the amp was easier
than falling off a log, and significantly larger improvements were
possible with more care.

As I remember, you hinted you had a method but said
nothing about what it could achieve because of concerns
over patenting.

However, as discussed before, that too is a
question of diminishing returns. Why spend ages building and tweaking a
couple of amplifiers to noise performances levels that nobody actually
needs, when you can more easily and consistently produce a more
economical device that still 'blows the socks off' the competition.

If you can improve noise by 20dB over current levels
then it would be of huge value

Since then I have, I admit, been 'looking for problems' on several
fronts. (Forewarned is forearmed.)

It is a good policy but impedance matching isn't
an area that is neglected, it is probably one of
the biggest drivers throughout the design process
and something you can take for granted will have
alread been optimised before you apply your new
technique.

However, I don't believe that line of enquiry is completely fruitless,
if it holds out a possibility of also reducing external noise, and
making the resultant LNA more easily and consistently manufacturable.

Use of standard components available to a far tighter
spec than any special goes a long way to achieving
that. The noise reduction benefit would be your selling
point.

Differences may arise because I am approaching this subject as a
physicist, whereas you are approaching it as an RF engineer.
Nevertheless, I believe a fusion of both approaches is required, to
achieve the optimum system solution.

My own degree is in physics and several of our RF
designers also followed that path.

George

wrote:
John (Liberty) Bell wrote:
George Dishman wrote:

Sure, but what is the source impedance of the pickup
in the guide/horn/diplexer/whatever? If it is a
standard component, it will have been designed to
have a common impedance (50 or 75 most likely) but
whatever it is the LNA will be matched to it.

Given the stated estimated budget for upgrading the DSN, I doubt that
those engineers would have necessarily been constrained to use standard
components.

I was not suggesting they were, read my comment again.

In that case, read MY comment again on posting 6 of the original
discussion "low-noise amplifiers for radio astronomy (was: Ranging
and Pioneer)". I was the one who first emphasised the necessity of
impedance matching the source and the amp, in this discussion.

snip

I note from this first reading, however:

1) that this amp does indeed appear to use standard components for I/O
(and 50 ohm coax between transistors)

The alternative is to get a manufacturer to set up
a production line to make special connectors. That
would be _very_ expensive, and the users would
ahve to buy special matching connectors.

I probably agree with you for the amp output, but not for the input,
since this can be just a wire placed at the focus of the telescope, as
suggested by the combined comments of you and Steve Willner. That wire
could itself be supplied as part of the amp which projects from the box
through a tiny bit of insulated sleeving. That would be still cheaper
again, but, as I have already mentioned, price is not the central issue
here.

2) the inductors are also conventional and of sufficiently few turns
that they can:
a) be wound by hand more accurately and cheaply than buying 'off
the shelf' components

Hand winding is less accurate and much more
expensive than off-the-shelf, perhaps $100 per
hour for a skilled coil winder compared to ~$0.2
for off-the-shelf to a guaranteed specification.
However, some parameters may require an
air-spaced coil but you would need to look in
more detail to find why they went that way. We
use a lot of such coils but only in transmitters
where voltages of many kV can appear if the
output isn't properly matched and PCB parts
would fry.

The photo of the LNA shows 3 turn and 5 turn air spaced inductors.
These are sufficiently easy and quick to wind that you could have many
hundreds made per hour on legal minimum wage, once you have determined
the number of required turns and diameter of each turn. However, I
admit that my original experience in such an area was with low
resistance wire wound power resistors. I found I could make dozens of
tight spec., custom resistance, high wattage devices from a single coil
of electric fire element, for the same cost as a single off-the-shelf
resistor, which often got delivered days later, and up to 50% out of
spec. Only problems we (a) you couldn't solder the ends for a
production run (b) when the idea was submitted to a popular electronics
magazine, they turned it into a four page feature article without
crediting me, or paying me a bean. : (

snip

That is precisely what I did at the beginning. This led me to conclude
that achieving a 20dB improvement in s/n ratio for the amp was easier
than falling off a log, and significantly larger improvements were
possible with more care.

As I remember, you hinted you had a method but said
nothing about what it could achieve because of concerns
over patenting.

There is no problem with discussing what it CAN achieve, only HOW. (I
have, by now, been down that route several times before, already.)

If I remember correctly, nobody actually bothered to ask me what it
could achieve, in quantitative terms. However, I have already answered
that unasked question by implication, by describing noise Ts of
resultant amps at an ambient (room) T.

If you can improve noise by 20dB over current levels
then it would be of huge value

I can, and more, if we are referring to amp, not system.

My own degree is in physics and several of our RF
designers also followed that path.

OK Let me ask you one final question pertinent to our continuing
apparent disagreement over ideal design impedances for a system, which
I am now going to take to extremes:

If you place two identical metal poles on at tall building, and connect
one to earth with a thick copper strip, and the other to earth via a 10
meg resistor, which will attract a lightning strike most? When struck,
which will then carry most energy to earth?

Now, explode a nuclear warhead in the stratosphere above them. Which
will now absorb and transfer the most electromagnetic (pulse) energy to
earth?

Looking forward, with anticipation, to your answer.

John Bell
http://accelerators.co.uk
(Change John to Liberty to bypass anti-spam email filter)

"John (Liberty) Bell" wrote in message
...
wrote:
John (Liberty) Bell wrote:
George Dishman wrote:

Sure, but what is the source impedance of the pickup
in the guide/horn/diplexer/whatever? If it is a
standard component, it will have been designed to
have a common impedance (50 or 75 most likely) but
whatever it is the LNA will be matched to it.

Given the stated estimated budget for upgrading the DSN, I doubt that
those engineers would have necessarily been constrained to use standard
components.

I was not suggesting they were, read my comment again.

In that case, read MY comment again on posting 6 of the original
discussion "low-noise amplifiers for radio astronomy (was: Ranging
and Pioneer)". I was the one who first emphasised the necessity of
impedance matching the source and the amp, in this discussion.

That's because I was taking it for granted, though I did
mention matching cables around the same time. It really
doesn't matter who first mentioned it, we both understand
that impedance matching in RF electronics is an essential
consideration. Bringing it up is like telling a professional
gardener that watering the plants is a good idea ;-)

snip

I note from this first reading, however:

1) that this amp does indeed appear to use standard components for I/O
(and 50 ohm coax between transistors)

The alternative is to get a manufacturer to set up
a production line to make special connectors. That
would be _very_ expensive, and the users would
ahve to buy special matching connectors.

I probably agree with you for the amp output, but not for the input,
since this can be just a wire placed at the focus of the telescope, as
suggested by the combined comments of you and Steve Willner. That wire
could itself be supplied as part of the amp which projects from the box
through a tiny bit of insulated sleeving.

And how would you ensure the impedance of the wire in
the hole was matched to the input of the amplifier?
If you look at the photograph in the article, you can
see connectors on the right hand side. These have gold
plated contacts, the insulator will be PTFE and of a
controlled dielectric constant, and the cross section
will have been modelled to ensure the impedance does
not vary through the connector. I would expect to be
able to find figures for measured insertion loss on
the manufacturers website provided by a third-party
test house.

That would be still cheaper
again, but, as I have already mentioned, price is not the central issue
here.

Exactly, so we buy connectors that we can trust instead
of "doing it ourselves".

2) the inductors are also conventional and of sufficiently few turns
that they can:
a) be wound by hand more accurately and cheaply than buying 'off
the shelf' components

Hand winding is less accurate and much more
expensive than off-the-shelf, perhaps $100 per
hour for a skilled coil winder compared to ~$0.2
for off-the-shelf to a guaranteed specification.
However, some parameters may require an
air-spaced coil but you would need to look in
more detail to find why they went that way. We
use a lot of such coils but only in transmitters
where voltages of many kV can appear if the
output isn't properly matched and PCB parts
would fry.

The photo of the LNA shows 3 turn and 5 turn air spaced inductors.
These are sufficiently easy and quick to wind that you could have many
hundreds made per hour on legal minimum wage, once you have determined
the number of required turns and diameter of each turn. However, I
admit that my original experience in such an area was with low
resistance wire wound power resistors. I found I could make dozens of
tight spec., custom resistance, high wattage devices from a single coil
of electric fire element, for the same cost as a single off-the-shelf
resistor, which often got delivered days later, and up to 50% out of
spec. Only problems we (a) you couldn't solder the ends for a
production run (b) when the idea was submitted to a popular electronics
magazine, they turned it into a four page feature article without
crediting me, or paying me a bean. : (

You can use them reliably with screw connector blocks
as long as they are made from high temperature plastic.
I used that method to make 8 ohm 100W loads out of 1kW
elements back in the 1970s when I designed disco amps.
You had to couple pairs in reverse to cancel the
inductance of course otherwise you tended to get a
high power RF transmitter ;-)

My current experience (this year) is with a 1kW HF
transmitter where we have air-spaced coils, but
they have to be held with rigid spacing between
the turns to avoid microphony and of course you
have to use silver plated wire because of the skin
effect. Production test has to tunes certain coils
by squeezing them, trial and error. If there was
an alternative we would take it.

snip

That is precisely what I did at the beginning. This led me to conclude
that achieving a 20dB improvement in s/n ratio for the amp was easier
than falling off a log, and significantly larger improvements were
possible with more care.

As I remember, you hinted you had a method but said
nothing about what it could achieve because of concerns
over patenting.

There is no problem with discussing what it CAN achieve, only HOW. (I
have, by now, been down that route several times before, already.)

If I remember correctly, nobody actually bothered to ask me what it
could achieve, in quantitative terms. However, I have already answered
that unasked question by implication, by describing noise Ts of
resultant amps at an ambient (room) T.

OK, I stand corrected, you did indeed give temperatures.

If you can improve noise by 20dB over current levels
then it would be of huge value

I can, and more, if we are referring to amp, not system.

Then you have a fortune at your fingertips. I am
sceptical of course since predicting random noise
is impossible in theory but there are many neat
tricks that appear to defy theory so I will have
to wait and see. No doubt you will make the
headlines.

My own degree is in physics and several of our RF
designers also followed that path.

OK Let me ask you one final question pertinent to our continuing
apparent disagreement over ideal design impedances for a system, which
I am now going to take to extremes:

Nowhere have I said anything about "ideal design
impedances", all I have said is that whatever the
impedance, you must keep things matched to avoid
power loss and reflections. The requirement is
that the output of each stage should be matched
to the input of the next. What I am saying is that
this is standard practice of course hence is a
distraction from the topic of your noise reduction
scheme.

If you place two identical metal poles on at tall building, and connect
one to earth with a thick copper strip, and the other to earth via a 10
meg resistor, which will attract a lightning strike most?

Both equally since the upward pilot is at low current.

When struck,
which will then carry most energy to earth?

Wrong question, in our system we are trying to get the
maximum power into our amplifier so the question is
which design will extract most energy from the strike.

Assuming the resistor is rated for the job, and taking
typical values of 10MV and 10kA capacity for the strike
and 1 ohm for the copper strap, I get 10MW (~10MV at 1A)
dissipated in the 1M resistor and only 1MW (~10kA at 10kV)
for the strap, the resistor is better. Using a 1k resistor
would produce the best result, 5MV at 5kA giving 25MW.
Note the 10MV at 10kA cloud output looks like a 1k source.

Now, explode a nuclear warhead in the stratosphere above them. Which
will now absorb and transfer the most electromagnetic (pulse) energy to
earth?

Looking forward, with anticipation, to your answer.

I guess you are referring to EMP in which case the
answer is probably neither. You have a vertically
descending pulse with essentially dipole receiving
antennas (ignoring any horizontal runs of wire
across the roof) trying to work in "near-vertical
incidence" mode. Dipoles have a null in the
direction of their axis.

George

wrote:
"John (Liberty) Bell" wrote in message
...

(Snip material long since agreed, on desirability of impedance
matching.)

And how would you ensure the impedance of the wire in
the hole was matched to the input of the amplifier?

Trial and error on pre-production prototype. Measure impedance after
using small drill bit to create hole, use larger drill bit if
indicated, then match amplifier to most sensible figure you can thus
achieve, using standard drill bits, the given sheet thickness of the
employed box, and the dielectric constant of your chosen insulator.

The photo of the LNA shows 3 turn and 5 turn air spaced inductors.
These are sufficiently easy and quick to wind that you could have many
hundreds made per hour on legal minimum wage, once you have determined
the number of required turns and diameter of each turn. However, I
admit that my original experience in such an area was with low
resistance wire wound power resistors. I found I could make dozens of
tight spec., custom resistance, high wattage devices from a single coil
of electric fire element, for the same cost as a single off-the-shelf
resistor, which often got delivered days later, and up to 50% out of
spec. Only problems we (a) you couldn't solder the ends for a
production run (b) when the idea was submitted to a popular electronics
magazine, they turned it into a four page feature article without
crediting me, or paying me a bean. : (

You can use them reliably with screw connector blocks

That is obvious. Screw connectors were explicitly covered in the
published article.

I used that method to make 8 ohm 100W loads out of 1kW
elements back in the 1970s when I designed disco amps.

That sounds precisely right. That was when my idea was published, and
dummy amplifier loading was the suggested primary application for the
idea. I can even give you the magazine name, if you don't remember.

You had to couple pairs in reverse to cancel the
inductance of course otherwise you tended to get a
high power RF transmitter ;-)

And that was NOT covered in the published article, which suggests it
was not obvious to the publisher. In fact, I don't think that it is
necessarily true anyway, given that an audio pickup is only supposed to
pick up the audio range, and a vinyl disc is definitely not modulated
at radio frequencies, even if the stylus & cartridge could pick such
modulation up, (which they can't). Sounds to me like your early amps
were unstable, and subject to HF oscillation (which is not at all
unusual).

snip

Production test has to tune certain coils
by squeezing them, trial and error. If there was
an alternative we would take it.

Obviously. But this is what you have to do with prototypes. Once you
know exactly what you want, via that process, you can churn the things
out using the detailed knowledge you have thus gained, for the
production run.


wrote:
John (Liberty) Bell wrote:

That is precisely what I did at the beginning. This led me to conclude
that achieving a 20dB improvement in s/n ratio for the amp was easier
than falling off a log, and significantly larger improvements were
possible with more care.

As I remember, you hinted you had a method but said
nothing about what it could achieve because of concerns
over patenting.

There is no problem with discussing what it CAN achieve, only HOW. (I
have, by now, been down that route several times before, already.)

If I remember correctly, nobody actually bothered to ask me what it
could achieve, in quantitative terms. However, I have already answered
that unasked question by implication, by describing noise Ts of
resultant amps at an ambient (room) T.

OK, I stand corrected, you did indeed give temperatures.

If you can improve noise by 20dB over current levels
then it would be of huge value

I can, and more, if we are referring to amp, not system.

Then you have a fortune at your fingertips.

I will not take that advice too seriously until/unless I have a
microwave engineer ready to do the 'obvious' twiddly RF detailing, and
potential customers clamouring for the product.

I am sceptical of course

That is natural and normal, at this stage, for every genuinely
inventive step that has ever been taken.

This is why Confidential Sight Agreements are invariably agreed for
much less than the invention is actually worth, to permit potential
partners, developers, and angels, an opportunity for informed
evaluation of the inventive proposal, prior to serious commitment.

OK Let me ask you one final question, which
I am now going to take to extremes:
If you place two identical metal poles on at tall building, and connect
one to earth with a thick copper strip, and the other to earth via a 10
meg resistor, which will attract a lightning strike most?

Both equally since the upward pilot is at low current.

Your response surprises me. You are claiming that all lightning
conductors I remember seeing on buildings as a little boy were, in
fact, a complete waste of time and money?

When struck,
which will then carry most energy to earth?

Wrong question, in our system we are trying to get the
maximum power into our amplifier so the question is
which design will extract most energy from the strike.

Not so. You are making unwarranted assumptions. If you want to second
guess what I am considering at present, you would be closer to the
mark if you equated the 10 meg resistor to the source impedance, not
the amplifier input impedance.

Assuming the resistor is rated for the job, and taking
typical values of 10MV and 10kA capacity for the strike
and 1 ohm for the copper strap, I get 10MW (~10MV at 1A)
dissipated in the 1M resistor and only 1MW (~10kA at 10kV)
for the strap, the resistor is better. Using a 1k resistor
would produce the best result, 5MV at 5kA giving 25MW.
Note the 10MV at 10kA cloud output looks like a 1k source.

Wrong answer to the described question (which you chose to ignore). The
question is: what is the resultant energy injected into the earth.

Now, explode a nuclear warhead in the stratosphere above them. Which
will now absorb and transfer the most electromagnetic (pulse) energy to
earth?

Looking forward, with anticipation, to your answer.

I guess you are referring to EMP

Yes, EMP is the acronym for ElectroMagnetic Pulse

in which case the
answer is probably neither.

Sorry, wrong again, since your conclusion is that neither will be
affected. EMP will fry everything in the neighbouring vicinity,
including solid copper and solid silver wire. In order to take this
question seriously, you thus need to assume that the explosion is
sufficiently far away that such catastrophic damage does not occur. In
which case, the following additional comments become pertinent.

You have a vertically
descending pulse

Nonsense. You have assumed, without justification, not only that the
warhead will explode exactly vertically, but also that it will explode
exactly vertically above two different poles, in two different places,
at the same time. That is impossible.

with essentially dipole receiving
antennas (ignoring any horizontal runs of wire
across the roof) trying to work in "near-vertical
incidence" mode. Dipoles have a null in the
direction of their axis.

Sorry, you have made yet another unwarranted assumtion. You have
additionally assumed that both poles are mounted exactly vertically. My
question said nothing about the orientation of the poles. In practice,
by far the easiest way to put poles on a roof is to lay them flat on
the slope of that roof, in whatever orientation is convenient.

Please try answering the question again, this time a little more
carefully.

[Mod. note: I feel this thread may be becoming slightly acrimonious:
please try to treat each other with courtesy, or alternatively take
the parts of the discussion with little direct relevance to
astrophysics research to private e-mail, where you can say what you
like -- mjh]

John Bell
(Change John to Liberty to respond by email)

wrote:
"John (Liberty) Bell" wrote in message
...

OK Let me ask you one final question pertinent to our continuing
apparent disagreement over ideal design impedances for a system, which
I am now going to take to extremes:
If you place two identical metal poles on at tall building, and connect
one to earth with a thick copper strip, and the other to earth via a 10
meg resistor, which will attract a lightning strike most?
When struck, which will then carry most energy to earth?

Assuming the resistor is rated for the job, and taking
typical values of 10MV and 10kA capacity for the strike
and 1 ohm for the copper strap, I get 10MW (~10MV at 1A)
dissipated in the 1M resistor and only 1MW (~10kA at 10kV)
for the strap, the resistor is better. Using a 1k resistor
would produce the best result, 5MV at 5kA giving 25MW.
Note the 10MV at 10kA cloud output looks like a 1k source.

The question was: "Which will then carry most energy to earth?", not
"Which will dissipate most energy en-route?".
Your answer says that a 1 ohm strap transmits 10kA , and a 1M resistor
transmits 1A . Since power is proportional to current squared, the
correct answer is that the strap will transmit 100,000,000 times as
much power as a 1 M resistor. (According to your own figures, impedance
matching instead, using a 1 K resistor, would result in a 75% loss of
transmitted power to the point under present scrutinisation.)

I apologise to both you and the moderator if I sounded a bit 'shirty'
in my earlier more comprehensive response to your posting. However, you
cannot afford to assume anything about what is inside the 'black box',
or why I consequently ask the questions that I do. Similarly, I should
make allowances for the fact that you don't actually know why I ask
such questions.


Regards
John Bell
(Change John to Liberty to bypass anti-spam email filter)

John (Liberty) Bell wrote:
wrote:
"John (Liberty) Bell" wrote in message
...

OK Let me ask you one final question pertinent to our continuing
apparent disagreement over ideal design impedances for a system, which
I am now going to take to extremes:
If you place two identical metal poles on at tall building, and connect
one to earth with a thick copper strip, and the other to earth via a 10
meg resistor, which will attract a lightning strike most?
When struck, which will then carry most energy to earth?

Assuming the resistor is rated for the job, and taking
typical values of 10MV and 10kA capacity for the strike
and 1 ohm for the copper strap, I get 10MW (~10MV at 1A)
dissipated in the 1M resistor and only 1MW (~10kA at 10kV)
for the strap, the resistor is better. Using a 1k resistor
would produce the best result, 5MV at 5kA giving 25MW.
Note the 10MV at 10kA cloud output looks like a 1k source.

The question was: "Which will then carry most energy to earth?", not
"Which will dissipate most energy en-route?".
Your answer says that a 1 ohm strap transmits 10kA , and a 1M resistor
transmits 1A . Since power is proportional to current squared, the
correct answer is that the strap will transmit 100,000,000 times as
much power as a 1 M resistor.

Yes, that's correct. However my understanding of an
analogy with an electronic circuit would be that this
would represent "ground bounce", a problem particularly
in digital circuits where the inductance of the ground
plane can cause spikes on the 0V pin of ICs. It would
manifest as a small amount of signal on the ground
plane near the source of the HEMT in an LNA and is
one reason why choosing a ground point for a scope
probe requires care when working at RF.

(According to your own figures, impedance
matching instead, using a 1 K resistor, would result in a 75% loss of
transmitted power to the point under present scrutinisation.)

The "matched" condition maximises power into the load,
it is not intended to maximise a problematic side-effect.

I apologise to both you and the moderator if I sounded a bit 'shirty'
in my earlier more comprehensive response to your posting. However, you
cannot afford to assume anything about what is inside the 'black box',
or why I consequently ask the questions that I do. Similarly, I should
make allowances for the fact that you don't actually know why I ask
such questions.

I will also apologise for not answering directly but in
the past I have been accused of taking questions too
literally as a way of avoiding a point. Drawing the
analogy with the circuit structure suggested the style
of my response. A literal answer would be that since
the earth connection is hypothetically a zero resistance
point, both methods would produce zero power which
would have been an equally useless answer. I'll reply
to your more detailed post later as I'm pushed for time
at the moment.

In the meantime try plotting a graph of SNR versus load
resistance for a given ource resistance. Remember for
a given badwidth, the noise power is independent of
the resistance hence noise voltage varies as the sqrt of
the resistance.

best regards
George

"John (Liberty) Bell" wrote in message
...
wrote:
"John (Liberty) Bell" wrote in message
...

(Snip material long since agreed, on desirability of impedance
matching.)

And how would you ensure the impedance of the wire in
the hole was matched to the input of the amplifier?

Trial and error on pre-production prototype.

I meant how would you ensure it stayed stable once
a unit was built but it ws really rhetorical. A
company in the business of manufacturing radios
doesn't want to worry about such details. In fact
though my local part of the business has about 500
employees with over 100 in the engineering side of
product development, we no longer have even a model
shop.

Measure impedance after
using small drill bit to create hole, use larger drill bit if
indicated, then match amplifier to most sensible figure you can thus
achieve, using standard drill bits, the given sheet thickness of the
employed box, and the dielectric constant of your chosen insulator.

What insulator vbg? You want something to support the
wire and it needs to be correctly shaped to provide that
support without distorting the field. You can find the
best shape by computer modelling but we are not a
plastics moulding company. Far simpler just to buy an
off-the-shelf part.

The photo of the LNA shows 3 turn and 5 turn air spaced inductors.
These are sufficiently easy and quick to wind that you could have many
hundreds made per hour on legal minimum wage, once you have determined
the number of required turns and diameter of each turn. However, I
admit that my original experience in such an area was with low
resistance wire wound power resistors. I found I could make dozens of
tight spec., custom resistance, high wattage devices from a single coil
of electric fire element, for the same cost as a single off-the-shelf
resistor, which often got delivered days later, and up to 50% out of
spec. Only problems we (a) you couldn't solder the ends for a
production run (b) when the idea was submitted to a popular electronics
magazine, they turned it into a four page feature article without
crediting me, or paying me a bean. : (

You can use them reliably with screw connector blocks

That is obvious. Screw connectors were explicitly covered in the
published article.

I used that method to make 8 ohm 100W loads out of 1kW
elements back in the 1970s when I designed disco amps.

That sounds precisely right. That was when my idea was published, and
dummy amplifier loading was the suggested primary application for the
idea. I can even give you the magazine name, if you don't remember.

I don't think I ever saw it anywhere, it was just an
obvious solution.

You had to couple pairs in reverse to cancel the
inductance of course otherwise you tended to get a
high power RF transmitter ;-)

And that was NOT covered in the published article, which suggests it
was not obvious to the publisher. In fact, I don't think that it is
necessarily true anyway, given that an audio pickup is only supposed to
pick up the audio range, and a vinyl disc is definitely not modulated
at radio frequencies, even if the stylus & cartridge could pick such
modulation up, (which they can't). Sounds to me like your early amps
were unstable, and subject to HF oscillation (which is not at all
unusual).

They were indeed initally, given an inductive load.
Pairing the segments solved it while I developed
the amp to the point where it didn't care.

snip

Production test has to tune certain coils
by squeezing them, trial and error. If there was
an alternative we would take it.

Obviously. But this is what you have to do with prototypes.

With air-spaced coils you have to do it in production
too :-( This kit has been made for many years.

Once you
know exactly what you want, via that process, you can churn the things
out using the detailed knowledge you have thus gained, for the
production run.

Trouble is they don't stay at the same value during
the handling and soldering processes. It is OK for the
less critical values but a pain in filters where to
have manually adjust and then lock with some suitable
varnish or proprietary compound.

If you can improve noise by 20dB over current levels
then it would be of huge value

I can, and more, if we are referring to amp, not system.

Then you have a fortune at your fingertips.

I will not take that advice too seriously until/unless I have a
microwave engineer ready to do the 'obvious' twiddly RF detailing, and
potential customers clamouring for the product.

That wasn't advice, just an acknowledgement of reality.
My advice would be that your first step should be to
produce a SPICE model (or whatever your favourite
simulator uses) to find out if it works before building
prototypes. Building shouldn't be too much of a problem
but measuring the noise level is going to be _really_
difficult (A.K.A. expensive!!!).

snip

OK Let me ask you one final question, which
I am now going to take to extremes:
If you place two identical metal poles on at tall building, and connect
one to earth with a thick copper strip, and the other to earth via a 10
meg resistor, which will attract a lightning strike most?

Both equally since the upward pilot is at low current.

Your response surprises me. You are claiming that all lightning
conductors I remember seeing on buildings as a little boy were, in
fact, a complete waste of time and money?

No. As a strike comes down it spreads out in many
threads like a river delta. The spike produces a
high field at the tip inducing breakdown and the
avalanche effect propagates that upwards. When
the upwards 'pilot' makes contact with the downwards
flow, the current increases ionising more air and
the charge from the remaining downward branches is
pulled back. Both initial flows are essentially
electrostatic and at relatively low current so
both your spikes will "attract a lightning strike"
equally, both designs would work.

Once the plasma path is complete, the current leaps
to tens of kA and the resistor would in reality explode
while the copper strap can take the current. You
asked "which will attract a lightning strike most" and
the _literal_ answer is that they will attract equally,
survivability is a different question.

When struck,
which will then carry most energy to earth?

Wrong question, in our system we are trying to get the
maximum power into our amplifier so the question is
which design will extract most energy from the strike.

Not so. You are making unwarranted assumptions. If you want to second
guess what I am considering at present, you would be closer to the
mark if you equated the 10 meg resistor to the source impedance, not
the amplifier input impedance.

OK, then it is a difficult analogy to follow.

Assuming the resistor is rated for the job, and taking
typical values of 10MV and 10kA capacity for the strike
and 1 ohm for the copper strap, I get 10MW (~10MV at 1A)
dissipated in the 1M resistor and only 1MW (~10kA at 10kV)
for the strap, the resistor is better. Using a 1k resistor
would produce the best result, 5MV at 5kA giving 25MW.
Note the 10MV at 10kA cloud output looks like a 1k source.

Wrong answer to the described question (which you chose to ignore). The
question is: what is the resultant energy injected into the earth.

OK, answering literally, zero in both cases since the
aerth point is rthe reference for measurements.

Now, explode a nuclear warhead in the stratosphere above them. Which
will now absorb and transfer the most electromagnetic (pulse) energy to
earth?

Looking forward, with anticipation, to your answer.

I guess you are referring to EMP

Yes, EMP is the acronym for ElectroMagnetic Pulse

in which case the
answer is probably neither.

Sorry, wrong again, since your conclusion is that neither will be
affected. EMP will fry everything in the neighbouring vicinity,
including solid copper and solid silver wire.

Only if a current is induced in the wire and the wire is
not rated to handle the current. Since we are talking of
lightning conductors, it is reasonable to assume they are
rated to take direct strikes.

In order to take this
question seriously, you thus need to assume that the explosion is
sufficiently far away that such catastrophic damage does not occur. In
which case, the following additional comments become pertinent.

You have a vertically
descending pulse

Nonsense. You have assumed, without justification, not only that the
warhead will explode exactly vertically, but also that it will explode
exactly vertically above two different poles, in two different places,
at the same time. That is impossible.

EMP is produced by the prompt gamma emission displacing
the charge in the ionosphere. To create it you need an
explosion substantially above that and generally if the
spikes were withing a mile of each other there would be
little difference in the unduction.

with essentially dipole receiving
antennas (ignoring any horizontal runs of wire
across the roof) trying to work in "near-vertical
incidence" mode. Dipoles have a null in the
direction of their axis.

Sorry, you have made yet another unwarranted assumtion. You have
additionally assumed that both poles are mounted exactly vertically.

That is necessary for a lightning conductor to work
otherwise the sharp point doesn't spray the charge
upwards towards the downcoming strike. The same
effect is why the conductor is as high as possible,
it needs to intercept the strike before any other
upwards flow from trees, buildings, etc..

My
question said nothing about the orientation of the poles. In practice,
by far the easiest way to put poles on a roof is to lay them flat on
the slope of that roof, in whatever orientation is convenient.

Certainly easier but completely useless if you are
trying to attract lightning.

Please try answering the question again, this time a little more
carefully.

OK, that's done above. To summarise, both will attract
equally but the resistor would in reality blow like a
fuse while the copper could take the surge.

[Mod. note: I feel this thread may be becoming slightly acrimonious:

I hope that doesn't happen. I made certain assumptions
in good faith and I have apologised to John if he got
the incorrect impression that it was an argumentative
device. That was not the intention, the answers I gave
should have illustrated the benefit of matching which
was the point in question. I have now answered directly
though I suspect they will again not be what John was
expecting and I can understand why John might feel
somewhat exasperated as a result. Having had to design
equipment to survive EMP, I probably know more about
the subject than is optimal for the discussion.

please try to treat each other with courtesy, or alternatively take
the parts of the discussion with little direct relevance to
astrophysics research to private e-mail, where you can say what you
like -- mjh]

It is certainly drifting off-topic and if John wishes
to switch to email I am happy to continue that way.
My address is at the top of this post. I don't bother
anti-spamming it as it has been publicly available
since before spam was invented :-(

George

wrote:
"John (Liberty) Bell" wrote in message
...
wrote:
"John (Liberty) Bell" wrote in message
...

You want something to support the
wire and it needs to be correctly shaped to provide that
support without distorting the field. You can find the
best shape by computer modelling but we are not a
plastics moulding company. Far simpler just to buy an
off-the-shelf part.

That would be true if that standard part gave similar performance,
which it presumably does in your designs. However we already know that
the detector and connector contribute a significant amount of noise in
their own right, in the arena of radio astronomy. That significance
becomes much greater when your preamp is a further 20 dB quieter.
Significantly improving s/n ratio before the preamp would obviously be
worth doing if this could be achieved for the modest cost of getting a
plastic moulding company to supply you with a run of insulating washers
of appropriate dimensions.

snip material going off topic. (I have already mentioned that this amp
design permits fairly relaxed component toleraces, and so there is
little point in discussing the need for tighter component tolerences in
other products, here.)

If you can improve noise by 20dB over current levels
then it would be of huge value

I can, and more, if we are referring to amp, not system.

Then you have a fortune at your fingertips.

I will not take that advice too seriously until/unless I have a
microwave engineer ready to do the 'obvious' twiddly RF detailing, and
potential customers clamouring for the product.

That wasn't advice, just an acknowledgement of reality.
My advice would be that your first step should be to
produce a SPICE model (or whatever your favourite
simulator uses) to find out if it works before building
prototypes.

As already indicated under Steve Willner's thread, I have not been
involved in electronic design for about 25 years, except to the extent
of recently applying my thus acquired knowledge to the problem of
further reducing preamplifier noise (as a consequence of our discussion
under Pioneer and Ranging). (Even when I was thus involved, this was
more in a research as opposed to manufacturing environment).
Consequently, I think I would be better advised to go into a
partnership with a company that is already 'tooled up' for RF/microwave
manufacturing.

Building shouldn't be too much of a problem
but measuring the noise level is going to be _really_
difficult (A.K.A. expensive!!!).

That was equally true for prior art LNAs.

In practice, testing that the idea works at particularly low
frequencies is easy and inexpensive. By chaining amplifiers together
(without an active input signal!) it does not take too long before you
can display the noise floor of both the prior art device and the new
device together on an oscilloscope, for comparison purposes.
Viability at higher frequencies is simply a matter of extrapolation.

snip additional material going off topic

The only reason for my final question was to demonstrate that the whole
subject of optimum impedance matching could change if your circuit
allows you to treat the sensor as a current injector, not as a voltage
source. However, it seems from your response that I may be the only one
to have implimented a design solution with a current injector before
(to the delight of the customer). Bearing this firmly in mind, perhaps
my question could have been more appropriately phrased thus: If you
terminate a TV arial (pointing towards the transmitter) with a 75 ohm
resistor between its two connectors, how much current flows between
those terminals. If you now connect a piece of copper wire between
those two terminals, how much current flows then? More, or less?

I don't bother
anti-spamming it as it has been publicly available
since before spam was invented :-(

What we have done in such situations is set up an autoresponder on the
original address, and then set up a somewhat different email address on
the email server that only genuine respondents can get to. You can
delete the autoresponder if and when you know there is negligible
chance of genuine respondents still using the original address. (We can
even supply you with a suitable email server, if your current host does
not provide that flexibility.)

John Bell
(Change John to Liberty to bypass anti-spam email filter)