[time-nuts] Cutler NAA on 24.0kHz....
paulswedb at gmail.com
Tue Sep 9 20:50:00 EDT 2014
A short update on NAA as some sort of reference.
Have completed a simple direct conversion circuit using a NE612. A few
minor changes because the system only needs 100 Hz or less. It is
essentially this circuit.
Its receiving NAA just fine with about 1 Vpp out and the official MSK is
there. No attempt to use the tracor 900 circuit yet. Also the LO is a HP
3335a locked to the station RB. Somewhat a massive system connected to a
small board. A start none the less.
Will not be hard to build the tracor d-msk-r to see what happens.
On Wed, Aug 20, 2014 at 9:01 AM, Bill Riches <bill.riches at verizon.net>
> NAA -50 dbm (1 MV) using a mini-whip. -60 dbm on the k9ay loop. Mini-whip
> is full of surprises.
> 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.
> 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
> 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
> 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
> > Best regards,
> > Charles
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