Re: Tek 2465b two probes showing slightly different voltages and phases


Tom Lee
 

Hi Mark,

Your data helps a great deal.

So, at 5MHz there is a 4% gain difference. For a 10:1 probe, that's "high frequency", so the gain there is sensitive to the probe's adjustment. If that 4% bothers you, just tweak one or the other probe. I doubt you'll see a huge effect on the 1kHz response you used to adjust the probe in the first place.

Terminating the probe in 50 ohms tells you very little besides "don't do that." The voltage divide ratios for two different probes will produce quite different scale factors at your relatively high 5MHz frequency.  It's important to keep in mind that a probe is not just a cable, nor is it a cable with an RC. It's a specially-designed lossy cable with (generally) an equalizer (in high-bandwidth probes). Different probe types have different equalizing networks, so they'll present different impedances. The voltage dividers formed with a 50 ohm termination will be different (and not at all flat with frequency).

Now, as to your phase difference: I just took note of the probe model numbers. I'm confused: The P6019 is a current probe. Is that a typo? Would you confirm?

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/30/2020 18:16, Mark Hatch wrote:
Tom,

Thanks for your reply. Let me try to summarize some more info:

General data:
1V/div
time base 100ns
DC coupled 1mohm input
triggered on channel 1, Signal ~5mhz sine out of the HP 652A.
Voltage and delta time read using cursors with 10x mag

1. Channel 1 (trigger) : P6019, 1.5m cable, 15.6pf. DC Coupled.
P-P Voltage 4V

2. Channel 2 : p6012, 3m cable, 12.5pf
P-P Voltage 3.84V

Channel 2 (longer cable( leading Channel 1 by 15 ns (10x mag)

There appears to be an impedance issue here too. I am using the 50ohm output of the 652a, and then I switch to the DC/50 mode on the Tek, I see Channel 1: 438mv on 100mv/div
Channel 2: 660mv on 100mv/div

Sweep here was also 100ns. Channel 2 continues to lead Channel 1 by ~ 15ns.

Note that in the 50/DC setting, Channel 2 is 200mv higher than Channel 1 where in the straight DC1mohm it was ~200mv lower.

I went down this rathole because the HF amp I was building suggested that I monitor the output of a transformer and that I should see the two ends as being 180 degrees out of phase.... Well they were not exactly...

Appreciate any suggestions here.

Regards,

Mark








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