Adding external reference to TM500 modules: impedance issues? - Page 1
Forum section: electronics / testgear

author: aarons
author_rank: Regular Contributor
timestamp: August 07, 2025, 03:20:08 am
content: Hello,I got a TM502A with SG503 and TG501 modules. Both modules had issues, and the problem with the SG503 ended up being the 1MHz crystal. Since I've been hooking all my test equipment up to my GPSDO, I thought it would be nice to add an external reference to these as well. I built a little 1:10 frequency divider using a MAX975 comparator feeding a 74HC390E decade counter. Initially my waveform looked like garbage, and in debugging it I ended up stripping it back to just the counter, a 7805 to regulate the 11.5V from the TM502 to 5V, a 100uF cap on the 5V rail, and an 82pF cap between the output signal and ground on the divider board. Probing the signal down inside the SG503 I saw it was no longer a square wave. I figured it was probably the signal reflecting off the high impedance input of the SN7473AN I had it tied to, so I added a 50 ohm resistor there.I thought all was well at first as the waveform looked like a nice enough square wave, but then noticed that my 1MHz signal was now 1.11MHz.I then put a 50ohm feed-thru terminator on the output of the divider board, since I had verified that worked when then the divider was connected right to my scope (which was also set to 50ohm input impedance) and that got me back to 1MHz, but the amplitude of the square wave was low now that there was a 25ohm load on it. I replaced the feed-thru with a decade box so I could experiment with it and found that somewhere around 400-600 ohms the frequency of the square wave would abruptly jump from 1MHz to 1.11MHz.Through experimentation I eventually landed on 200Ω at the divider output, and 200Ω inside the SG503, which results in a 3.5Vpp square wave at 1MHz with minimal ringing. But I am left wondering, what was going on that caused that abrupt shift in frequency? How could I have determined those resistor values from theory instead of experimenting, and are they they even the "right" values?

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2025, 06:08:24 am
content: I suspect that reflections combined with excessive ringing was double clocking the 7473 (7493?), and your DSO is not properly displaying the signal. I would want to see a much more detailed view of the clock edges. Incidentally, is Keysight trying to make their DSO screen captures inscrutable? Is there competition for the most user hostile display?How long was the 50 ohm cable between the source and 7493?Double termination is not suitable for TTL inputs if it drops the signal level too much. Maybe drive the cable directly and use only a parallel termination? Keep in mind that most TTL outputs will not properly drive a 50 ohm cable.The alternative is proper signal conditioning at the 7493 end, usually with a comparator. There are all kinds of ways to handle this.A better frequency reference for the SG503 is a waste of time considering the resolution of its built in frequency counter and the intended applications for the SG503.

author: aarons
author_rank: Regular Contributor
timestamp: August 07, 2025, 07:53:12 am
content: Yes, 7493.The cable from the reference to back of the TM502A was 24",then there was a 6" rg316 jumper to the divider board, another 6" jumper to the edge connector backplane, and finally a 3-4" piece of rg316 from near the edge connector to the 7493.I know there isn't much point to the better reference for the sg503, but I was waiting for the replacement 1 MHz crystal to arrive, and I did want a better reference for the tg501 since that is used for calibrating my other scope. The tg501 was designed to have an external reference fed into it, but not a ttl signal apparently since it has a reversed diode in series with the input signal which blocks the ttl entirely. I jumpered over that (and set the other jumpers according to the service manual for the external reference) and that is now dead on (second/third pic). The problem with that module is that sometimes if you look at it funny it the output goes nuts. It was doing that with its internal reference though too. I think it's just a poor contact somewhere in the unit though. That's the next thing I'm going to be looking at.As for the sg503 though, I wanted my modifications to be as simple as possibe, which is why it ended up as just a bit of cable and one lifted pin. I'm not sure I understand your theory about the 1.11MHz though. The probe was on the input to the 7493, so how would it's interpretation of the clock signal matter when talking about the input square wave abruptly jumping to 1.11MHz? Also, while I didn't do the math, the frequency measured by the sg503 counter would also abruptly jump down by what appeared to be the correct amount given the increased clock.

author: Hexley
author_rank: Regular Contributor
timestamp: August 07, 2025, 03:53:25 pm
content: Quote from: aarons on August 07, 2025, 03:20:08 amTwo clues: (1) your waveform in this image is not actually square. The signal is low longer than it is high. (2) The circuit is dividing by 9, not by 10.So it looks like the circuit was initially set up as a BCD counter with the output taken from Qc. For some reason the divider gets reset when it reaches the 9th count. Possibly noise coupled into the CLEAR pin when Qd goes high.----It looks like the later scope capture shows a true square wave. This suggests that the counter was reconfigured as a bi-quinary counter, which is good. It looks like the magenta trace is the quinary output, and the output must be from Qa now. Note that if the input clock is the blue trace, the amplitude is marginal. An HC390 running at 5V needs a 3.5V high level on the clock. The scope image looks to be a bit shy.---A detailed schematic would be helpful, btw, if you want further comments on termination, etc.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2025, 07:26:15 pm
content: Quote from: aarons on August 07, 2025, 07:53:12 amMy guess is that the 7493 is going to be subject to noise from pattern dependent jitter in its ground and supply, so not every input edge is going to misbehave. Buffering the input with another TTL gate, or some transistors or whatever, to true up the external clock will likely fix it. Disconnecting R468 and applying your reference signal to the 74L00 buffer at pin 12 and 13 should work better.Schematic 3 shows that the 74L00 is supplied from a different branch of the +5.2 volt supply and ground from the 7493. (1) I would probably add decoupling directly across their supply pins on the back of the board with 1 microfarad tantalums or 0.1 microfarad ceramics, or both. The power and ground connections may be compromising the noise rejection of the 7493 clock input.(1) Of course I could just look. I do not remember exactly why I did what I did, but apparently I added collet sockets for almost every IC in my SG503; I assume it had failing Texas Instruments edge wipe sockets. Looks like the 74L00 and 7493 are supplied from different branches of +5.2 and ground, presumably for good reason.Quote from: Hexley on August 07, 2025, 03:53:25 pmAttached Below

author: aarons
author_rank: Regular Contributor
timestamp: August 07, 2025, 08:07:21 pm
content: Quote from: Hexley on August 07, 2025, 03:53:25 pmYou know, I think at one point one of the CLR lines was floating, so perhaps something was triggering that reset and that is why I was getting the 1.11MHz. The counter was always configured as bi-quinary.The 10MHz trace was 3.3V, but that's with a 50ohm load. The amplifier (BG7TBL) doesn't produce a nice square wave with a high impedance input. I put a 100Ω load across it and that cuts way back on the overshoot and the amplitude is about 3.6V. But it does seem to work at 3.3V anyway.Quote from: David Hess on August 07, 2025, 07:26:15 pmYeah, that would be better, I'm going to give that a try.