[time-nuts] DMTD Ideas

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 2 02:18:20 UTC 2009


Brian

Brian Kirby wrote:
> I have toyed with the idea of building a Dual Mixer Time Difference
> setup for testing oscillators.  I have attached a drawing I made and I
> have a few areas I need to clear up.
>
> At Point #1 on the drawing (the output of the mixers) I expect to see
> 20 mhz and the 100 hertz beat note.  My plans were to put a 3 db
> attenuator here and expecting it to provide a 20 mhz termination/match
> for the 20 mhz part of the signal.  Do you think this would be an
> adequate way to terminate or does it need a better system ?
>
Usually NIST use a capacitive termination to reflect the sum frequency
back into the mixer.
This reduces the loss and increases the amplitude of the beat frequency
output.
Small value resistors in series with the RF and LO ports are then
required to improve the input VSWR.
Follow the mixer termination with an LC filter and finally an RC filter.
> The Op Amps at Point #2 would be a something like LT1000s or so and
> they basically would only allow passing of the beat signal as they do
> not have the response to handle the 20 mhz signal.  They would have to
> have a lot of gain and also would simple RC low pass filter say for
> 120 hertz be good enough ?  The DBMs have 7 db conversion loss, the RF
> port goes into 1 db compression at +2 dbm.  The LO ports can take +13
> dbm absolute, and are recommended drive at +7 dbm.  What voltage range
> can I expect out of the mixers (next question would be how much gain
> do the op amps need....).
>
Measure the mixer output noise spectrum before deciding on appropriate
opamps.
You can do this with a sound card and a suitable low noise preamp if the
RF and LO ports are driven in quadrature.
You can test the effect of various termination schemes on the zero
crossing slope and the output noise spectrum.

Usually one would saturate both the LO and mixer ports.
Read up about the Collins hard limiter.
There is a spreadsheet to allow the effect of the number of stages used
etc to be explored at:
http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html
<http://www.ko4bb.com/%7Ebruce/ZeroCrossingDetectors.html>
> At Point #3, I thought about using some type of Schmidt Trigger like a
> 74AC14.  But is it necessary ?  The 5370B can be set to trigger from
> zero to 2 volts or so.
>
Use a comparator its jitter will be adequately low ( and interfacing
will be simpler) as the noise at the output of the mixer even when
optimally conditioned will result in a jitter of 1ns or more.
> If the 5370B can see one shot performance of 20 pS (2x10-11 at one
> second) then a 100 hertz beat note should give 2x10-13 at 1 second,
> 2x10-14 at 10 seconds, 2x10-15 at 100 seconds and so on.......correct ?
Your Schmitt trigger will have more output jitter than that.
You need to use an optimised Collins style hard limiter between the
mixer output and the Scmitt trigger input or the jitter will be large.
>
> Brian - KD4FM

Bruce




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