HP 437B Sensor Compatibility Issues

[NicholsMatt]

Hi all,

Working for one of my professors as a research assistant this summer. Having some issues with an HP 437B power meter and getting it to work with an HP 8481A power sensor. I found a listing that said the meter is compatible with the 8480 series of power sensors and I have the correct model selected in the shift/sensor menu. The 8481A does work with another Agilent power meter so I'm fairly confident the sensor isn't broken. The Agilent 8485A sensor worked fine with the meter, however, it seems it was damaged at some point (only displays "please zero" and then fails to zero). Hence the desire to get the 8481A sensor working.

Any advice or ideas would be appreciated. Thanks!

A few images attached for additional clarity:

[alm]

They should be compatible, indeed. What are the issues you have getting it to work?

[Tony_G]

As ALM said - They are compatible - One thing you should know, and apologies if you already do, is that the 8400 series sensors don't communicate with the 437B (HPAK did later release power sensor & meters that replaced the 437B that do in fact communicate with the sensors, e.g. E4418A/B and the E9300 series sensors).

The 437B shipped with a set of "factory default" calibration factors for various 8400 series sensors so bringing that cal factor set up doesn't indicate that sensor actually works.

What happens when you try to zero and calibrate the sensor?

TonyG

[NicholsMatt]

One thing you should know, and apologies if you already do, is that the 8400 series sensors don't communicate with the 437B (HPAK did later release power sensor & meters that replaced the 437B that do in fact communicate with the sensors, e.g. E4418A/B and the E9300 series sensors).

The 437B shipped with a set of "factory default" calibration factors for various 8400 series sensors so bringing that cal factor set up doesn't indicate that sensor actually works.

I did not know this! The primary issue is that I am unable to zero the sensor or do anything meaningful with it, really. Attempting to zero only results in a "cannot zero" message. The manual only suggests that I ensure RF power is not applied to the sensor (which it isn't). If I try to calibrate it says "cal failed" and then suggests I zero again.

[Tony_G]

Unfortunately, many sellers on eBay will sell you a sensor that doesn't work and say that they "couldn't test" or sell it "as is" - There really isn't much you can do with a sensor that can't zero or calibrate - Typically if you aren't paying somewhere in the US$400 range (don't hold me to that price, I haven't looked in a while), and the seller doesn't offer a guarantee or return policy, then you should just assume that the sensor is faulty and they're hoping that you won't notice in time or that they win the byzantine eBay dispute policy.

Now that said, it is possible that the 437B could have a problem and the sensor may be fine - Normally this is worked out by just using a "known good" meter to test the sensor. If you don't have one then you can try the process that is covered in the 8400 series manual (available for download from the Keysight website):

Troubleshooting - Eliminating the Power Meter and Sensor Cable
Where a “known good” power meter and/or sensor cable is unavailable, another
means must be used to isolate the fault to the Power Sensor.
This is done by ensuring the power meter is providing the correct 220Hz drive signal. Check the following levels of the square wave with an oscilloscope.
• At the black/white wire: -0.05 ± 0.05 Vdc (top of square wave).
• At the brown/white wire: -9Vdc (bottom of square wave).
If the levels are incorrect, then the power meter or sensor cable is at fault. Refer to
the power meter service manual for troubleshooting information.
If the levels are correct then the Power Sensor is at fault.

This should let you at least get a feeling for whether the 437B is working.

Actually, I'd probably recommend returning the unit & sensor to the seller as not working and trying again. The actual 437B's are almost free these days (the price is in the sensor). I would try to look for a sensor sale like this eBay auction: #255151934714 - To be clear I'm not suggesting that this seller is good or that I would pay this for the sensor but that this listing has the hallmarks of a good seller - Sensor tested, offers return policy, etc.

Happy to help out further if you get something to work - I think that the 437B is a great unit and if I didn't move to a more recent version of the meter then I'd still be using it. In fact I still have a 435A because the design is just a classic and they work with the 8400 series sensors.

TonyG

[Tony_G]

It occurs to me that I should also comment on how these sensors get broken - It's almost always due to feeding in too much power (though there was just a thread here on the forum where it appears that the damage is due to shipping/dropped/etc - I had never seen that before).

The majority of the 8400 series are thermocouple based and are broadband devices so you have to be aware that your signal might contain other things outside of the frequency you're interested in that would actually deliver more power than expected to the sensor - A probably inaccurate way of describing this simply is that you have a signal you can about that is 10dBm but you have an array of harmonics that add up to another 10-15dBm - In total, your sensor is now exposed to ~25dBm which would overange it and possibly damage the sensor.

Again, apologies if you already know that but I thought it might be worth sharing. I'm sure other posters here on the forum have stories/theories/data on how these things get blown so often.