Re: parametric detector

"doug dwyer" <dd@xxxxxxxxxxxxxxxxxx> wrote in message
news:98vvxpAYmT0CFwn1@xxxxxxxxxxxxxxxxxxxxx
> In message <Z6Qze.7334$Dq.3916@xxxxxxxxxxxxxxxxxxxx>, colin
> <no.spam.for.me@xxxxxxxxxxxx> writes
> >Hi,
> > Im looking at using a parametric method for a phase detector,
> >The source (~4mhz) is already very strong but the sideband, @ 1-10hz, is
> >extremely low, I am considering using a quartz crystal as the most
sensitive
> >slope detector but to get a significant signal to noise ratio I would
need a
> >crystal with a Q of over a million. I dont expect acheiving such a high Q
is
> >practical, unless cooling it drasticaly would help ?
> >
> >The main noise problem is 1/f noise especialy as a high Q circuit would
need
> >a high impedance amplifier and a mosfet would have too high 1/f noise at
> ><10hz.
> >
> >However I cant see a way of using a paramatric method for a detector, at
> >least not with any gain, I havnt realy looked at them much before,
although
> >I know the basic principles of operation of using as using variable
> >reactance in a ac voltage dividor or charge/voltage pump.
> >
> >I came accros an old parametric amplifier by AD the 310/311 but I doubt
they
> >make this any more, at least something like this might be usefull after a
> >diode detector, although I am not sure about how much the diode noise
> >particularly 1/f noise would be.
> >
> >Colin =^.^=
> >
> >
> I understand that you intend to apply a 4MHz signal with low sidebands
> to a high Q crystal offset slightly so that pahse modulation will appear
> as amplitude modulation

Hi, thanks, yes thats right :)

> and then perhaps employ a low frequency
> spectrum analyser to look at the phase noise. Is this the idea?

well it is actualy a modulation signal im trying to detect rather than just
noise but it is very similar problem, of course the phase noise of the
reference oscillator is a big issue but I intend to try and null most of
that out I hope.

> If so:
> A good quality 4MHZ crystal wcan have a Q approaching 1000000.
> All depends on the diameter/contour surface polish and the quality of
> quartz used.

I'l go looking for some nice shiny quartz rocks... I chose 4mhz as it seemed
they had the lowest series resistance for the frequency coresponding to
highest Q although manafacturers dont list Q on a per frequency basis, its
generally 20k for off the shelf types, im not sure if its advantagous to use
an overtone or not. Im not sure if i can get any better ones, but the ones i
bought seem to demonstrate a higher Q than anticipated from the simulation.

> The crystal could be operated at its series resonance and terminated in
> 50 ohms so that a <1nVroot Hz amplifier could be used.
> The technique is not unknown

aha, I have looked at using it in parrellel mode trying to use the
considerably high voltage generated from resonance to improve snr before
detection, although I was woried about destroying the crystal I can easily
see 50v pk-pk.
I will experiment with the series mode in ltspice, the problem with most
1nv/hz^-2 amplifiers is 1/f noise is stil quite high as much as 1uv total
below 10hz, not that many manafacturers seem to specify noise below 10hz,
but certainly it has meant i have had to rethink things, I was hoping a
parametric amp would avoid the 1/f problem unless i am mistaken here ?
I have managed to get significant gain using the reactance of a varactor in
a tuned circuit, but there still the problem of the preceeding detector
diode.


> Marconi manufactured a tuneable cavity in
> the 400MHz range for the purpose.

so could a 400mhz cavity have a higher Q ?

> The orthadox approach would be to mix a high q crystal reference
> frequency with the "unknown" and explore the LF result with an analyser.
> I did well with this approach employing the steep side of an ssb crystal
> filter to attenuate the resultant mixed carrier whilst looking at the 0
> to 3kHz ssb noise.

Ive seen test aplication where the same type of oscillator is used as the
reference if it is a VCXO of course.
Ive tried using a PLL with a OCXO but this proved disapointing, of course
one never fuly knows if its the topology used or just unlucky construction
with such low snr, from what I can gather I think 1/f noise can be degraded
with handling/soldering etc.

> With careful use of low noise gain blocks stepped attenuator to overcome
> the dynamic range limitations of the 141T I could see down to -145db
> from the carrier.

Thats quite impressive.

> For another approach see error multipliers , these circuits are seldom
> seen now but you may be able to buy an old Tracor ? for the purpose.

I will look into this with interest.

> Finally a counter with low noise reference, high clock frequency and
> multiperiod capability can be programmed to convert jitter to ssb noise.
> See "AllensTime"

thnks

Colin =^.^=


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