Electronic Control for Bob Cordell's THD Analyser?

[Chris56000]

Hi!

I'm new to this, so old hands please excuse me!

I would like to design an Electronic Front End for Bob Cordell's THD Analyser, with two press–buttons for Frequency Up/Down – ten discrete frequency steps, plus another two buttons for Range Up/Down (four range outputs), a single toggle button for each of the two changeover switches, plus a colour TFT displaying each discrete frequency step, and red/green indicators on the  LCD for the status signals (input low/high, frequency low/high, driven by LED outputs).

Can anyone recommend a good DIP uC that will do this, and where to find tutorials on:–

1) Writing push–button interfaces,

2) Designing a nice colour TFT interface (I don't need touch – screen interface)

3) Controlling relays and interfacing LED–level signals to show a status on a colour TFT

The original design used innumerable banks of yaxley wafer switches, which I'd like to replace with small standard PCB relays mounted near to the switched components on the analogue PCBs.

Chris Williams

PS!

I'll be posting my PCBs, code, etc., etc., for discussion and assistance if needed!

[rhb]

Please excuse my asking, but why would you not use a good sound card and an FFT?  You should be able to measure to 0.0001% or better with a modest amount of  software using Octave or MATLAB.

[Chris56000]

Hi!

I'd rather learn by working out my own electronic controls, PCBs, coding, etc., etc., to finish up with a laboratory –grade instrument where everything is a known quantity, not least of which Bob Cordell's circuitry is genuine "old–skool" that anyone here can discuss!

Also I suspect good FFT software and a sound–card with quality electronics capable of measuring THD + N down to the level Bob Cordell's circuits can won't be cheap either!

Doing it a bit at a time the way I'd like to won't be too taxing on time or my wallet as it can be done in (hopefully small & easy!) instalments!

Chris Williams

[rhb]

Fair enough.

FWIW To do it with a sound card and FFT:

create a WAV file with a sinusoid
Play that through the sound card output and record on the input
Compute the FFT of the input and the output files
Add a very small constant to all the frequencies in the output to prevent divide by zero
Compute IN/OUT
Record your DUT
FFT the DUT recording and multiply by IN/OUT
compute the sum of the amplitudes at all frequencies divided by the amplitude of the sine wave and subtract 1.0

Octave is free software and will do all the above so cost is just a sound card.  In any case, if you've not read it, read "Max Wien, Mr. Hewlett and a Rainy Sunday Afternoon" by Jim Williams.

Have Fun!
Reg

[thm_w]

Fair enough.

FWIW To do it with a sound card and FFT:

Or in other free audio measurement software (ARTA, etc.): click Spectrum analysis, start, and observe the live FFT measurement result.
http://www.artalabs.hr/index.htm

[rhb]

Fair enough.

FWIW To do it with a sound card and FFT:

Or in other free audio measurement software (ARTA, etc.): click Spectrum analysis, start, and observe the live FFT measurement result.
http://www.artalabs.hr/index.htm

Yeah, but you learn a lot more doing it in Octave ;-)