Hello Ross,
I've used HP 8590 series analysers for EMC pre comp testing for 30 years and have also had an SSA3032X for 6 months.
In the early days I used a free running (not synth) HP 8590A with a home made crystal marker for 1, 10 and 100 MHz with comb generator. Used home made biconical and dipoles for radiated emissions. Happy days.
Had HP 8591EM for 20 years now. You will need the HP 08590-10027 Measurement Card with Setups, Limit Lines and Correction Factor tables. These are often separated from the analyser, but do appear on ebay individually for a couple of hundred US $.
To your question, a few thoughts ...
The basic HP 8591E comes a lot cheaper than the EM variant, now well under US $1000 for a 8591E. You loose the 9kHz and 120 kHz EMC RBW, but 10 and 100 kHz of the standard 8591E are fine for pre-compliance needs. You loose the CISPR spec average detector, although video averaging is good for precomp, you loose the CISPR QP detector and that's the main hardware loss really and probably a must have. The card setups are lost too. It may be a useful try and see how you get on option at very low cost?
In 30 years of almost daily operation of 3 HP 8590 series, I had one PSU fault on the old HP 8590A. There are gazillions of old units out there for spares now. Watching friends experiences I wouldn't recommend buying older Advantest instruments because of reliability issues, sorry to Advantest champions
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At under US $2000 the HP 8591EM is looking great value to me. The Siglent with standard preamp has a much lower DANL and your going to need an external preamp on the HP 8591EM for Class B radiated emissions tests. But that's not too costly.
I use HP E4444A Bench Link software for screen captures. This is an area where the Siglent excels by comparison. I don't have the Siglent EMC software so can't comment on that.
You don't mention your experience with EMC testing so I might be preaching to the converted, but you do not need a screened room to carry out meaningful tests. What you need is experience and patience and an interest in what you're doing. If you want to automate tests and have them run by a relatively inexperienced tech, then you need a semi anechoic chamber.
In summary ... I bought the Siglent for general purpose radio dev apps and for portability. If my HP 8591EM broke I'd try and find another to replace it for my EMC work, I think they are a bargain
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Then again if I was 30 years younger and not wedded to the old kit, maybe I'd make a different choice, the Siglent 3 year warranty is REALLY attractive!
Like I've said on other threads, the Siglent SSA is a very nice instrument
Martyn.