What instruments to get next

[klewis]

I'm trying to decide which test instrument(s) to get next. I currently have an HP digital scope and an Tek analog scope (both 100MHz), a function generator, a Juli@ sound card, Pete Millet's sound card interface. I'm planning on building several AN67 oscillators at 10k and 20kHz.

I'm interested in looking at HD 2 through 5 with a 10 or 20kHz fundamental (based upon Bob Cordell's comments), so, was thinking about a signal analyzer and maybe Bob distortion multiplier. But, I don't know... I also don't know if cost effective signal analyzers have the resolution to see much in -120 to -100 db range.

Budget is about 1k. What do you think?

Ken

 

[tomchr]

Building an oscillator with THD lower than 0.01 % is not an easy task. But it can be done.

Measuring THD below 0.01 % (-80 dB), generally, requires modern equipment. My HP8903A goes down to 0.002 % on a good day. The only equipment I know that go below that is the Audio Precision AP-2700 and possibly the AP-1.

However, a good, external computer sound card may be able to measure THD in the -120~-100 dB range. Those sound cards are generally limited, though. That's where Millett's sound card interface comes in.

That said, I get an incredible amount of mileage out of my 8903A as well as my HP 3562A spectrum analyzer/dynamic signal analyzer. Both are great instruments, but the dynamic range is limited to 90 dB or so (spec says 80 dB but mine measures better).

~Tom

 

[klewis]

Building an oscillator with THD lower than 0.01 % is not an easy task. But it can be done.

Measuring THD below 0.01 % (-80 dB), generally, requires modern equipment. My HP8903A goes down to 0.002 % on a good day. The only equipment I know that go below that is the Audio Precision AP-2700 and possibly the AP-1.

However, a good, external computer sound card may be able to measure THD in the -120~-100 dB range. Those sound cards are generally limited, though. That's where Millett's sound card interface comes in.

That said, I get an incredible amount of mileage out of my 8903A as well as my HP 3562A spectrum analyzer/dynamic signal analyzer. Both are great instruments, but the dynamic range is limited to 90 dB or so (spec says 80 dB but mine measures better).

~Tom

Tom,

Thanks for the help, I just picked up a HP 3562A on Ebay for about 700. How is the 8903A different from a sound card?

Ken

 

[macboy]

Tom,

Thanks for the help, I just picked up a HP 3562A on Ebay for about 700. How is the 8903A different from a sound card?

Ken

Well for one thing it has an AC measurement bandwidth of over 500 kHz. So you can (more) accurately measure higher-order harmonics when using high fundamental frequencies like 20 kHz, which by your post, you plan to do. Even a soundcard with 192 kHz sampling is limited to roughly 80 kHz or so; a loopback frequency response test will confirm that. So you will only see the first 3 harmonics of 20 kHz (2nd, 3rd, 4th), with the 4th quite possibly being rolled-off substantially. You can't get a good idea of the high frequency distortion with that limitation. (Of course I am ignoring the fact that even 80 kHz is well above human hearing... and I don't want to turn this thread into a debate about the importance of ultra-sonic distortion).

The 8903A/B/E is also a great AC True-RMS millivoltmeter, capable of measuring microvolts of AC at at least 100 kHz. (it has a .0000 - .3000 mV range).