[AMRadio] Re:Re: Use of an oscilloscope to monitor am audio


doxemf at aol.com doxemf at aol.com
Sat Mar 18 19:37:55 EST 2006


Geoff et al,
A possible easy circuit to implement could be what is used for audio peak indication in most good recorders and mixing consoles. It responds to the slightest DC present in a sampled audio signal.
 Not the threashold comparator type of circuit but a true clipping indicator. I have worked on audio gear where the flat top wasn't even noticable on the scope but the circuit was triggering. That could sound a chirp or other tone for indication.
  My complete monitoring setup was a Philips 10 mhz dual trace O Scope that for receive, the triggering channel was the receiver audio out and the other channel was the IF out. On transmit, controlled by the plate switch of the VK II, a set of coax BNC relays with suitable in line attenuators to match levels would give me the mic audio to the triggering channel and the long wire rf to the other channel.
 This way the scope was always triggered from the audio and any phase shift or flat top/clipping was clearly visable. This was used for 40 and 75 meters. Another relay could switch in the output from my Heath Cantenna dummy load for testing and tune up.
  Everything was controlled from the antenna relay plug on the rear of the transmitter with the 120 vac output from that going into a stepdown transformer to whatever voltage was needed by the coax relays.
  It was the start of a much more complex monitoring system that was never carried out to include other tranmitters and receivers since I moved before finishing it.
 
Bill KB3DKS/1



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