Re: parametric detector

In message <xzmAe.8581$Dq.7318@xxxxxxxxxxxxxxxxxxxx>, colin <no.spam.for.me@xxxxxxxxxxxx> writes 
"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.
Intrinsic acoustic loss is per cycle so ltimate q reduces from about 10000000 at 1 MHz to 100000 at 100MHz.
Modern crystals use grown quartz which can be much lower Q however the best grown approaches the ultimate.
A 5 MHz 5th overtone will have a q of 1 to 2oooooo but cannot be driven hard.


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 =^.^=



--
dd
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