Another rack completed

Initially I thought this rack would be a few potentiometers in an otherwise rather empty box, but of course it turned out to be a bit more involved...

Well, that completes all the main electronics - for the time being - special functions are next - but before that I'll do a bit of the hardware to hold the three completed racks, plus making up the bus cable interconnects and some miscellaneous wiring (e.g. the two meter outputs are buffered and will appear at BNC sockets on the rear panel...)

Coefficient Attenuators

Allows coefficients to be set in the range -1 to +1. There are fifteen of these circuits. Each uses a 10 turn potentiometer. Based on the circuit from Ulmann's book, Fig 4.26, right.

Of course nothing being as straightforward as it seems, it turned out that there was a problem when the gain is set to zero - i.e. with the potentiometer wiper halfway along its track. Had my retrospectoscope been fully operational maybe this would have been obvious (i.e. the op amp is attempting to find difference of two large voltages - not a problem at DC, but...). At several kHz noise becomes a big problem and the output isn't zero, but a noisy, relatively large amplitude (> 100 mV) waveform. Oddly, it doesn't seem connected with the fact the potentiometers are wire wound (and hence inductive).

Apparently, making an op amp circuit with zero gain is not as easy as it sounds, at least at several kHz. After much thought and some experimentation, the problem was solved with the addition of a small amount of capacitance from the non-inverting pin to ground. It's very specific - an 82 pF plus trimmer (5 - 60 pF) in parallel does the job. Adjusting the trimmer gets the output noise right down to a few mV, with the 10 turn potentiometer set halfway.

The switching (via relays) allows the op amp's output to be sent to the panel meter with either (a) input switched to an internal reference voltage (1 V or 10 V), or (b), input from panel jack. There is an (on) off (on) momentary switch on front panel to do this, which switches the relays. One switch for each op amp. Relays needed because an (on) on (on) double pole switch seemed impossibly difficult to procure, whereas I could obtain a huge number of dpdt relays via eBay at about 20 pence each(!). 

The switching idea is taken from Jackson (Analog Computation, 1960), Fig 12-41 (c).

And here's one of the trimmers, with 82 pF capacitor behind...