[time-nuts] question Alan deviation measured with Timelab and counters

[Bob Camp]

Hi

Wow, that guy did a *lot* of work on all that. Congratulations to the him and those kind enough to translate it.

Per an off list request there’s at least one additional circuit to add to his list:

Wire the RF as shown on page 17 of the MAdS slides but:

1) Use an RPD-1 (or modern equivalent) as the phase detector. 
2) the 50 ohm on the filter input goes to 500 ohms. 
3) Run an OP-37 (or modern equivalent) on +/- 18V 
4) + input of the OP-37 goes straight to the lowpass filter output 
(Bruce has correctly recommend protection here if you don’t trust your supplies)
5) The - input of the OP-37 gets 100 ohms to ground and (roughly) 1K ohm to the output 

You now have a low(er) noise amp with fewer parts (other than the -18V supply).  It also has no low frequency cutoff. That makes 
it useful for a few more things. There may / will be some peaking in the lowpass filter on the mixer. Exactly how much depends on the parts 
you use. You can either measure the peaking or model it.

I would put blocking caps in between the mixer RF inputs and the J1 and J2. Mixers seem to last longer that way. John has suggested 
running blocking on both sides of the BNC’s to nuke a ground loop. That may also be a good idea. You might simply try running 
the mixer using it’s input transformers for isolation. That may or may not work depending on what’s really inside. 

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The other comment is that the “lock box” portion of the circuit for pages 17 and 18 is missing. The lock box drives the EFC on one  of
the OCXO’s you are testing. Without it you can’t really do a phase noise test. Yes, it’s not an exciting circuit, but you do need it
to make the thing work. Given that the guy already had a massive number of topics to cover, it’s understandable that it’s missing.

What you need:

1) A DC pickoff point from the preamp. In the circuit above, the output of the OP-37 is fine. 
2) Some means of varying the gain. A pot or switched resistors both work. 
3) An ability to crank in an offset. Again a pot works pretty well.
4) The ability to invert the signal if needed. 

The amp, pots, switches, drives the EFC on one of your test devices. You fiddle the offset to get them locked with zero DC on the preamp output.
You fiddle the gain to get the low end of your measurement down far enough to be useful. I’ve seen people do this all with a couple of pots and
a 9V battery. I typically do it with a pair of op-amps. 

You can add caps to the circuit. You can model the cutoff of the circuit. You can noise load and measure the impact of the circuit. All of these 
enhancements help at the low frequency end. None are needed if you are just starting out. 

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Once you have this preamp (the OP-37 stuff) built up, it also works as the input for a single mixer ADEV setup. You need to put a limiter on the 
output, but there is no more RF work to do. With a little planning, you can have a phase noise and ADEV front end all in one box / all on one pc board. None
of it is really critical. I’ve made them up a number of times on a chunk of copper clad. It will probably take you more time to build up a low noise / clean / reliable /
ground isolated power supply than to make up the rest of the circuit. A ground loop through any of this will drive you a bit nuts. 

I would make up the power supply from scratch. Current required is nearly nothing (< 100 ma). You want an isolated supply, and if if faults you have big problems. Good 
old 7818 and 7918 regulators are plenty quiet enough. Op amps are good at rejecting audio frequency noise. Resist the temptation to hook anything else to 
the supply that feeds this box. Depending on the op-amps you pick, your supply may be +/-12 up to +/- 20V. The idea is to run as much gain as you can in the preamp
without getting into clipping. The idea is *not* to blow out them out with a higher than rated supply. 

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Yes there’s a bit of calibration involved in getting this running. You can play with other mixers, they won’t be 500 ohm output. There is nothing special about an op amp 
that’s old enough to vote. This isn’t quite a single weekend project. It is if you have all the parts on hand and have done it a couple times. It should not be more than a couple 
weeks of fiddling to get one running. Cost wise, you should be able to do it for < $100 using perf board and copper clad. If you go the full up ADEV route that probably tacks on 
another few bucks and another weekend or two.   

Bob