Tek P6860 Flying Leads adapters

[mbalmer]

I spotted this thread regarding flying lead adapters for the Tek P6860 logic analyzer compression probes and was considering the possibility of making some of my own that break out to the 36-pin HP flying lead sets used on their 40-pin cables (HP part number 01650-61608 -- the housing mates with a 40-pin IDC connector but only has 36 of the pins populated).

I know that folks have broken them out to the Tektronix P6810 flying lead sets, but I already have an absolute butt-ton of HP cables and leads, so to me it makes sense to use them.

Is there a schematic that has been drawn up already for these? Is there a symbol & footprint library for them? Has anyone done this before? Are there any special concerns I should take into account when doing so?

[switchabl]

The schematic for the HP adapters can be found in this document https://www.keysight.com/us/en/assets/7018-06707/data-sheets/5968-4632.pdf (p. 47).

You will have to check if the built-in termination network is compatible with the Tektronix analyzers.

[mbalmer]

The issue for me isn't the schematic for the HP leadsets, it's the footprint for the P6860. I don't have a ton of experience doing footprint modeling, and the only version I've seen is an old EAGLE library, so making a new footprint in KiCad feels a little daunting for me, especially at the tolerances we're talking with this probe.

I wouldn't be doing super high-frequency measurements, so I'm less worried about the termination network, but I'm also getting a little confused over terminology. HP refers to an "isolation network" -- I'm guessing this is the same thing as a termination network, but I want to be clear. The Tek probe manual doesn't mention an isolation network or termination network for the P6860 at all (nor does it mention one for the P6810, which is the 8-channel per pod flying lead version). What the Tek specifications mention is a 0.7pF capacitive load, along with 20kΩ input resistance.

However, they also mention an "equivalent load", which is clearly spelled out as the "load model" in the Tek probe's manual.

Again, I'm not doing measurements on high-frequency signals (for the most part, I can really only see myself looking at signals in the sub-100MHz range) so I don't know that it's critical, but I want to make sure I'm doing the right thing.

[alm]

If the termination is wrong, the DC attenuation could make the LA unable to trigger if it's attenuated too much. I'm not very familiar with the Tek logic analyzers, but reading this datasheet, it looks to me like the P6860 and P6810 both connect to the logic analyzer, and both likely include a termination network. If you have access to the logic analyzer and/or the probes, you could measure the input resistance of the logic analyzer, and the series resistance of the probes. So whatever you connect in series with the P6860 would be looking into a 20 kOhm // < 0.7 pF input. The ~90 kOhm series resistance of the HP pods would result in a 20 kOhm / (91 kOhm + 20 kOhm) = ~5.5x attenuation of the input signal. I'd say this is going to make triggering pretty difficult. The match could be better if you could directly connect to the logic analyzer without the P6860 in between.

HP also had probes that did not include termination and that would rely on termination in the target system (see page 66 of the document switchabl linked to). If Tektronix had those, those might be easier to adapt to the HP pods, depending on the impedance of the logic analyzer.

[mbalmer]

If you have access to the logic analyzer and/or the probes, you could measure the input resistance of the logic analyzer, and the series resistance of the probes. So whatever you connect in series with the P6860 would be looking into a 20 kOhm // < 0.7 pF input. The ~90 kOhm series resistance of the HP pods would result in a 20 kOhm / (91 kOhm + 20 kOhm) = ~5.5x attenuation of the input signal. I'd say this is going to make triggering pretty difficult. The match could be better if you could directly connect to the logic analyzer without the P6860 in between.

I will have to look and see if the termination is in the cables or the flying lead ends -- It seems to me that the replaceable plug-in lead ends don't really have much space to include even the small network they describe, but anything's possible. I can measure the leads when I get home.

[alm]

For the HP analyzers the termination is in the pods or flying leads (I believe in the leads). The cables are resistive, but only a few hundred Ohm to reduce ringing. For the Tek it seems the P6860 contains termination.

[mbalmer]

For the HP analyzers the termination is in the pods or flying leads (I believe in the leads). The cables are resistive, but only a few hundred Ohm to reduce ringing. For the Tek it seems the P6860 contains termination.

Turns out the tiny 0.1" pin modules that go into the flying lead sets contain the termination networks, as opposed to the pod cables themselves. Looks like I have some work to do to figure out an alternative -- short of something built into the circuit to divide the resistor termination network down to something negligible, or just make a series of flying leads.