advertisement


Spectrum analyser

Mike P

Mike P

Trade: Pickwell Audio
Some high end vintage CD players using Burr Brown DAC chips such as the PCM58 have the facility to adjust the DAC output for best linearity by means of several small trimmer potentiometers.

I have only recently starting looking into this but as far as I understand it the basic method is to play a test tone from a CD disc and observe the output with a spectrum analyser. You then tweak the adjuster for the MSB to give the lowest THD and move onto the next most significant bit and so on.

Lab grade spectrum analysers are very expensive bits of kit but I have seen that it's possible to buy a USB spectrum analyser for use with a PC for under £100.

Here's an example:

http://cpc.farnell.com/velleman-ins...7AbMk3dXTBBFkfQF-inkkff7U50pKNwAaAltIEALw_wcB

I'm trying to find out if something like the Velleman is suitable for the task or if I'll need something more 'serious'.
 
Thanks for the reply. I've just been looking at the price of decent external soundcards and they seem to be about £60 broadly comparable in price to the Velleman. I suppose I'd also need some software to use with a sound card to get the THD measurements I need. The Velleman looks like a nice neat solution providing it's good enough to do the job.

As a general rule I'm all in favour of buying an older higher quality tool rather than a new cheap one. I already have a Racal Dana Frequency counter and a Tektronix CRT scope which are both a pleasure to use. However, a 'proper' spectrum analyser is likely to be at least £700 second hand which I can't justify for a tool I'll only use very occasionally. There's also the worry that if it goes faulty or needs calibration it'll be very expensive to put right.
 
Perhaps someone can answer this question I have about using audio cards.

I know most PCs and audio software will easily do 24/192kHz sampling, but do they all still have audio frequency (20-20k) brick wall anti-aliasing filters on the front end or are they switchable? I'm curious because I'd like to see the spectrum above 20kHz for stability purposes.
 
I've routinely used USB ADCs to capture signals for analysis. You can see examples of results here
http://www.audiomisc.co.uk/HFN/ADC/USBrecording.html
http://www.audiomisc.co.uk/HFN/ADC/HighQualityADCs.html
as well as elsewhere on my webpages, etc.

I wrote my own DIY scope/specan software for Linux and RO to go with them You can find it here
http://www.audiomisc.co.uk/software/index.html

The cost of the ADC depends on how good a result you want. But for general audio the 2i2 may be fine. If you want excellent results, go for Benchmark (albeit at a much higher cost, but cheaper than most dedicated specans).
 
Even cheap modern PC microphone ports measure far better than a HP3561A, which only had 80dB dynamic range.
They also can measure up to around 50 kHz
 
Thanks Jim, there's a wealth of information there. I've borrowed a 2i2 to have a play.
 
I'll take a proper hardware instrument over a software one all day long... no contest. I hope to maybe get a HP3580A some day (older and lower spec than the HP3561A which davidrsb mentions). It's personal preference for me... I'll always take an analogue hardware anything over a modern digital one. no matter what the specs say the hardware ones are generally much more intuitive and useful in practice IMO. Yes I'm biased!... to the extent that I will pay £500 for a 1974 hardware one rather than £200 for a "better on paper" new digital one in the blink of an eye.

My HP8558B spectrum analyser has proved most useful in locating VHF sproggies when doing R&D which just don't show up on a scope. I have digital spectrum analyser kit but have never needed to use it.
 
MJS said:
Thanks Jim, there's a wealth of information there. I've borrowed a 2i2 to have a play.
Click to expand...

The 2i2 I have goes up to 96k/24. I've been told that they now sell an updated version of the 2i2 that goes to 192k/24, but I've not tried it. So I'll be interested to hear how you get on. I find it quite handy to use with a laptop as a portable capture device.

FWIW I also sometimes use an old Tascam HD-P2 recorder. This goes to 192k/24 *and* had digital (co-ax) inputs and outputs as well as analogue ones. It can also 'lock' onto the incoming spdif or AES-EBU. So it handy for clean digital capture as well. Has really nice peak reading meters as well. Only snag is that it records onto CF cards which aren't that convenient these days. But it can run off its own internal batteries, so is even more handy for capture rather than as a 'see as you go' scope/specan.
 
I've done more reading today and it looks like a USB audio interface mated up to appropriate software will do the job.

So can anyone recommend suitable examples of each?

Ideally I'd like RCA inputs on the interface to partner up with a CDP.

I still lust after a big ol' bench spectrum analyser.
 
Right mark audio analyser software 25euros and a sound card of your choice, I use a U24XL, works fine.
 
You may want to look at AudioTester software. It has a very reasonable spectrum analyser for real time distortion work, along with various interesting sweep modes.
 
The trouble with the 2i2 is you can't bypass the preamps on the inputs. Whether that will colour your signal or not I don't know...
 
Cheap on board ADCs are far better than a Vintage HP3861a. If you want better, you need a R&S audio analyser. I have two of these, both with broken and beyond repair power supplies, grrr.
 
davidsrsb said:
Cheap on board ADCs are far better than a Vintage HP3861a. If you want better, you need a R&S audio analyser. I have two of these, both with broken and beyond repair power supplies, grrr.
Click to expand...

The HP3561a is actually too modern for me (mid 80's) and I probably wouldn't have one as it has processors etc in it. No it's a much older HP3580a for me.
 
You would be much better off with a proper USB spectrum analyser. Issues of input impedance, sensitivity and overload protection...
 
Arkless Electronics said:
You would be much better off with a proper USB spectrum analyser. Issues of input impedance, sensitivity and overload protection...
Click to expand...

I'm not sure this is the case, it depends on what you are trying to do. Resolution comes at a high price, most affordable USB instruments that can do FFT analysis (e.g. Picoscope) only have 8 bit resolution. They do win over a sound card for bandwidth and, as you say, input protection. The Velleman unit mentioned by the OP is only 8 bit and 512 line FFT resolution. I doubt it's anywhere near as linear as a decent sound card, which is liekly to be important when you are trying to measure the linearity of something, you need the measurement apparatus to be as clean as possible.

For measuring over the audio band it's hard to beat a decent USB interface like the Focusrite 2i2, that will measure flat to 48k for the first gen units which have 24/96k capability. The second gen can sample @ 192k, but I don't know if it has wider anti-aliasing filters to benefit from that bandwidth. Based on my 2i2 there's a good chance. The 2i2 has high linearity and will give you >130dB of dynamic range to play with. If you want to do sensitive measurements it's not hard to build a simple instrumentation amp input buffer, with input protection, that can resolve down to -170dBv.

A friend of mine has just bought a cheap Behringer UMC202HD, which claims 24/192 capability - I'll compare this soon with the Focusrite, I'm hoping it's decent as it costs circa £55 and could make a cheap measurement system.
 
Thanks again for the responses folks.

I've had a few recommendations now for the Focusrite 2i2 so I may well go for one of those.

The Sony service manual calls for a 1kHz sinewave at -60dB but I've read elsewhere that a very low level signal at -60dB should be used.
 
You must log in or register to reply here.


advertisement


Back
Top