[time-nuts] Noise floor test of my homemade DMTD

[Pluess, Tobias]

Dear colleagues

As some of you may remember I built my own DMTD in December and I struggled
with some problems. It had quite a bit of stability issues.
A noise floor test with some 10 MHz sources gave a 2e-12 noise floor at
1sec, not so good!


[image: grafik.png]

Well, after I had this result, I was a bit disappointed and set the project
aside for a couple weeks (months, actually) and worked on other stuff.
Recently, I decided to go back to this DMTD project and tried different
things. I found out that my first OpAmp in the amplifier/limiter for the
zero crossings didn't like the fact that it was clipping! so I reduced its
gain slightly. My zero cross detector has now a gain of 20 in its first
stage (previously: 100) and a gain of 100 in the second stage (unchanged).
The IF bandwidth is 16 Hz. And after this small modification, the first
OpAmp in the zero cross detector does no longer saturate, stays in its
linear region, and I get a quite good result: around 2.5e-13 at 1sec!

[image: grafik.png]



also note that this test was a very "quick and dirty" test, i.e. nothing is
mounted in a housing, and I didn't let my instruments warm up before. I
used a HP 5335A as TIC, and a HP 8663A 10 MHz out for the DUT, and let the
HP 8663A generate 10MHz+10Hz for the REF. Just connected everything and
recorded from the beginning. In my opinion, under these circumstances,
2.5e-13 is a quite acceptable result, isn't it? it might not be good enough
to measure a BVA, but it should definitely be OK to characterise some
"normal" OCXOs. After all, I am quite happy with the result, or should the
floor be even lower?

The next step is definitely to build all four channels. With a 4 channel
DMTD, I can compare 3 DUTs against each other (three corner hat!) and use
the fourth channel to estimate the noise floor!

BR
Tobias
HB9FSX
-------------- next part --------------
A non-text attachment was scrubbed...
Name: grafik.png
Type: image/png
Size: 46442 bytes
Desc: not available
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20210426/29494147/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: grafik.png
Type: image/png
Size: 39495 bytes
Desc: not available
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20210426/29494147/attachment-0001.png>