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

paul swed paulswedb at gmail.com
Tue Aug 19 21:50:43 EDT 2014

Did measure NAA near Boston 8000uv using a dipole for 80 meters.
Looking at various vlf receivers it looks like a LPF or maybe a BPF filter
to a ne602 mixer followed by a tl081opamp LPF makes a direct conversion
receiver. Then hit the tracor d-msk-r.

On Mon, Aug 18, 2014 at 2:34 PM, Charles Steinmetz <csteinmetz at yandex.com>

> Paul wrote:
>  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.
>> Robert wrote:
>>  It's simple, but not obvious. The LM3900 is a Norton amplifier, and
>>> while it has differential inputs they are current driven.   *  *  *   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.   *  *  *
>>> combining 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?
> The circuit as a whole operates as a frequency doubler using full-wave
> rectification and filtering.  The rx LO is 100Hz below the nominal carrier
> frequency, so in "normal" (non-MSK) mode, the IF frequency is 100Hz.
> Referring to the MSK addendum, a received 200 baud MSK signal is 50Hz below
> nominal, and a 100 baud MSK signal is 25Hz below nominal.  With the LO 100
> Hz below nominal, this makes the IF frequency 50Hz when receiving a 200
> baud MSK signal, and 75 Hz when receiving a 100 baud MSK signal.  After
> doubling, these become 100 Hz (200 baud) and 150 Hz (100 baud), so the BPF
> is switchable between 100Hz and 150Hz.  They used a FET to chop the 150Hz
> (100 baud) signal with a 50Hz square wave.
> I can't say I'm impressed with the design, even for the era.  The whole
> instrument is built mostly with LM3900s, which makes it thousands (maybe
> even millions) of times noisier than it would be if it had been properly
> designed with standard op-amps.  It may work more or less, but it's a fugly
> way to get there.  There are other questionable choices (like the FET
> chopper, an overall design that depends on lots of one-shots, etc.).  The
> designers knew about the LM301 (there is one in the unit), so there was
> really no excuse for using LM3900s.  Yeah, the 301 was more expensive --
> but this was supposed to be a state-of-the-art measuring device for
> characterizing good OCXOs down to PPB or below.
> I simulated the MSK board in LTspice.  Let me know (OFFLIST ONLY, please)
> if you would like the files to play with (662kB ZIP file).  (Note that
> these won't do you any good if you're not an LTspice user.)  Again, please
> do not clutter the list with requests for files -- OFFLIST ONLY, please
> (check your headers carefully before you hit "Send").
> Best regards,
> Charles
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