I need a distortion analyzer,,,,I think

[Hextejas]

I am trying to refurbish a HP 3311A function generator and the service manual says that I will need a HP3 333a distortion analyzer.  I saw one listed on EBay for about $200.00.the specs say that it measures 10Hz to 600k Hz
Aren't there less expensive alternatives ?  Or failing that, maybe put the $200 to better use and buy a better function generator?

All suggestions are welcome.

[Tomorokoshi]

It's somewhat about the technology that would have been available at the lab at the time. What they're really doing with the distortion analyzer is measuring the purity of the sine wave output. In 1974 when that manual was written, spectrum analyzers were not readily available.

Notice that they specify distortion is "less than 3%; i.e. greater than 30 dB below the fundamental."

If you have an oscilloscope with an FFT function, you can measure that directly now.

At those levels, if the sine wave on an oscilloscope looks good, then it's probably within that specification.

[Berni]

A digital scope with FFT can get you to about 40 to 50 dB of distortion so you can use that. For any lower distortion the scopes 8bit ADC and the analog front end tend to become limitations.

These days if you want to make good distortion measurements you can use a good high end USB sound card and get distortion measurements down to -100dB by again FFT.

[David Hess]

Function generators have pretty high distortion anyway so the FFT on an oscilloscope or a sound interface will work fine.  If you want to build something, then twin-T notch filter to remove the fundamental will allow the use of any oscilloscope.

[vk6zgo]

It's somewhat about the technology that would have been available at the lab at the time. What they're really doing with the distortion analyzer is measuring the purity of the sine wave output. In 1974 when that manual was written, spectrum analyzers were not readily available.

Notice that they specify distortion is "less than 3%; i.e. greater than 30 dB below the fundamental."

If you have an oscilloscope with an FFT function, you can measure that directly now.

At those levels, if the sine wave on an oscilloscope looks good, then it's probably within that specification.

Actually, there were plenty of spectrum analysers around, but not many targeting the  lower frequencies
Standard Audio generators used in Broadcast service produced distortion figures around 0.01% "standing on their heads"---- many could do much better!

The normal companion to generators like those was the "Noise & Distortion Meter" or "N & D set".
These were made by many companies, so you are not stuck with HP.

In operation, the device reference was set in "wide band" mode, then the fundamental was "nulled out " using a very sharp tuneable rejection filter.
Any remaining reading was the THD (plus noise) of the DUT.

It was easy enough to measure the DUT noise separately, then subtract that from the result.
(Some industries measured both parameters in dB to simplify this step.)

Noise was usually so low that it could be neglected, especially with sources.

The Hearing Industry used a different approach, & used a selective millivoltmeter to tune to each of the harmonics in turn, thus giving the user a good idea of the frequency distribution of Harmonic Distortion, when calibrating Audiometers

[Cubdriver]

If you decide that you want to use this as an excuse to buy a 'low frequency' spectrum analyzer, note that in HP parlance they were commonly known as 'Dynamic Signal Analyzers'.  The 3561A, 3562A, 35660A and 35665A are some examples of the breed.  Be forewarned they tend to be both pricy AND boat-anchory.

-Pat

[Berni]

Yeah these low frequency signal analyzers can be quite the boatanchors, but they do have lots of dynamic range and good THD specs. Often these things also have a signal generator output that can be used as a tracking generator. This lets you also use it for plotting the frequency and phase response of audio circuits or other low frequency filters. Essentially a poor mans network analyzer for low speed, but also offering 1M input impedance.

I got a 89410A (Vector Signal Analyzer) that i scored for cheep(They normally cost quite a lot on ebay) and its a pretty nice instrument.