[sdiy] Cheap audio spectrum analyzer

Neil Johnson neil.johnson71 at gmail.com
Wed Oct 28 12:18:03 CET 2015

Hi Rick,

> http://www.cnx-software.com/2013/07/24/359-red-pitaya-board-combines-an-oscilloscope-a-spectrum-analyser-a-waveform-generator-and-more/
> ~360 US$

I wrote up my thoughts on this board last year on muffwiggler:



I think I'm about to be flamed for the following. This project is an
over-priced half-baked time-waster. Allow me to justify that.

Firstly, the price. I just checked the RS website, and including VAT
one of those would cost me £378.00 + postage. For that I get a fragile
board that might be good as a wideband noise source, but not much
else. To make it a useful instrument, one that could be fairly
compared to "old T&M gear on eBay" it would need (a) a case, (b) a
power supply, (c) input and/or output interfaces, (d) a display. And
if you're serious about your hobby then it will also need calibrating.
Once you add up all those, plus the time taken to design/assemble/test
I think you'll find you have significantly less money to spend on
synthesizers, and significantly less time to play them.

Secondly, the half-baked. It's like holding up a steering wheel and
proclaiming "I has a car!". No, before this board could be even
remotely comparable to even a cheap old 20MHz dual trace scope from
eBay or somewhere like Stewarts it is going to need at least two input
pre-amps, and an external trigger input. Those pre-amps need to have
switchable gains, AC/DC coupling, very wide bandwidth (DC-20MHz or
so), over-voltage/ESD protection, and so on. And that's the easy bit.
Reliable triggering is what really makes or breaks a scope - Tektronix
got this nailed, HP spent years trying to get it right. Now, surely
the point of using a fancy doodad CPU is you can control it all in
software. So, off to RS or Farnell and buy some expensive Teledyne
signal switching relays to do the gain switching. And then put it all
together and make it work.

Suppose you only want a signal generator? That's a bit simpler, since
you only need output drivers, able to cope with short circuits and
externally-applied volts (e.g., plugging the cable into a VCO output
or bench PSU...oopsie). Depending on what you intend to use the signal
for you might also need to investigate additional filtering to ensure
a clean output.

Finally, the time-waster. Ok, so you got the board. Then you need to
put it in a case (you wouldn't want that bare board laying on a bench
with pliers, components, clipped leads, solder splashes, etc). Next
you'll need to add proper interfaces, as mentioned above. Then, as you
say, load and maybe modify applications to do what you want. The
power! Great. So instead of debugging the real problem (VCO not O'ing,
whatever) you're debugging your tools.

Don't get me wrong - this has the potential to make a great component
within a piece of test equipment. But it is only a part, like a
steering wheel. On its own it is a long way even from cheap T&M kit
from eBay. Given enough time, effort, expertise and expense you could
make something comparable to what you can get on eBay. The project's
kickstarter page has a great sales pitch BTW!!!  *sheesh*

Or put another way, what can I get for £378?? Recently sold on eBay (uk);
* Hameg dual-trace 20MHz scope, £50-£60
* Thandar TG-501 function generator, £60
* HP 3478A 5.5-digit bench multimeter, £90
And have plenty of cash left over, or maybe choose higher-spec
instruments (Tektronix scope, get the 3478A calibrated, etc)

Beyond that and you're into higher end, such as:
* HP 3580A spectrum analyzer, £320
* HP 8903B audio analyzer, £214
* HP 8594E RF spectrum analyzer, £550
* HP 34401 6.5-digit bench multimeter, around £300
* HP 3325A synthesizer function generator, about £200
None of the above can be achieved by the redpitaya without significant
time and expense - in short, the above options are way cheaper if you
really need them.

Personally I would rather spend my limited hobby time designing,
building, repairing, and playing synthesizers than futzing around with
building standard test equipment. YMMV of course



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