HP 8590A 75 Ohm option - Page 1
Forum section: electronics / testgear

author: tatus1969
author_rank: Super Contributor
timestamp: March 07, 2019, 10:22:40 am
content: 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.

author: RFasic
author_rank: Contributor
timestamp: March 07, 2019, 11:52:23 am
content: 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.

author: tatus1969
author_rank: Super Contributor
timestamp: March 07, 2019, 10:57:10 pm
content: Quote from: RFasic on March 07, 2019, 11:52:23 amThanks, sounds like an easy task. I'll do that!

author: RFasic
author_rank: Contributor
timestamp: March 08, 2019, 01:14:39 am
content: And the nice thing is that you can sill use it outside when you need your 75 ohm SA again (ie on TV network)

author: TheSteve
author_rank: Unknown
timestamp: March 08, 2019, 01:25:33 am
content: I see it is also a DC block so perhaps the pad should be replaced with a 50 ohm DC block.

author: Emo
author_rank: Regular Contributor
timestamp: March 08, 2019, 10:39:46 am
content: 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----------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 Ohm0590-1251 NUT HEX 15/32-32 NONE08590-20182 CBL AY-RF INPUT 08590-2015708590-60179 RF INPUT 1250-2191NONE ADPT M N - F BNC 1250-07806960-0150 PLUG-HOLE 6960-014808590-60028 Cable, Cal Out 8120-50525062-6452 (75ohm) CABLE, CAL Out-RF In 8120-2682 (50ohm)0955-0453 A24A4 ATTENUATOR, TG OUTPUT 0955-045308590-20179 J2 MATCHING PAD NONEAgilent 8590B Dress Panel75 Ohm Description 50 Ohm08590-00039 HARD KEYPAD 08590-0003808590-00046 RUBBER KEYPAD 08590-00045HP 8591A Dress Panel75 Ohm Description 50 Ohm08591-00002 HARD KEYPAD 08591-0000108591-00014 RUBBER KEYPAD 08591-00013Common 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 Ohm0698-7239, 1.33 KOHM A9R20 0698-7237, 1.1 KOHM0698-7216, 147 OHM A9R21 0698-7212, 100 OHM0698-7216, 147 OHM A9R23 0698-7212, 100 OHM9135-0076, 39 NH A9L2 9135-0070, 24 NHThe 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 013If 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

author: Teun
author_rank: Regular Contributor
timestamp: March 08, 2019, 11:47:32 am
content: I think I remember seeing a youtube video on calibrating one of those somewhere.

author: tatus1969
author_rank: Super Contributor
timestamp: March 11, 2019, 07:45:41 am
content: 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...

author: tatus1969
author_rank: Super Contributor
timestamp: March 11, 2019, 07:47:49 am
content: Wait, I have a 8950>A<, does this still mean that I have to change the resistors on the converter?

author: Emo
author_rank: Regular Contributor
timestamp: March 11, 2019, 07:28:46 pm
content: 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