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

Bill Riches bill.riches at verizon.net
Wed Aug 20 09:01:58 EDT 2014


NAA -50 dbm  (1 MV) using a mini-whip.  -60 dbm on the k9ay loop. Mini-whip
is full of surprises.

73,

Bill, WA2DVU
Cape May, NJ

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of paul swed
Sent: Tuesday, August 19, 2014 9:51 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Cutler NAA on 24.0kHz....

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.
Regards
Paul
WB8TSL


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

> 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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