[time-nuts] How can I measure time-delay of a cable with HP 5370B time-interval counter?

Bill Byrom time at radio.sent.com
Thu Nov 1 02:31:50 EDT 2018

Sorry for the delayed response, David. It took me a while to do a bit of research and I have some clues to why you measured those confusing results. Please refer to the HP5370B operation and service manual at the following link. 
The specifications are on pages 1-3 and 1-4 (pages 12-13 of the PDF). 
The input amplifier schematic diagram is on part three of page 8-85 fold out (page 256 of the PDF). 
You have the following connections, following the cable from the source to the termination:  
(1) Sinewave generator: If you want to measure the low frequency characteristics of the cable, you will need to use a sine waveform. If you use a fast risetime pulse or square wave you will measure the characteristics at frequencies related to the risetime of the pulse, not the period or width of the pulse. 
(2) BNC TEE connector at the Start input of the HP5370B: You have the switches for the Start channel set to 1 M ohm, divide by 1, AC coupling, and rising edge. The input impedance of the Start input is 1 M ohm in parallel with <50 pF, and the BNC TEE adds a few more pF and also a few mm of propagation distance between the center of the Tee and the reference plane at the Start connector. At low frequencies these effects can be safely ignored, but I'm guessing that at 30 MHz you might see an affect if you are measuring with 3 digit resolution. The AC coupling capacitor and 1 Mohm termination produce a highpass RC filter with a corner frequency of about 16 Hz. At 10 MHz this is inconsequential, but at 1 kHz this will cause some noticeable phase errors. 
(3) BNC termination at the Stop input of the HP5370B: You have the switches for the Stop channel set to 50 ohm, divide by 1, AC coupling, and rising edge. The combination of AC coupling and 50 ohm termination used at low frequencies is one of your main problems. The combination of the 10 nF (0.01 uF) series AC coupling capacitor and 50 ohm termination after that coupling capacitor produces a high pass RC filter with a corner frequency of about 318 kHz. At the corner frequency the Stop input amplifier amplitude is 0.707 of the voltage at the BNC TEE, and at 10 kHz the counter voltage comparator has a very small input swing due to the highpass characteristics. The phase is affected greatly, and at the corner frequency of 318 kHz the counter sees a 45 degree phase error due to the AC coupling highpass filter. That's a 393 time delay error due to the AC coupling at that frequency, and the effect gets worse as the frequency is reduced. 
Skin effect below 100 MHz: In addition to the methodology (wrong measurement tool and setup), I question the premise behind the test. I believe that the low frequency (below 30 MHz) dielectric properties of common RG-58 and similar coaxial cable are reasonably stable versus frequency. But the skin depth depends on frequency, and this will change the resistance and more importantly the internal inductance of the center conductor. At very low frequencies the AC fields penetrate throughout the center copper wire conductor and this causes internal inductance. As the frequency is increased the skin depth produces a significant reduction in the cross sectional area where conduction can occur, so the inductance is lowered. 
The crossover frequency where the AC resistance (due to skin effect) equals the DC resistance of a copper conductor depends on the wire diameter as shown in figure 4 of this informal paper: 
Figure 7 in that paper shows a graph of the calculated delay (ns per meter of cable length) of RG-58 coax between 1 MHz and 1 GHz on a logarithmic frequency scale. You can see that the calculated change in the propagation delay is about: 
 1 MHz: 1.630 ns per meter 
 10 MHz: 1.573 ns per meter 
100 MHz: 1.555 ns per meter 
 1 GHz: 1.550 ns per meter 
Are you really distributing low frequency sinewave signals, or pulses with a low repetition frequency? 
Bill Byrom N5BB 
On Mon, Oct 29, 2018, at 10:25 AM, Dr. David Kirkby wrote: 
> On Mon, 29 Oct 2018 at 09:43, Tom Van Baak <tvb at leapsecond.com> 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 
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