"ralph sansbury" wrote in message
...

"George Dishman" wrote in message
...

"ralph sansbury" wrote in message
...
George, I know you are a superior EE and that

We are talking about the size of the intermediate frequency
range
relative
to the original range 1MHz is small relative to 200MHz but
not
to 1Hz

The terms "narrow band" and "wide band"

But the size of the intermediate frequency relative to the
original
range is what we are talking about. You seem to have your own
subjective
read of what others say and on what you say without understanding
that words are ambiguous and you have to say out loud what you
mean
or what you think is meant before going off halfcocked.

This is your original phrase:

A resonance tuner picks out the difference frequency ..

and my reply:

Wrong, it doesn't pick out one frequency, it passes a
complete band of frequencies to the FFT.

If you had said "By _the_ frequency I meant the IF band, we
would not be arguing. Instead you said:

OK relax. It is a small band of frequencies around the single
difference frequency. This is always understood.

There is no such thing as "the single difference frequency"
and if you start thinking in terms of a single frequency, you
will get entirely the wrong understanding. I realise you are
not familiar with much of this, few people are, but that should
make you more amenable to help instead of fighting to keep
using misleading ideas.


What you are saying is very different to what is being
done. It may be that this is because you are using terms
in an unconventional manner but you will then hit problems
in referring to your text books.

No. I am using terms and descriptions of mixers as in my
1985 Shrader Electronic Communication text which shows
a (tuned)resonant inductor and capacitor circuit for the
intermediate
and different ones for the sum and the input frequencies.

That technique works well for single frequencies. For example
if you were trying to pick out Radio Luxemburg and reject the
rest of the medium wave band, it was ideal. If tuned to the
Luxemburg frequency, it would boost that and reduce all the
other stations.

I dont want to keep arguing this point but what I am saying
is in principle what is being done.

No it isn't. What the DSN is trying to do is exactly the
opposite. Their task is like building a repeater on a hill
to re-broadcast the medium wave band into a valley. In that
case the equipment has to amplify the whole band because
different people want different stations at the same time
and if it boosted the BBC for some people, it would swamp
Luxemburg for others. The aim in this case is to amplify
all frequencies across the band equally. That is what the
DSN early stages do and treating it like a tuned circuit
is entirely inappropriate and misleading.

You are obscuring the essence
of what is being done which is the use of Fourier's transform to
obtain a Fourier series representation of the noisy received
oscillations
You also seem to have changed your understanding of the nasa
documents to come around to my initial impression

Go back and read my emails. I spent about six months trying
to get across to you that there was no resonant circuit in
these stages. I can show you at least a dozen mails where
I said that.

Nothing in my understanding of the documents has changed in
any way.

Again it may be clearer but it is wrong. It is not just
the carrier oscillations that are digitised, it is the
whole signal, oscillations plus random thermal noise
and any other sources such as the galactic background.

Your understanding is wrong. I did not say CARRIER
oscillations
You can't change the meaning of
'oscillations' to mean only the
part due to the spacecraft transmitter

"oscillations" means something regular,

Not necessarily. And obviously not in this context

Elsewhere you talk of a "sequence of voltages". If you stick
with that terminology which is entirely accurate, the
confusion won't arise.

The final stage is the carrier PLL, not the FFT. All
the FFTs are removed from the chain once the PLL locks
on and they play no further part in the process. It is
the PLL that tracks the drifting signal and gives us
the accurate measurement.

The FFT as I was using the term includes the PLL.

The two are entirely diffeent and separate.

Not the way I am using the term. Note I say how I am using the
term.

For everyone else, and in the DSN documents, FFT means Fast
Fourier Transform while PLL means Phase Locked Loop. If you
mean something else by them, you will have to define your
usage, but you cannot expect me to know that unless you say
so.

.. The PLL locks on and tracks the carrier and it
uses a digital phase comparator that probably treats the signal
as 1s and 0s.

That is what I thought initially and you said I was wrong.

Ralph, check the emails. I explained to you how a phase
comparator works and gave you the simplest example of
an exclusive-or gate. I also explained what JPL mean by
type 2 and type 3 comparators.

Evidently
you have changed your mind.

No.

It doesn't matter however for the
purposes of showing the essence of the procedure and the
rationale
as to why it is reliable.

Agreed.

There are several levels of processing that you are skipping
over
which are very important in establishing that the signal is
genuine and from the right craft. Ultimately that is your main
concern, isn't it?

Yes. I welcome your pointing this out. But I deplore
the obscure and argumentative way that you are doing it.

Most of what you say is OK but you still treat the initial
stages as if they worked at a "single frequency" (your phrase)
which will lead you astray later if you don't correct it, and
in these posts you glossed over several of the important later
stages. These are the ones that are responsible for finding
and tracking the signal so need to be dealt with accurately.

and I mentioned that the
movement of the Earth etc requires different patterns to be
obtained successively but the point is that the FFT procedure
finds the underlying pattern and it is this that is used to
compare to the given sequence of 1s and 0s.

No it isn't. The final FFT is only used to set initial
frequency for the carrier PLL. If that locks, the
bandwidth is reduced to improve the signal/noise ratio.


You are saying the same thing that I was saying. I think
it is clearer to say it without the jargon.

Clearer but completely wrong.
No clearer but not detailed.

The FFT does not compare
Again I did not say this.

You said above:
.. the point is that the FFT procedure
finds the underlying pattern and it is this that is used to
compare to the given sequence of 1s and 0s.

I read that as saying the FFT is used to compare a pattern
of bit to a given sequence.

I said that after the
FFT procedure finds the dominant sine function,
this function is then compared to the observed set
of values which I thought you said earlier was reduced to
a set of 1s and 0s and that this was compared to
the corresponding observed set to get the degree of
error.

The FFT finds the frequency with the highest amplitude, that
is correct. The computer then passes that measured frequency
to the PLL which is a completely separate system. It contains
a circuit that generates a known frequency and a phase
comparator. The phase comparator is described as "digital"
and probably uses only the polarity information, treating
the signal as 1 or 0 as you say. I was pointing out that it
is part of the PLL that does this, not the FFT.

Again
that has to lock before the signal can be decoded using
a phase detector. Then it gets decoded through the error
correction scheme. There are many critical steps after
the FFT, and in fact the FFT plays no part in the decoding
process whatsoever.

Again I did not say that it did.

It reads that way since you talk of the FFT comparing against
"the sequence of 1s and 0s".

It is very inaccurate when the DSN document tells you
the analog band is the digitised band is 110MHz wide and
the signals of interest are of the order of 1Hz wide.

You are quoting the wrong document. We are talking about the
intermediate frequency
being smaller that the original frequency. Is that so hard for
you to understand.

What matters is how wide the frequency range is compared
to what you are looking at.

No what matters in this context is the size of the
intermediate frequency relative
to the size of the original frequency.

The band is 110MHz wide. You said "A resonance tuner picks out
the difference frequency" and clarified that as "It is a small
band of frequencies around the single difference frequency."
You cannot treat a 110MHz wide flat band as a "single frequency".

This makes no sense. Electrical oscillations add by the law of
superposition;

Yes, which is why it takes a special ciruit to get around that.

They dont multiply.

Dual gate fets and other methods of implementing mixers do

If they do then how do they.

I thought you wanted to discuss principles? There are many
different techniques but most use some sort of non-linearity.
A Field Effect Transistor for example inherently passes a
current that is proportional to the square of the voltage
because of the underlying physics. Diode mixers use their
exponential relationship between current and voltage.

Your text book should cover "diode mixers". Does it have
a chapter on wide bandwidth designs?

All I can see is superposition and
then various filters to extract the desired frequency or range of
frequencies

Superposition means the voltages add so it does not change
the frequency. BTW, superposition is a term usually applied
to EM waves rather than signals on wires.

Perhaps you have and analogue to digital converter to change
the incoming
frequencies to digital and then multiply them and then convert
this back instead
of the filter part of the mixer I see in my 1985 text.???

No, the frequency is too high to be digitised directly. The
purpose of changing the frequency is to bring it down to
something slow enough for the analogue to digital converter
(ADC) to work with.

because that is their intended function and we poor designers
have to make them do it well. It's what engineers get paid
for (though I personally work on the digital side).

The mathematical fact that a sum of sine
and cosine functions can
be represented as a product of related sine and cosine functions
has to be mentioned dont you think?

Only if you don't already know it.

Yes. And if I already knew it well we would not be having
this discussion would we?

You do know it, we discussed it by email for several weeks so
I felt you didn't need that explanation. Since you introduced
it in this thread I think I guessed correctly.

You seem to have your own subjective
read of what others say and on what you say without understanding
that words are ambiguous and you have to say out loud what you
mean
or what you think is meant before going off halfcocked.

If you use an acronym like FFT to mean something other than
what everyone else means but don't say so, you shouldn't be
surprised. When we are dealing with a band of signal and
noise less than 1Hz wide and you call the original IF which
is 100 million times wider "the single frequency", you must
expect your readers to be confused. Threads drift but look
back at the quotes above and that is what you are now
claiming you meant.

George