[time-nuts] Cutler NAA on 24.0kHz....

Robert LaJeunesse lajeunesse at mail.com
Sun Aug 17 16:35:43 EDT 2014


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 for 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" <kgordon2006 at frontier.com>
To: "paul swed" <paulswedb at gmail.com>
Cc: time-nuts at 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



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