After you’ve been building audio gear for a while you begin to acquire all sorts of miscellaneous test equipment and I am no exception. So today I was looking at a couple of audio function generators that I use to test amps and preamps. One is a relatively modern Tenma 72-455 and the other an older vacuum tube unit; a Precision model E-310. Neither of these units have been calibrated in all the time that I’ve owned them except for some rough checks involving a speaker and a tuning fork. So today I began thinking about calibration.

There are a couple of ways to do this. One way is to take a known frequency standard, mix the signal generator output with the standard, and monitor the beat frequency on a spectrum analyzer as you calibrate the generator to the standard. This approach has some drawbacks that make it unattractive to me. First, it requires a large number of fixed high accuracy frequency standards (usually at least two per range) and second, it requires a good quality spectrum analyzer. I have neither. The second method is simply to measure the output frequency on a fairly accurate frequency counter and use this to calibrate the generator. This is clearly the easier solution, so I started to look into what it would take to get a decent quality frequency counter.

After several hours of research I was fairly discouraged. It looked like I wasn’t going to get my hands on a reasonable quality frequency counter for less than several hundred dollars. It was going to cost even more if I wanted one that was itself recently calibrated. My hopes for calibrating my signal generators were quickly fading.

Then it hit me! I have sitting on my bench a nice Fluke model 179 multimeter. Now normally I think of multimeters as voltage, current, and resistance meters. But the Fluke 179 has a few other functions that are very handy. One is a capacitance measuring capability that will measure from 0.001µf to 10,000µf. But it also has AC or DC coupled frequency measurement capability from 0.1Hz to 100kHz. Exactly what I need to calibrate my audio signal generators! Checking the manual I found that the stated accuracy of the meter is 0.1% of the reading. This is more that sufficient to do the job for audio equipment. The Fluke 175, 177, & 179 meters all have this capability.

The moral of the story is that modern equipment is not what it was even twenty years ago. There is a lot of functionality built into things these days that we can take for granted if we’re not careful. So before you go looking for that next piece of equipment, carefully look at what you’ve already got. A solution may well be closer than you think.