Hi again Bob I tried to do some measurements with a DMTD! In my junk box I found a little PCB from earlier experiments on that topic, with a power splitter and two SRA-3H mixers, it was even already wired for the DMTD configuration. So I gave it a try! As "transfer oscillator" I used my HP 8663A signal generator, and set it high in frequency by 10 Hz. To the two mixers, I connected the two 10MHz signals and at the mixer outputs, I put a little lowpass filter with 100Hz corner frequency. The output signals from the two SRA-3 mixers are almost 0.5Vpp, so I tried to feed them directly into the HP 5335A TIC and used the TI mode to measure the delay between the two signals. This gives 10 readings/sec, which I try to process with TimeLab. It does give some interesting graphs, but I don't know yet how to correctly set up TimeLab for this kind of measurement. I.e. now, I get an ADEV in the order of 1e-4 (at tau=1sec) to 1e-5 (at tau=500sec). So does that mean I simply need to multiply this with 1e-7 to get the *real* ADEV at 10MHz? this would mean that my real ADEV is in the range of 1e-11 to 1e-12, which is indeed my target value, BUT I expect that things are not that simple. (i.e. what if I didn't set the transfer oscillator high by +10Hz but only by 9.9Hz for example). Can you give some hints on that? Of course I also did the noise floor test (i.e. I fed the 10MHz signal into a power splitter and connected the two outputs to my DMTD with two different lenghts of cables. This gave results starting at 1e-4 going down to 1e-7, maybe it would have gone even lower but I measured only for a couple of minutes.)
Can you give some hints on that? Best Tobias HB9FSX On Fri, Apr 3, 2020 at 1:45 PM Bob kb8tq <[email protected]> wrote: > >> Hi >> >> The quick way to do this is with a single mixer. Take something like an >> old >> 10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz. >> >> Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone. >> That tone is the *difference* between the 10811 and your device under >> test. >> If the DUT moves 1 Hz, the audio tone changes by 1 Hz. >> >> If you measured the 10 MHz on the DUT, that 1 Hz would be a very small >> shift >> ( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change >> in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase ). >> >> *IF* you could tack that on to the ADEV plot of your 5335 ( no, it’s not >> that >> simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1 >> second. >> >> The reason its not quite that simple is that the input circuit on the >> counter >> really does not handle a 10 Hz audio tone as well as it handles a 10 MHz >> RF signal. Instead of getting 9 digits a second, you probably will get >> three >> *good* digits a second and another 6 digits of noise. >> >> The good news is that an op amp used as a preamp ( to get you up to maybe >> 32 V p-p rather than a volt or so) and another op amp or three as >> limiters will >> get you up around 6 or 7 good digits. Toss in a cap or two as a high pass >> and low pass filter ( DC offsets can be a problem ….) and you have a >> working >> device that gets into the parts in 10^-13 with your 5335. >> >> It all can be done with point to point wiring. No need for a PCB layout. >> Be >> careful that the +/- 18V supplies to the op amp *both* go on and off at >> the >> same time …. >> >> Bob >> > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
