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  1. HP8924-Family-Spectrum-Analyzers
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Adjustment of the power reading on the HP 8920A

 

Hello all,

I have finally got my instrument going well, NO FAILURES? :)

The one thing left is to adjust the reading to agree with the value of watts at the rf input, measured with my Bird 43 watt meter.

I presume the adjustment is in the "Configure" menu ... ?

Can someone advise me as to the procedure to make the correction? 27.6 watts in @ 144 Mhz, reads about 21 on the screen.

Regards

Bill M. ??? VK6IC
 
Edited

Your Bird 43 is likely far less accurate than your HP. The elements include a potentiometer that is prone to shifting or going ¡®rusty¡¯. Some even fail open circuit. I took an element apart once and was shocked at how highly geared the 10 cent pot was - from memory needing about 20k of 100k full range.

Find a properly calibrated power meter before you touch your 8920. Mine was within a percent or two after over a decade without cal.

If anything, adjust your Bird. But then you have two challenges: fiddling with the element, or the meter, as they both have adjusters.

73, Colin G4CWH

On 8 Jan 2024, at 07:11, William Macham via groups.io <westwill1@...> wrote:

?Hello all,

I have finally got my instrument going well, NO FAILURES :)

The one thing left is to adjust the reading to agree with the value of watts at the rf input, measured with my Bird 43 watt meter.

I presume the adjustment is in the "Configure" menu ... ?

Can someone advise me as to the procedure to make the correction? 27.6 watts in @ 144 Mhz, reads about 21 on the screen.

Regards

Bill M. VK6IC
 

Hi Bill,

I listen to the normal gaggle of 2m repeaters so you can find me there. Tied up with WARG (again). Re your calibration against the Bird, is the Bird a good reference??

Happy to have a chew of the rag :)

73 de?
VK6XRE
Robyn
 

I would tend to trust the 8920 more than the Bird.

IIRC correctly the Bird wattmeters are +-5% of full scale ...not sure what slug you have in the Bird .? If that's a 100 W VHF slug you are +- 5 Watts? which pulls those numbers pretty close.

As was mentioned I would test/compare? the 8920 with some other device.? ?Usually an easy check is? a known good signal gen from @ 0, 10 and 20 dbm.? ?Make sure you zero the 8920 power meter first and set the scale to dbm as well.

Normally I have a number of power meters? and the 8920's I fix are very close.

You may have an issue on the RF i/O assembly ...not really an adjust per se on the watt meter.

I have had to replace leaky diodes in the power detect circuit? ...they actually met spec at higher levels but below 0 dbm the leakage became a factor.

Most of your typical through line wattmeters? are not supper accurate ....and then you have your cabling and terminator that needs to be verified as well.

I have? at least two of everything? so that if anything looks off I have another one to validate with?
 

Tru Dat!?
I have found as much disagreement among a handful of Bird slugs and/or cables.? There is actually a closed loop test for the 8920 to verify your cables first.


On Mon, Jan 8, 2024 at 06:36 AM, John Yelmgren wrote:
I would tend to trust the 8920 more than the Bird.

IIRC correctly the Bird wattmeters are +-5% of full scale ...not sure what slug you have in the Bird .? If that's a 100 W VHF slug you are +- 5 Watts? which pulls those numbers pretty close.

As was mentioned I would test/compare? the 8920 with some other device.? ?Usually an easy check is? a known good signal gen from @ 0, 10 and 20 dbm.? ?Make sure you zero the 8920 power meter first and set the scale to dbm as well.

Normally I have a number of power meters? and the 8920's I fix are very close.

You may have an issue on the RF i/O assembly ...not really an adjust per se on the watt meter.

I have had to replace leaky diodes in the power detect circuit? ...they actually met spec at higher levels but below 0 dbm the leakage became a factor.

Most of your typical through line wattmeters? are not supper accurate ....and then you have your cabling and terminator that needs to be verified as well.

I have? at least two of everything? so that if anything looks off I have another one to validate with?
 

On the 8924, which is like 8920's, there is a PERCAL or periodic Calibration internal routine. You need well calibrated 6 digit voltmeter & one of the steps is to enter reading of + & - rails on the measurement unit. This calibrates power meter and basically everything the measurement unit measures. The only other thing is frequency calibration & I can't recall if it was part of PERCAL or another routine. The 8920 will have same/similar routines as the measurement unit is the same along with almost everything else except the CDMA boards..
 
Edited

Thanks.? I have replaced the load resistor in the A24 assy., so It is a lot better than it was before. It looked like the user had left the TX on while he went for lunch!

I think I will believe the 50 ohms I measured whilst installing the new load, and the calibration in the HP.? Bird had a 'D' slug in it.

It is not that critical any way to ham radio applications such as mine. I do appreciate your comment though.

Many thanks

Bill M.

?

On 8/01/2024 10:36 pm, John Yelmgren wrote:
 
Edited

Normally the A24 attenuator is easily verified.? If it's the standard 60 Watt input than you should see 14 dB.

But if brings the measurement back in line than you should see something different.? ?You should also be able to verify the resistance values.
 

Thanks John, I'll check it.

Thanks for your response

Regards

Bill M.

On 11/01/2024 1:23 am, John Yelmgren wrote:

Normally the A24 attenuator is easily verified.? If it's the standard 60 Watt input than you should see 14 dB.

But if brings the measurement back in line than you should see something different.? ?You should also be able to verify the resistance values.?
 

Look here, and scroll down for RF power reference:

I built one years ago, and my HP8920B showed -10 dBm, within less than 1 dB. I gave it away, so I may have to build another one.
 

To boost that -10 dBm tester as decribed in the documentation, you can buy an inexpensive RF amp on amazon, ebay, etc. See this page for details:

I bought two of the same design, but different layout. The square one did not sit flush onto the heat sink. Some grinding fixed that. The rectangular one did sit flush. Both performed almost the same. From 9-12 V, you'll get about 30 dB gain (or more), depending on input frequency. Of course, you'll also get lots of harmonics, so a LPF is needed.

These modules are sold as 2-3 W, but are really only good for 1 W, unless you want to burn it up. With 0 dBm input and the correct voltage, you'll get 1 W out (harmonics included). Also, my two modules worked up to 700 MHz, then the output fell off rapidly.

Here is a picture of the two that I bought:
 

If you want further attneuator calibration for 892x power meter

In the group files section is a folder called Attenuator Calibration. It has generic calibration files for 6/14/16/20dB & a program that installs. If you use a editor, you'll be able to see what differences there are in those files and make a custom attenuator file for your kit...

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