On Mon, 29 Oct 2018 at 09:43, Tom Van Baak <[email protected]> wrote:
> David, > > Just to see if your setup is working: > > 1) Set the pulse generator to as fast a risetime as possible; ns or less. > Use a low pulse rate (100 Hz is fine). > Unfortunately, I don't have such a pulse generator, so I can't run that test. But it is clear the system is sensitive to the trigger levels, so I guess is the problem. I have done all the confidence checks in the manual on this TI counter before and it was fine. Also, I am interested in the delay of the cable at low frequencies, as I suspect that might depart significantly from the usual figure based on the "velocity factor". Certainly the impedance of coax rise at low frequencies because the normal formula Zo = sqrt(L/C) is not valid before a few MHz. A more accurate formula is Zo = sqrt ( (R + j w L )/ ) / (G + j w C)) where R = Resistance per unit length L = inductance per unit length G = Conductance per unit length C = Capacitance per unit length. So feeding in short pulses brings the validity of such a test into question. > 2) Use a BNC tee at the generator, into two equal 2 meter cables, each one > into a 5370B input. > 3) Set manual trigger, 50R, 1.0 V, DC > 4) Now collect time interval data in block/stats mode. You should see a > mean of under +/-1 ns and a stdev in low ps. > > /tvb > Thank you. I will look for a pulse generator. I would use the 1 pps from the GPS receiver, but my HP 58503A GPS receiver has decided to pack up. I need to have a look at that, but it is not the highest priority task just now. This possible trigger issue metioned by Hal Murray is probably a result of the knobs not exactly lining up with the positions they are in. It is fairly clear that the marker on one of them is vertical at 0 V, but the other is not. I need to try to get the knob back on the shaft in a slightly different position. But they were both set to preset, but it is clear that the reading is sensitive to the trigger points. I have a 100 MHz scope, and can borrow a 300 MHz scope, but I don't have anything really fast. I have a VNA which can make measurements of phase difference down to 300 kHz, but don't trust those because of the fact one calibrates with a 50 ohm load, 50 ohm calibration standards, yet I know the impedance will rise well above 50 ohms at low frequencies. I was looking for a different approach than a VNA, to make comparisons with a VNA. Dave _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
