Below your post are posts I copied from last year from me and others about the pseudo-random phase modulation in certain Tektronix counters from the 1980's and 1990's. You can read more about metastability at these links:
http://www-classes.usc.edu/engr/ee-s/552/coursematerials/ee552-G1.pdf http://www-s.ti.com/sc/psheets/sdya006/sdya006.pdf My main point is that averaging quantized values (contents of a counter at the end of a gated input) doesn't always work as intended if there is some bias in the quantization process due to metastability or other timing nonlinearities or if the random timing jitter is very small. Here are some examples: (1) Let's say that the 1 PPS signal is is generated by a process which has NO sawtooth timing edge delay errors (jitter) due to resynchronization. We use that signal to gate a counter which is clocked by a 10 MHz signal with very low jitter. If the gating and counting circuits use fast logic and they react to the rising edge of both signals, then as long as the timing relationship between these two input signals is stable and the rising edge of the 1 PPS signal is coincident with the falling edge of the 10 MHz clock, there will be no metastability and the counter will always register 10^7 counts per gate. The system is deterministic over time intervals of many years. See Figure 8 (on page 6) of the second link above (TI application note). In this case, averaging has no effect. Over time intervals longer than the lifetime of a human the count will always be 10^7. (2) Now let's say we adjust the delay of the 10 MHz clock so the 1 PPS rising edges coincide with 10 MHz rising edges. We now have the metastable situation shown in Figure 2 (on page 2) of the TI application note. We now don't know if the first clock rising edge within the gate interval will be counted. Similarly, we don't know if the last clock rising edge within a specific gate interval will be counted. If neither edge is recognized the count will be 9,999,999. If one edge is recognized the count will be 10,000,000. If both edges are recognized the count will be 10,000,001. The system may jump between these three cases slowly in a manner which is not completely random. So if you average you might get the correct result (remember - we are assuming that the frequencies and timings of the external signals are perfect), or have an average result anywhere between -1 count and +1 count from 10,000,000. (3) Finally, consider the case of a GPS 1 PPS output with sawtooth timing jitter due to the manner the 1 PPS signal is generated by one clock sampling another signal. If there is any bias in the sawtooth delay my guess is that averaging might not produce the desired result, especially if some process in the accumulation of the counter results does not perform end-to-end data collection and only averages some of the counts. With regards to error introduced by the purposeful phase jitter imposed on the clock: Voltage digitizing systems often add known analog dither noise before an A/D, then remove that noise digitally from the result. This can improve the accuracy of systems by reducing the effect of differential nonlinearity and other system errors. -- Bill Byrom N5BB Full disclosure: I am a Tektronix Application Engineer ----- Original message ----- From: Hal Murray <hmur...@megapathdsl.net> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Cc: hmur...@megapathdsl.net Subject: Re: [time-nuts] uncertainty calculations Date: Sat, 15 Apr 2017 21:46:22 -0700 t...@radio.sent.com said: > * You might have any possible phase relationship between the two > signals. If they are exactly related by a 10^7 ratio, it's possible > for the 1 PPS edges to exactly coincide with the 10 MHz edges. > Depending on the type of gating circuit, you will have jitter and > possibly metastability resolving whether which edge occured first. ... > * To stay away from such problems, most precision counters add a small > amount of controlled jitter (phase modulation) to the clock. ... My experience with metastability is that it's really hard to make it happen. Are the clock and data in a good lab setup really stable enough to make metastability a problem? How much does the added jitter screw up measurements where it isn't needed? -- These are my opinions. I hate spam. _______________________________________________ ----- Original message ----- From: Bill Byrom <t...@radio.sent.com> To: time-nuts@febo.com Subject: Re: [time-nuts] How does sawtooth compensation work? Date: Sun, 07 Aug 2016 21:36:21 -0500 A slight correction to a typo in the description below (sorry for the long delay). The correct Tektronix model numbers of these counters start with DC (not TM). The Tektronix TM500 (manual control) and TM5000 (GPIB or manual control) instruments which used the National Semiconductor MM5837 noise generator chip were the following: DC509 DC510 DC5009 DC5010 These models were manufactured from the early 1980's until 1995. The first two digits of the instrument model number designated the type of instrument. So: DC = Digital Counter DM = Digital Multimeter SG = Signal Generator PG = Pulse Generator AA = Audio Analyzer PS = Power Supply TM = Test Mainframe (which includes the power supply for the plug- in modules) Typically the 10 MHz internal standard or proportional oven timebase (or external 1/5/10 MHz rear edge connector timebase input) is dived down with 7490 TTL /5 and /2 divider sections to 1 MHz. The 1 MHz reference then drives a X100 PLL (using an ECL oscillator with varicap frequency control and 4044 phase detector) to create the 100 MHz main internal clock. The MM5837 pseudo-random noise generator phase modulates the PLL in some instrument modes of operation so that time interval averaging works correctly if the input signal is synchronously related to the clock. -- Bill Byrom N5BB Tektronix Application Engineer On Tue, Jul 19, 2016, at 01:52 AM, Mark Sims wrote: > The Tektronix TM509/5009 (and I think the 5010) counter modules have a > National Semiconductor noise generator chip in them. It injects noise > into the counter to get around counter oscillator/input frequency > synchronization. I was once given a TM509 with a bad noise generator > chip... Some Very Smart People spent ages thinking the > problem was in > the device they were working on until they tried a different counter. > The Very Smart People (too smart to RTFM) could never figure > out why the > counter was flakey and finally tossed it. A little dumpster > diving a few > minutes with a scope yielded a very nice little counter. > ---------------------------- >> Universal timer/counters and equivalent time sampling DSOs can have > this problem when their timebase ends up synchronized with the signal > they are measuring. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
