David asked:
Where does this non-linearity get corrected?
What test modulation patterns could I use to verify and calibrate the
[TPLL] gadget?
What other tests would you recommend?
Of course you could try and do some form of post data processing (before
filtering), but the KISS answer for a high end TPLL tester is to just limit
its lock range.
I set up my TPLL circuit so that it can Phase lock only over ~ +- 2e-8 freq
range and limit the analyzing range to around +- 1e-9
To calibrate, I use a 10 MHz source that can be offset by small known fixed
freq amounts.
First I zero the TPLL's DC output with exactly 10 MHz applied, then apply an
offset of +- 1e-11, +-1e-10, +-1e-9 and finally +-2e-9.
A Tbolt and LadyHeather makes a great offset reference generator for this.
It is done by changing the Dac voltage while in holdover, using LH's
filtered phase and freq plots to verify the amount of freq offset.
To further check both DC and dynamic performance of the TPLL,
I use a stable OXCO with known EFC gain at 10MHz and apply a small know
attenuated signal into it's EFC at various waveforms, amplitudes, and
frequencies.
Only need a simple audio function generator and a TBolt to calibrate and
verify that the TPLL is working correctly.
TPLL advanced note:
Have to be careful of anything that will roll off the response of the
controlled or test oscillator's EFC input.
Just the capacitance of the EFC's shielded cable of a dual oven 10811 is
more than enough to screw things up if you are not careful.
ws
From: Tom Van Baak David,
The 10811 linearity may not be that good rail to rail, but how linear is
yours over the narrow EFC range you use during a stability measurement?
I would expect it to be very good over any given interval of a few or
tens of mV.
/tvb
**
From: David Hooke
Subject: [time-nuts] TPLL linearity and other questions
Hi All,
I've built a measurement PLL along the lines of W. Riley's and Warrens's
designs. It's working, but I have a hole in my understanding of them.
The transfer function of the OCXO (10811-60159 in my case) is decidedly
non-linear, implying that that the voltage output is also not
proportional to frequency. Where does this non-linearity get corrected
before analysis in either Warren's or Bill's systems?
Also, without access to a 5062C as a noise source as some lucky folk do,
what test modulation patterns could I use to verify and calibrate the
gadget? I have an HP8662A, 3325A, 33120A and 8648C which I can rig
together to generate various phase or frequency modulations. Simple FSK
gives the expected square wave output (which is where I noticed the OCXO
non-linearity). What other tests would you recommend?
Also, I have 24bit, 192k sound cards which work down to around 1Hz. Are
there any analysis packages I should look at to explore PN output, or is
spectrum lab the best best?
Thanks from a beginner.
david
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