Krohn Hite 3800 Tracking Filter

[1audio]

I have one. I just ressurected it, did the calibration/alignment and verified its function. Some pictures below.

Now that its working its not suitable for what i needed- to measure a 100 uV signal amplitude accuractely. It does have bandwidths down to nothing- 1 milliHertz. However even in cal mode its not level accuracte and its minimum sensitivity to lock is too high for my application. This all left me wondering just what would this be useful for? Especially with AGC how are you getting info on the signal when you already know a lot about it to find it?

I think I'll sell it if it really isn't that useful for signal analysis for me.

[Performa01]

Nowadays we would use a DSO for signal analysis - this is indefinitely more powerful.

If we can get a sufficiently strong copy (signal fidelity not important, only frequency and phase matter) of the signal to be analysed, that can be used as trigger, then the DSO can accurately analyse signals down to the single digit microvolts in averaging acquisition mode. And other than any BP-filter, this preserves the harmonics, hence the true signal shape, which is a prerequisite for true signal analysis.

If there is no trigger signal, then  the advanced FFT in modern DSOs can still quite accurately measure spectral components down to about -100 dBm (= 2.24 µVrms = 6.32 µVpp). The advantages compared to a tracking filter are:

• Weak signals, even totally burried in noise can be analysed
• We can see (and measure) all signals or signal components instantly and at the same time respectively
• This is not limited to audio, but applies to the full bandwidth of the DSO