Cal Error in HP E4418B

[jonsikul]

I am fairly new to the HP E4418B power meter + sensor business.
I repaired one E4418B that didn't show anything on the display, so far so good.
I purchased a used HP 8484A power sensor with a cable which is supposedly good (but not tested by the seller, good source he claims).

Now when I connect the HP 8484A power sensor to the power meter it detect is and measures around -70dBm. I can zero but I cannot calibrate it, the power meter gives Cal Error when I try.

Now, when I repaired the E4418B I removed the backup RAM battery so it defaulted to E4418A, and I am still waiting on a USB-GPIO adaptor to correct this.
When I turn on the reference source the power meter measures -37dBm, so it's definitely doing something. I measured the S11 roughly and it shows below -10dB return loss at least up to 3GHz. I have also measured the reference source on my scope and sure enough it gives 50MHz with stable output.

I don't have another power meter and I don't have another power sensor to swap out so I am not sure which is broken. Is there anything else I can try to narrow down where the fault lies?

Regards
Jonsikul

[Miek]

The 8484A only measures up to -20 dBm, so it can't be calibrated by connecting it directly to the 0 dBm reference on the power meter. The sensor would have originally been sold with an 11708A 30 dB reference attenuator, which should be used for the calibration.

Here's the operating manual for more detail: https://www.keysight.com/gb/en/assets/9018-05414/user-manuals/9018-05414.pdf

[jonsikul]

Sorry, I didn't mention that I do in fact have a HP 11708A between the power meter and the calibration port.

[Miek]

Ahh OK.

Depending on how rough your S11 measurement was, that might indicate a problem with the sensor if it's not much below -10 dB, power sensors generally should have very good return loss. My 8484A measures below -30 dB up to around 6 GHz, and below -20 dB to 9 GHz. I have a blown 8482A and that measures around -10 to -15 dB.

[Tony_G]

Stepping back a bit on this, there are really 4 things I think that can cause this issue:

• 8484A sensor
• 11708A attenuator
• E4418B reference output
• E4418B meter itself

Out of all of these, the most likely is that the 8484A sensor itself is actually broken - I have generally seen that unless you pay a couple of hundred dollars for the sensor and buy from someone who says it is working and offers a refund then you're going to be buying a dead sensor.

Now, that said, the manual for the 8481A does provide a very basic way for you to determine if it is the meter or the sensor - This will at least help you get in the rough area for where things might be wrong.

When you measured the reference output, as long as it was somewhere near the 50MHz 0dBm then the calibration process should work. Measuring the 11708A might be harder given what you have but if you can measure S11 then you should have the equipment available to confirm that the attenuator is 20dB near 50MHz, again as long as it is somewhere near that then the process should work even if the actual numbers are highly uncertain.

TonyG

[jonsikul]

Thanks for the reply.

I have now measured S21 of the 11708A to be spot on -30dB so we can rule that out.
The reference output gives about 240mVRMS into 50 Ohm so that is about 1mW.

The sensor was cheap and untested so that is sure a possible option.
The E4418B which I claimed to have repaired seems not fully repaired in 2 ways.
First, The display goes blank after a while from turning on (while can be several minutes up to 60 min). Turning it off and on again makes it come back to life.
It should be noted that it was not (not only at least) the famous RAM problem or the power button that was my issue, but a bad Flex PCB connecting from the display to the main PCB.
Second, the fan is ON all the time while plugged in. I'm sure that is not supposed to be the case. I probably need to disassemble the unit once more.

Any ideas on these E4418B issues are also welcome.

Regards
Jonsikul

[Tony_G]

I've only ever had to replace the battery (I think it's been a while) and the membrane keypad so I don't have any real experience in the specifics of the meter and I haven't seen the schematic for it (mostly I've worked on the 435-438 series).

However, I'm pretty sure the fans in my E4418B & E4419B run all the time - When I get back from the cabin I can plug them in for you to check.

The display powering off after some time does seem to indicate a sick meter - If you tried the checks from the 8481A manual to roughly check if it is the meter, cable, or sensor and it's still not clear then I'd suggest asking over on the HPAK groups.io channel to see if anyone has seen that behavior before.

TonyG

[jonsikul]

Hi Tony,

That would great, if you could check if the fan is supposed to be ON all the time. I just find it a bit strange that it should be that way while on standby.

The Cal Error may have been a user error. I added an offset of 30dB to account for the 11708A calibration attenuator and then I could calibrate the 8484A power meter. Now when I turn the offset off (according the manual, E4418B ignores the offset anyways during calibration) and it calibrates successfully. Not really sure why it works now, I don't like it.

If yes to the calibration and fan question, then my only problem for now seems to be the display the suddenly becomes blank. (the keypad still works, as I can see I can turn the reference generator on and off while having a blank display).

Regards
Jonsikul

[Tony_G]

Happy to. Will let you know.

While it wouldn't be the first time that a HPAK manual was incorrect, it does seem that along with the shutdown issue, to imply that there is something off with the meter itself.

Maybe the ROMs have gotten corrupted?

TonyG

[Tony_G]

Sorry, I forgot all about this - I did plug my unit in and the fan on mine also runs all the time.

Apologies for the delay on that - Hopefully you've already fixed it.

TonyG

[jonsikul]

Thanks for checking! It does seem to be working now.

[Tony_G]

Great to hear it working - If you can get the EEPROM sensors then it makes things way easier to do measurements - No cal table entry required.

TonyG

[TERRA Operative]

For reference, here's an app note on testing the power sensors.

[Tony_G]

Did they ever document the "Proceed with other failure troubleshooting"?

[TERRA Operative]

There might be more info in the respective power sensor service manual, otherwise it may just be code for 'pay us here at Keysight to fix it for you"....

[Tony_G]

As far as I can tell there isn't so I suspect that you're bang on with 'pay us here at Keysight to fix it for you'.

TonyG