HP 8590A 75 Ohm option

[tatus1969]

I just scored one of these for 550€ on ebay, and noticed only after the purchase that it has the 75 Ohm option installed. This isn't a problem right now as I am going to use it for EMI sniffing with near field probes and a 30dB amplifier, which is highly frequency dependent anyway. And I hope that the amplifier doesn't go nuts when being terminated improperly. I could add a resistive matching network to its output otherwise, and the amp will be placed right at the analyzer's input via a BNC to SMA adapter.

My question now is, does anyone know if this 75 ohm option is removable, or if it could be modified to 50 ohms? That would make my instrument more versatile, as I have no plans that would need 75 ohms whatsoever.

[RFasic]

Hi,
it is only an internal standard 75 to 50 BNC/BNC pad after the front panel input, you just have to remove it and launch the internal calibration.

[tatus1969]

Hi,
it is only an internal standard 75 to 50 BNC/BNC pad after the front panel input, you just have to remove it and launch the internal calibration.

Thanks, sounds like an easy task. I'll do that!

[RFasic]

And the nice thing is that you can sill use it outside when you need your 75 ohm SA again (ie on TV network)

[TheSteve]

I see it is also a DC block so perhaps the pad should be replaced with a 50 ohm DC block.

[Emo]

Hi,

In order to make the complete chain 50 ohms you also need to exchange the calibration part. I did the conversion some time ago and it is relative easy. The official guide was posted in a key sight blog. It looks complicated but is actually adjusting some resistors and BNC chassis parts(be careful, the center pin on 75 ohm version is different from 50 ohm)


Found here : https://community.keysight.com/thread/5431
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When using an external matching pad or network, the analyzer does not know that it is connected, so you have to compensate for that 5-6 dB front end loss for sure.  If you plan to convert the analyzer, you can find the details below.  Since your analyzer is very old, you may not be able to get the conversion parts needed, not sure.

Agilent 8590B and HP 8591A Conversion for 50 or 75 ohm Operation
________________________________________
Parts listed for the impedance input required. Order one of each.     

75 Ohm      Description      50 Ohm     
0590-1251     NUT HEX 15/32-32      NONE     
08590-20182     CBL AY-RF INPUT     08590-20157     
08590-60179     RF INPUT      1250-2191     
NONE      ADPT M N - F BNC     1250-0780     
6960-0150      PLUG-HOLE      6960-0148     
08590-60028      Cable, Cal Out      8120-5052     
5062-6452 (75ohm)     CABLE, CAL Out-RF In     8120-2682 (50ohm)     
0955-0453     A24A4 ATTENUATOR, TG OUTPUT     0955-0453     
08590-20179     J2 MATCHING PAD     NONE     


Agilent 8590B Dress Panel
     
75 Ohm      Description      50 Ohm     
08590-00039     HARD KEYPAD     08590-00038     
08590-00046     RUBBER KEYPAD     08590-00045     

HP 8591A Dress Panel
     
75 Ohm     Description     50 Ohm     
08591-00002     HARD KEYPAD     08591-00001     
08591-00014     RUBBER KEYPAD     08591-00013
     
Common to the HP8590B and the Agilent 8591A conversions.
To convert a 50 ohm 3RD CONVERTER, p/n 08590-60192, 08593-60021 or 08591-60081 to 75 ohm, order the parts listed in the 75 ohm column and replace the parts listed in the 50 ohm column.
To convert a 75 ohm 3RD CONVERTER, p/n 08590-60193 or 08590-60344 to 50 ohm, order the parts listed in the 50 ohm column and replace the parts listed in the 75 ohm column.

     
75 Ohm      Description     50 Ohm     
0698-7239, 1.33 KOHM     A9R20     0698-7237, 1.1 KOHM     
0698-7216, 147 OHM     A9R21     0698-7212, 100 OHM     
0698-7216, 147 OHM     A9R23     0698-7212, 100 OHM     
9135-0076, 39 NH     A9L2     9135-0070, 24 NH     
The RF input cable (W10) that connects to the rear of the RF Input connector remains the same for both configurations since it attaches to the rear of the Minimum Loss Adapter or the N connector.
Refer to "STOR PWR ON UNITS" in Chapter 13 (Service Softkey Descriptions) in the service manual for setting the Impedance and Power-On units of the analyzer to 50 or 75 ohm input.

Special considerations for 75 ohm spectrum analyzers.
If the final configuration is 75 ohms (Opt. 001), use only the 75 ohm cable (5062-6452) for calibration and use only 75 ohm cables for signal input. Using a 50 ohm cable can damage the 75 ohm input connector which is the Minimum Loss Adapter (MLA). The replacement cost of the MLA is currently $220. The MLA is not field repairable.
The recommended 75 ohm F to 75 ohm BNC connector p/n is 1250-0076, using Radio Shack type connectors WILL eventually damage the input connector because it is a 50 ohm BNC connector.
If converting an Option 011 (75 ohm TG) to 50 ohms, remove the MLA (TG Output Adapter, 50 to 75 ohm) and re- place with the type N (f), p/n 1250-2191, connector.

Special Considerations for the Agilent 8590B with a Count Lock, Option 013
If the 8590B has a Count Lock Option (Option 013), the output attenuator (0955-0453) will have to be added to the TG output or the TG Cal will fail. The reason is the analyzer will now ID as an Agilent 8591A, so when the Cal TG test routine is run the firmware will attempt to change the TG attenuator and test the results.

Regards,

Eric

[Teun]

I think I remember seeing a youtube video on calibrating one of those somewhere.

[tatus1969]

Thanks for that! Looks very comprehensive but doable. It seems that it only involved standard RF parts like cables and connectors, and some resistors. The same attenuator (0955-0453) seems to be used for both variants, unless that Keysight guy has made a typo. I'll save that for later though, as it looks like I'll have disassemble major parts of the units, and I need it desperataly for EMC compliance measurements. We have been 30dB above the limit in the test chamber...

[tatus1969]

Wait, I have a 8950>A<, does this still mean that I have to change the resistors on the converter?

[Emo]

Hi,

The 001 option can be reversed fully. Also on the 8590A besides the cabling and 75-50 ohm input unit the decider in the third converter needs altering in order to make calibration work again using the internal 299 Mhz signal.
I have no link to the original source of the images.

Regards,

Eric