[sdiy] Cheap audio spectrum analyzer

[rsdio at audiobanshee.com]

Excellent article, Neil.

For the same reasons you've described, I've not built my own SMD oven out of a used Toast-R-Oven plus Arduino. I'd rather pay the local assembly shop $30 and have them use the right tools for the job (not to mention, they have pick-and-place robots in addition to the SMD ovens).

I'd rather design things that don't exist yet, or at least don't exist any more (vintage synths). Designing things that already exist is a great exercise for learning, but it's a bad choice if you need to get other things done.

Brian


On Oct 28, 2015, at 4:18 AM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> 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:
> 
> https://www.muffwiggler.com/forum/viewtopic.php?p=1576218&highlight=#1576218
> 
> -------
> 
> 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
> 
> -------
> 
> Neil