Interstage Distortion Measurements

[rdf]

Hi all. It's one of those topics that strings common words together, resulting in searches which return nothing or everything. Is anyone doing interstage noise and distortion measurements with a sound card? I'ld like to get a better picture between the input and output stages of circuits I'm playing with but the 10 kohm input impedance and 1 VRMS input limit of my M-Audio makes it a dicey thing. One possible option is 'tapping' off the grid resistor with a cap, however it only appears useful where voltage swing is sufficient to allow a low enough tap to present a low impedance drive to the card.

Any recommendations? Thx in advance.

[anatech]

Hi rdf,
Low distortion buffer time. Many points before the output are high impedance points and you don't want to load them.

This could be as simple as a FET input op amp after your high impedance attenuator.

-Chris

[SY]

I use a high speed buffer amp (BUF-03) with a 1M input impedance and a 10x scope probe. Works a charm. A second BUF-03 is used between the soundcard and any nasty load (like the low input Z of a step-up interstage); source Z can then be easily adjusted with a series resistor.

[richwalters]

Originally posted by anatech
Hi rdf,
Low distortion buffer time. Many points before the output are high impedance points and you don't want to load them.

This could be as simple as a FET input op amp after your high impedance attenuator.

-Chris

Ah...interesting....As SY mentions low point loading.....I use 10Meg min. It may sound old fashioned but for push-pull amp work I always use matched probes and straight into a god quality XY scope (Lissajous). Any capacitive loading or effects of crummy layout becomes phase sensitive and immediately apparent as 0°/80°/360° line on scope bows out of line assuming the oscilloscope amps are matched within a few %. This is only one of a raft of sim measurements one can do.
SUprisingly I couldn't find a description of the Lissajous method in the Radiotron designers handbook 4th ed......I presume the quality of dual channel scopes in early days wasn't good. I seem to remember that the Tektronix 545 series had this facility.

richj

[rdf]

Thanks everyone! I think I like SY's approach. The only downside is both the BUF-03 and -04 are now obsolete product. AD recommends the AD811 video buffer as a replacement and it appears to have suitable distortion performance. Cheap too. In your experience is there any reason one of these video buffers can't maintain performance into a 1 kohm ten-turn? The M-Audio drivers disable the Windows mixer leaving no way to adjust input level in software.

[SY]

I used the BUF-03 because John Curl gave me a couple. "Free" has always been an engineering desiradatum around here. Any high speed buffer with high input Z, low bias current, and low distortion will do.

[EC8010]

Video op-amps are designed to be able to drive 150 Ohms without distortion (that's a 75 Ohm load via a 75 Ohm series output resistor), so 1k is giving it the day off.

[anatech]

Hi richj,
Nifty idea. Thanks.

SY, I'm so embarrased. I should have thought of using a 'scope probe instead of building and compensating the attenuator each time - duh!

-Chris

[SY]

Yeah, the trick is to match the scope's input resistance and capacitance; that way, you know you can get the probe compensation adjusted properly. With a 10x attenuation network (100k + 900k) as the input to the buffer, you can get a 100x reduction, very useful for those of us who often build high voltage driver stages.

[anatech]

Hi SY,
20 puff is a nominal value expected by most probe manufacturures (X10), wouldn't you say? I like the added 10X tap idea.

-Chris