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Switching amp hash filter

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John Roberts {JR}:
Distortion analyzers are generally measuring THD + N(oise). Noise can significantly alter THD+N readings.


Frank Koenig:
I looked at the service manual for the HP 8903B audio analyzer, which dates from the early 1990s, and reveals a good bit about the design of the notch filter. It is an analog tour de force in the form of an electronically-tunable, 2nd-order, state-variable analog filter. The tuning elements are analog multipliers that get a coarse tuning input from a frequency counter that measures the frequency of either the internal generator or the input signal. A fine tuning input comes from phase and amplitude detectors that compare the input and bandPASS output of the filter so as to maintain the bandreject filter's null at the spectral peak of the signal. Using the bandpass output provides a robust signal for the phase measurement -- very clever. The coarse tuning ensures that the phase detector hunts for the correct peak.

I assume they used very high quality components and advanced design ticks, which they do not detail in the manual, to get the high Q and stability needed to make this work. I think this might be one of those examples of an old technology that reached its climax well after it was being supplanted by its successor. Steam locomotives in the 1940s, piston-engine airliners in the 1950s, CRT displays in the 1990s and rotating magnetic storage today come to mind as other examples.

The block diagram shows the residual output downstream of the notch filter, as expected. I'll revisit the measurements one of these days when I get that test bench urge. The weather is too nice right now.


John Roberts {JR}:
back in the 90s I had an 8903 on my test bench at Peavey... it is a solid citizen.... The more expensive computer controlled test gear often ended up out in factory doing automated testing (ATE). While the 8903 was too expensive for me to own back before Peavey hired me.  8)


PS: For TMI I discovered a design flaw (?) in a competitors SKU during bench testing when I switched the 8903 output impedance to 600 ohms and saw the frequency response of the DUT change.   ::)


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