On Mon, 29 Dec 2014 17:32:16 -0700, Phil Allison
wrote:

...snip...
1. Ability to adjust the output level to well below 1mV and smoothly up
to several volts.

2. Circuitry not linked to the AC safety ground to avoid hum loops.

3. Have a fixed level output to connect to the external trigger input on
a scope - so the trace remains in synch despite noise, hum and harmonics.

3. Square wave output for checking tone circuits and amplifier stability.

4. Frequency from 10Hz to 100kHz, preferably in four overlapping ranges.

5. Amplitude remains steady when frequency setting or range is changed.
6. THD not more than 0.1% with low order harmonics.

..snip..

You can do all this with some very inexpensive stuff. To cover the whole
range from 10Hz to 100kHz [well more like 10Hz to 89kHz, or any specific
smaller range you want] with a little C/C++ coding and the ASIO interface
to directly work with a 24 bit soundcard, like Creative Labs EMU1212 [old
and cheap], you can make a generator to pretty much accomplish everything
you want.

You can drive the generator with variable amplitude, with a set of
simultaneous tones that will show you passband, phase, and distortion. Use
a scope, or the soundcard's inputs to get a lot of information. From
memory the drive is low noise, better than 16 bits with amplitude fairly
well calibrated. The output is like a pure tone accurately/repeatably set
to selectable amplitude.

For passband, I suggest driving with an independent set of tones, not
square wave, but tones that just cover the complete range. Monitor the
amp's drive with one input, monitor the amp's output with the other, do
FFT's on both channels preserving the phases. Then comparing the FFT's
will provide a great deal of information regarding not just passband and
phase shift, but also the amount of distortion AND noise in the amp. Plus,
you can even measure the spectral noise across the band, spot excessive
[well, relatively excessive] noise peaks, like AC mains related.

I have my own set up and software for this, but more readily available
with a simple to use GUI try Bob Masta's SW and use your own soundcard
already in your system. The SW is small and non-obtrusive to a system. I
think there's a free trial time for the full blown and if purchased you
keep with rights to ALL future downloads. If you opt out after trial you
get to keep some features anyway, like the generator capability, a very
useful generator. Also this version has LCR Meter capability.

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
http://www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!