On Saturday, October 24, 2015 09:03:21 AM Charles Steinmetz wrote: > Bruce wrote: > >The comparator circuit measured was the front end of David Partridge's > >divider. I merely measured the 10MHz output. > > The MAX999 and ADCMP600 are the two comparator options noted on > David's schematic. Both parts suffer from a number of the design and > die-level issues I noted in my previous message, and I have never > obtained particularly good PN with either one. Also, even the > relatively direct path to the 10MHz output goes through two 'AC04 > inverters and an 'AC541 line driver, which contribute additional PN. > > >One thing that I have found is that at low offset frequencies the measured > >PN is substantially reduced when air currents and other sources of thermal > >fluctuations are reduced. Even the effect of a thin piece of paper used as > >an air current shield can be easily seen. > >With careful shielding from thermal fluctuations I measure the low > >frequency offset PN to be substantially lower than the datasheet values. > >I've seen this effect with everything for which I've measured the PN. > > Agreed. Whether or not it is explicitly stated, I take "all > circuitry to be enclosed and protected from drafts, and allowed to > stabilize thermally before testing" as a given with any sensitive > time or voltage circuit. > > >One problem with comparators when attempting to measure their PN is > >that they don't have sufficient output to drive the TimePod input directly. > >An amplifier is required. > > The spec sheet says both TimePod inputs accept -5 to +20dBm into 50 > ohms. -5dBm is less than 0.4Vp-p, which requires less than +/-4mA > from the source, so a 0-5v comparator output feeding a > capacitor and a 560 ohm series resistor should work fine as long as > the comparator can source and sink at least 4mA. The fly in the ointment is that with such low level inputs (the LTC6957-4 evaluation board will deliver +4dBm into 50 ohm) the Timepod phase noise floor is uncomfortably close to the phase noise floor of the LTC6957. > > Alternatively, a 0-5v comparator output could be buffered with three > 'AC04 inverters in parallel, or an 'AC line driver -- but that adds s > the PN of the gates. > Not if one uses a pair of drivers one to drive the Timepod Ch0 input and one to drive the Timepod CH2 input. > >A resistor from point A to ground in the Wenzel style shaper you attached > >has little effect on the output symmetry due to C4. > > It has just enough effect to correct the very small (<1%) asymmetry > due to the unbalanced drive. (With no resistor at Point A, the duty > cycle is ~51%/49% high/low.) > > >However it does allow the output amplitude to be adjusted. > > According to the simulation, the resistor has no effect on the output > amplitude until it is well below 1k ohms (at 1k ohm, the symmetry has > been WAY overcompensated and the duty cycle is ~45%/55% high/low. > Not true even 10k increases the output signal amplitude by 130mV or 2.6%. However that is smaller than the tilt/sag in the high level output due to feedthrough via Cbe of the input transistor when it is off. > Best regards, > > Charles > It would perhaps be useful to measure the PN characteristics of several comparators and other sine to square converter circuits using a Timepod or equivalent.
Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
