Nat Semi App Note 72 page 18, par. 6.4 shows the configuration for bandpass active filter. This matches the last LM3900 stage, so you would seem to be correct. The shift in filter frequency for 200bps is because the higher modulation rate results in a greater frequency shift. It's like 50hz instead of the 25hz of the 100bps rate.
Paul On Aug 17, 2014, at 4:35 PM, Robert LaJeunesse <lajeune...@mail.com> wrote: It's simple, but not obvious. The LM3900 is a Norton amplifier, and while it has differential inputs they are current driven. (Most older op amps are voltage driven.) The LM3900 is powered from 10V, so I think of that as just above the maximimum output voltage. Both the upper amplifier and the second lower amplifier have 1M feedback resistors, and + inputs fed 10V by 1M bias resistors. That would bias the output at near the supply rail, turning these stages into something like half-wave rectifiers. Since the first lower stage has a 2M bias resistor it idles at about half supply, and behaves as a simple inverter. If my analysis is correct (and I worked at National when the LM3900 came out, a friend did apps for this odd new part) then the combining of the two outputs produces a negative going full wave rectification of the signal. The fourth LM3900 stage looks like an inverting bandpass filter, but I'd have to dig out some reference books to determine its behavior in more detail. As f or the 100-200 switch I'm confused, why would the bandpass frequency be lowered for the higher modulation rate? Bob LaJeunesse Sent: Saturday, August 16, 2014 at 2:56 PM From: "Kenneth G. Gordon" <kgordon2...@frontier.com> To: "paul swed" <paulsw...@gmail.com> Cc: time-nuts@febo.com Subject: Re: [time-nuts] Cutler NAA on 24.0kHz.... On 16 Aug 2014 at 13:35, paul swed wrote: > > Kenneth on the opamps that is correct. > But I put little U's to indicate phase. They actually represent the top half > of > the input cycle. Yes, I saw those, but unless I am mistaken, you didn't add a "U" after the second opamp, which would have returned the phase to the input's. > In the top path it inverts once. I see twice: once through the first op amp and again through the second one. The second one then outputs to the IF. Anyway, to me, it is a very interesting and simple circuit. I LIKE "simple". I am a great believer in the KISS principle. Ken W7EKB _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
