if you use analog way to divide the 120kHz that will prevent an
incidentally flip of the phase
73
Alex
On 8/17/2014 2:21 PM, paul swed wrote:
Robert
Yes indeed the lm3900 is a great part. The last opamp is a 100hz BPF.
The RCs perform a phase shift and I will tend to believe that at the
bandpass filter it is a full wave rectified signal. Only a guess.
Here is the part I don't get. How does that remove the msk? Mask is FSK and
you can see the shifts in spectrumlab.
Rick per your comment yes the doubling of the carrier does remove the BPSK
that was the earliest approach studied applied and then rejected as when
teh carrier was returned to 60KHz any method used left an ambiguity that in
fact could flip randomly due to noise. Not pretty on the strip chart.
But back to this its msk. I am missing the secret math or something.
Do I believe this will work if I build it. Absolutely and a 24 Khz rcvr
ain't all that bad to build.
Regards
Paul
WB8TSL/1
On Sun, Aug 17, 2014 at 4:35 PM, Robert LaJeunesse <[email protected]>
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 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" <[email protected]>
To: "paul swed" <[email protected]>
Cc: [email protected]
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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