[time-nuts] Some questions related to time-interval measurements and "modulation domain analysis"

[Ole Petter Ronningen]

Hello, all!

This will be a long post, my apologies..

A while back I scored a HP E1740A time interval analyser. This unit has a
48.8 pS resolution, which is great, but it can only measure up to 3.2uS
with that resolution, which is a bit limiting. It can also store up to 500K
readings internally, so it can measure pretty fast.

While thinking about interesting things to use the instrument for, I
stumbled across "modulation domain analysis" - which this counter should be
perfect for. I am just starting to scratch the surface of the topic, but I
ran a couple of experiment that surprised me. That is, the results are far
"better", or at least far more sensitive, than I would have expected. So I
am looking for advice on what I may have done wrong..

The setup is basically this: a Rohde&Schwartz SMIQ-03B RF generator
provides the signal. The generator has a decent OCXO reference. The signal
was a 10Mhz sine at 7dBm, AM modulated at 10KHz, 0.5%(!) modulation. I
verified the signal on an HP8563A, and observed a nice, low peak 10Khz from
the carrier, as expected. Repeat on a Rigol DSA-815, same result.

The e1740A was configured to take 200k time interval measurements of the
signal on channel 1, paced at 25 edges, triggerlevel 0v. I.e. I should get
200K measurements of how much time has elapsed from one rising edge, to the
25th following rising edge. These measurements are back-to-back, i.e. the
stop-edge of one measurements is the start-edge of the next measurement. So
the counter captured a gap-free time-record totalling 500mS.

I (slowly!) downloaded the samples to my computer, and basically did an
FFT. The resulting spectrum showed a clear peak at 10KHz, growing and
shrinking as the AM modulation index was varied. Only the first 1/5 or so
of the spectrum gave much information, as the noise grew pretty quickly on
higher fourier frequencies.

So here is my first question - since this signal is amplitude modulated,
the only way this can (should) show up on the counter is if the triggering
is not "perfect" - that is, it does not trigger at precisely 0v + whatever
DC offset may or may not be on the signal. Is my undestandig here correct?
A perfect zero-crossing detector would be immune to AM?

If so, this was a very valuable lesson in AM/PM conversion, I think. I
would not have expected to so clearly see AM as little as 0.5%..

But there is a second conondrum in my experiment. I also saw an unexpected
much bigger peak at 64KHz in the spectrum.

In order to verify my methodology, I repeated the measurement on an Agilent
53230A, continous timestamp at 100Khz samplerate, and FFT the result. The
resulting spectrum was pretty much identical, including the large peak at
64KHz. This was also a little surprising, since the resolution of the
53230A is a lot better, but I suppose resolution is less important with so
many datapoints.

There was also a much bigger peak at 50KHz which I suspect is originating
inside the 53230A, since it does not show up on the e1740, nor on the
8563a. I plan to investigate this spur further.

I stuck the signal back on the spectrum analyser, to see if the 64Khz
signal was in fact real, or an artifact of my measurement setup, which I
was suspecting. Nothing on the DSA-815 nor on the 8563a. Not untill I have
set RBW 30Hz, videoaveraging 80+ measurements on the 8563a do I see a peak
at 64Khz from the carrier, at around -80dBc. So the signal seems to really
be there.

I suspected a switching power-suppy might be coupling on to my cables, so i
switched the RG-58 out for double shielded LMR-240, with no difference at
all in the measurements. The peak was there both on the e1740A and the
53230A. So I am lead to believe the 64Khz spur is real, and originates in
the signal generator, since is is visible on three separate instruments,
and two separate measurement methods.

My question is this - how in the world can the counter pick up such a TINY
signal?? Does this make sense at all?

I suppose it is possible that the spur originates in my lab somewhere, and
is in fact quite strong, strong enough to couple into the counters with a
more reasonable signal - and only appearing to be weak on the 8563a because
it is better shielded?

Thanks for any insight!

Ole