parametric detector

[colin]

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 =^.^=

 

[doug dwyer]

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 and then perhaps employ a low frequency
speactrum analyser to look at the phase noise. Is this the idea?
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.
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, Marconi manufactured a tuneable cavity in
the 400MHz range for the purpose.
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.
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.
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.

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

note demon email address invalid

 

[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 =^.^=

 

[doug dwyer]

Hi, thanks, yes thats right


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.


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.

 

[colin]

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.

Hi again,

I looked at using series resoance with a 50ohm resistor but the much lower
voltage means a much smaler signal even tho the slope is higher, I did
consider a curent transformer aproach but think it wpould be pushing things
a bit to actualy get more SNR.

I looked at auto zero amplifiers and found they auto cancel the 1/f noise
along with the dc offest, the AD8552 looks superb, very low voltage noise
1-10hz and very low input curent/noise too, so i can use a highly tuned high
impedance/high voltage section to give maximum signal, so i got one of these
now, (I already had one in my parts box !) and boarding it up now.

I couldnt find much on error multipliers, maybe il look some more ...

Colin =^.^=

 

[doug dwyer]

I couldnt find much on error multipliers, maybe il look some more ...

I looked and did not find, the technique was to multiply a 1MHz test
signal to 10MHz mix with 9 MHz clean reference filter the resultant 1MHz
and multiply again to 10MHz mix with the reference again can be done 4
0r 5 times.with increasing sidebands the exact opposite to dividing.
Various manufactrurers in the 60s