FFT spectrum analyser- what to buy? - Page 1
Forum section: electronics / testgear

author: Nomen luni
author_rank: Newbie
timestamp: June 23, 2013, 09:11:08 pm
content: Hi guys,I'm looking to buy a unit to replace an old Agilent HP3562A. Requirements are:-FFT spectrum analyser-2 input channels-0-100KHz frequency range-Sensitivity similar or better than 3562AThere doesn't seem to be much to choose from for FFT spectrum analysers.Agilent have an updated model, but it still looks dated.http://www.home.agilent.com/agilent/product.jspx?cc=GB&lc=eng&nid=-536902437.536881997Any recommendations? I'm wondering if this kind of thing has been replaced by FFT functions on oscilloscopes, but do any of these offer all the features of a dedicated FFT analyser?Cheers

author: edavid
author_rank: Super Contributor
timestamp: June 23, 2013, 09:49:33 pm
content: How about the Stanford SR780 or SR785?http://www.thinksrs.com/products/SR780.htm

author: vk6zgo
author_rank: Super Contributor
timestamp: June 24, 2013, 02:58:37 am
content: AS an RF person,I tend to sneer at FFT facilities on Oscilloscopes,but I realise that your application is a specialised one.Spectrum Analysers of any kind optimised for that bandwidth are few & far between,but I think you are better to go with an instrument designed for the job.Can you borrow an Oscilloscope with FFT & see if it is satisfactory?If it is,you then have the benefit of the 'scope for general purpose work as well.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: June 24, 2013, 03:43:40 am
content: I have an HP3562A, and have used the SR785. The SR is pretty decent, if not somewhat odd in the interface department.I think the reason that analyzers in this range have become so rare is because of FFT on oscilloscopes. I have yet to see a scope FFT implemented well, but they do work.

author: Unknown
author_rank: Unknown
timestamp: June 24, 2013, 04:17:07 am
content: Even my old Tek TDS-340A has a FFT function. Not a very fast one, but it's there nonetheless.

author: nctnico
author_rank: Super Contributor
timestamp: June 24, 2013, 10:08:40 am
content: The problem with an oscilloscope is the limited dynamic range. An oscilloscope has an 8 bit ADC which theoretically should give you 48dB of dynamic range.

author: G0HZU
author_rank: Super Contributor
timestamp: June 24, 2013, 07:41:27 pm
content: By oversampling an 8 bit scope should be able to display a greater range than 48dB on an FFT display. i.e. you effectively grow extra bits by oversampling. One of the penalties is reduced bandwidth and also many scopes have alias issues that can confuse the user.However, I would also consider using a decent PC soundcard as this may just about meet the requirements and there is plenty of DSP software out there for soundcards, some of it is free

author: nctnico
author_rank: Super Contributor
timestamp: June 24, 2013, 08:07:04 pm
content: Quote from: G0HZU on June 24, 2013, 07:41:27 pmOversampling needs specific conditions to work. Just imagine a DC level sitting between two ADC values. No amount of oversampling will ever get you any extra resolution in such a situation. The conditions required for oversampling are not always there if you use an oscilloscope. IOW you can get funny results when using the oversampling feature of a scope. And then there are the linearity errors...Perhaps a high quality soundcard (with some mods) could give better results. Most do to 192kHz at 24 bit nowadays.

author: Unknown
author_rank: Unknown
timestamp: June 24, 2013, 08:09:15 pm
content: I wouldn't be surprised if sound cards also used oversampling to get the high resolution. (decent) DC performance is not required anyway.

author: G0HZU
author_rank: Super Contributor
timestamp: June 24, 2013, 09:44:13 pm
content: Quote from: nctnico on June 24, 2013, 08:07:04 pmI guess we don't know what measurements the OP wants to make. I assumed that because the HP3562A doesn't quite measure down to DC that DC performance wasn't that critical. Oversampling gets used on scopes and real time analysers but I guess that not many will be able to provide the small signal sensitivity or dynamic range of the old HP analyser. One other aspect of the HP analyser is that it is presumably possible to quickly capture the raw data with a PC and post process it using modern DSP.

author: edavid
author_rank: Super Contributor
timestamp: June 24, 2013, 09:53:43 pm
content: Quote from: nctnico on June 24, 2013, 08:07:04 pmSure it will, you just need to add dither. However, that is not going to get you from the 8 bits of the scope ADC to the 14 bits of the HP 3562A.QuoteThey use delta-sigma converters, so... sort of.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: June 24, 2013, 09:55:52 pm
content: I had a brief encounter with a dScope recently, I didn’t get a lot of time with it (but I will be using it again). It seemed extremely flexible. If the OP needs pro-grade equipment, it could be worth looking into one of these, but there are not cheap.