[time-nuts] Cutler NAA on 24.0kHz....
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.
Cape May, NJ
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.
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. * * *
>>> 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
> 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,
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