Tag Archives: coaxial switch

HPAK 8763B switch test

The 8763B is a very useful device, a 4-port coaxial switch, and has been sold for many years by HP, Agilent, and still sold by Keysight today.
It is single-ended terminated, and has two latching switches.

8763b switch

Two of these will give a nice transfer switch, for auto-calibration (through-connection vs. DUT) of the attenuator test setup.

These switches are specified up to 18 GHz, and have a “max. 0.03 dB insertion loss repeatability”. Now, the big question is, what is the actual repeatability. Knowing the manufacturer, it can be 10 or 100 times better, but you never know. This is fairly critical, because a combined uncertainty of 0.06 dB, for the two 8763B forming the full transfer switch would be not acceptable for the purpose of calibrating attenuators to better than 0.1 dB precision/linearity.

So, quickly hooked this up to the not yet auto-calibrating setup, and recorded power traces, 40 points each, 1 measurement per second, and switching the 8763B in and out every 10 seconds (vertical lines).

This was done at 4, 8, 12, and 18 GHz, and for all ports of the switch.

The setup
8763b test setup
(green item on the right hand side is the feed line directional coupler, connected to the Micro-Tel SG-811 source; light blue test cable on the left is going to the Micro-Tel 1295 receiver).

The results (two examples; same finding at all frequencies) are not very difficult to interpret:

8763b test 1

8763b test 2

– There is not really any switching visible, and one can only judge that the repeatability is actually +-0.002 to +-0.004 – the noise of the measurement.

It seems the only way to get more accuarate data will be to measure the repeatability with the two switches in series, in the final setup. Even though I’m using high quality microwave test cables, 0.002 dB amplitude stability, at 18 GHz is a challenge.
Will need to let the source and receiver fully warm up and stabilize, and use long integration times, like several minutes per switching event, to get data of 0.001-0.002 dB resolution. For now, it seems the switches will add much less uncertainty to the setup as initally thought.

Equipment selection: switching matrix

There are quite a few coaxial switches around – I figured that I need two transfer switches to accomplish the task of “through” calibration, and reflection/insertion loss measurement.
Any unused ports should be automatically terminated with 50 Ohms, when switched out.

Looking around, I found that the HP/Agilent/Keysight (will call it HPAK from now on, and add further letters, with next name change of this wonderful company) HPAK 8763B transfer switch, offers really good data, especially on repeatability. 0.03 dB – for millions of cycles.
Determining this switching reproducibility will be the first task for the attenuation calibrator!

They go for USD 813 each (August 2014), but you can find them much cheaper elsewhere. Preferably, get a unit that doesn’t have 10 million+ cycles yet!

These are of latching type – so we will have to device some drive circuitry to switch them, 24 V positive supply. Won’t be too difficult.

Interconnections will all be rigid coax, and precision SMA to N test cables to connect to source/receiver.

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Note the Sage 0.5-18 GHz coupler, left of the switches. This will be used to get a sample of the SG-811 signal – stay tuned.
For this coupler – this item was found on xbay, quite reasonably prices for its bandwidth. However, the coupled port has a little damage of the SMA connector – rendering it non-usable for its original destiny, but will now be very handy for this project.

To the outside world, the interface is a pair of HPAK SMA (3.5 mm) to precision N panelmount transitions. These are the best and most reliable know to me to date.

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