Ulrich Bangert wrote: > I built a DMTD and made measurements with it on the few good oscillators > that I own. While my experiments have shown that the principle works
Ulrich, I am happy to hear you got the project working. I know last time we talked you had your doubts... > First Pitfall of DMTD: Transfer oscillator effects do NOT cancel out > completely ... > the two down mixed signals at the SAME time! This is due to the fact > that at the same time the transfer oscillator is in the same state > concerning both channels. Yes, this is one factor that determines the noise floor of a DMTD implementation. Do the math to decide if the level of noise is within your design spec. Note that the better the transfer oscillator, the less effect this "pitfall" has on your error budget. If you decide to use a 1 Hz beat, then it would seem that the 1 s ADEV is the parameter that dictates which xfer osc to buy. Note also that, based on my limited experience, most commercial mixer implementations use a much faster beat note: 10, 100 Hz, even 1 kHz. A faster beat note may help your concern #1 above, and #2 below. > Second Pitfall of DMTD: Decreasing slope to noise ratio counteracts the > magnifying effect of down mixing ... > Had we a noise-free signal available then there were no problem at all > because the noise-free signal crosses the zero line at a sharp defined > point in time. However, noise-free signals are an idealization not given > with real-world signals. There is always a certain amount of noise, > sometimes more, sometimes less, a fact that documents itself in the well > known signal to noise figure. Don't lose sight that you are building a DMTD -- whose sole purpose is to measure noise. Thus one would expect a large amount of AM and PM noise at the zero-crossing. The noise you see is some sort of rms sum of ref osc noise, transfer osc noise, other internal instrumentation noise, and of course, DUT noise. So the better your design, construction, and measured choice ($) of parts the more the DUT noise comes through compared to all the other noises in the system. > Third Pitfall of DMTD: Phase corruption due to mutual crosstalk ... > If you buy a good coaxial cable, this may have a shielding effectiveness > of 80 dB at radio frequencies. If you spend some bucks more you can get > a shielding effectiveness of 90 dB. 100 dB shielding is top and only > possible with double shielding and I do not remember to have seen a > shielding effectiveness been advertised better than 110 dB. So, 100 dB This sounds like a normal quality-of-engineering issue. I don't have design experience but I know from taking apart lots of gear that high-end, low noise systems seem to use hardline instead of coax. They also place PCBs inside their own shielded or solid brass boxes. Perhaps you could do this too if your noise floor isn't as low as you need. Someone else on the list can tell you what the typical isolation numbers are for this type of construction vs. a cheap single PCB and coax design. ... > channel? Perhaps equipment like the TSC 5110 uses a very high isolation > switch to keep the second signal completely out of the box while it No switches; the ZCD rate is somewhere between 100 Hz and 1 kHz. Have a look at the TSC data sheets and design papers. /tvb _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
