Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:Harry, are you using the circuit as below? Bearing in mind this was designed for audio, the LF response is lacking, hence the curve on the staircase wave form. Increase the input 1u cap to 100u. (Note the capacitor polarity is the wrong way around). This circuit for the use you are putting it to could be vastly improved without making it more complex. It's a typical internet bodge by someone who has no idea of what they're doing.
The input 1u combined with the 1k resistor is -3db down at 160Hz, ideally NBTV should go to 2Hz at -3db, but this is acceptable for telephone quality specch, not NBTV.
Steve A.
Steve Anderson wrote:Have a look at the waveform at the emitter of the BCxxx, it should be a very close copy of the input waveform without the curve. The same at the collector, though it will be inverted. If they look good then for the time being this 'test transmitter' is OK.
Steve A.
Steve Anderson wrote:Harry, are you using the PMT pre-amp I did for Steve O's colour camera or something else? My version was DC-coupled with no capacitors in the signal path, a response down to DC.
Another thought, how are you generating the -HV for the PMT?
Steve A.
Steve Anderson wrote:I very much doubt the change to NE5534's from TL071's would make that much difference, it's the bias current in the TL071s is much lower as they are FET op-amps as opposed to bipolar. That is unlikely to be the cause of the non-linearity (the curve), but you never know.
The PMT supply looks OK too.
This is a bit of an odd one. Either the transmitter circuit is still misbehaving or there's a defect in the PMT (let's hope not).
What colour LED are you using?
Steve A.
Steve Anderson wrote:PMTs are very sensitive to blue, far much less for green and as good as totally deaf to red. Change to the blue LED. I have a suspicion you're overloading/saturating the PMT with too much light. The output of the first op-amp should only be 100mV or so, if higher you're running the risk of damaging the tube.
If you can see the blue light from the LED in a darkened room (and I mean totally dark) and your eyes have become dark-adapted - that's far too much light.
These things can detect a single photon depending on the design. We can't.
Steve A.
I'm also wondering about the 'transmitter circuit'. I assume the author used a red or IR LED which have a far lower forward voltage than a blue one. This requires thinking about.
Klaas Robers wrote:I fear that the sensitivity for blue was great and the PMT was saturated from the far to bright light. For red the sensitivity is low, so it was less saturated and you had the idea that it was working better. So see that you can lower the light output (emitter resistor large, more than 22k) and start with the lowest brightness. You just see it giving light, if the room is darkened.
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