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

Thu Sep 11 15:06:10 UTC 2014

Update on NAA.

The tracor d-msk-r was completed last night and tried it out.

Someone on this thread said I would find that the lm3900 was acting as a
half wave rectifier and he was right. The whole thing is a frequency
doubler. But the last stage removes the higher frequency. Its a bandpass
filter.

The magic goes like this.
NAA is at 24,000Hz and the LO is at 23,900 Hz. Result a 100 Hz baseband
signal modulates + and - 50 Hz MSK. The doubler multiplies 50 Hz to 100 Hz.
the 150 is multiplied and rejected by the bandpass filter. Result, a 100 hz
signal only for one of the MSK carriers. The one at 23950 Hz. (Because of
the low side LO injection that may be flipped. But no real effect.)

I barely looked at the stability of that 100 Hz. The fact is I have no 100
Hz locked reference. There is a bit of humor everyday. Never thought I
would need one. Easy enough to make and I will dig in further tonight.

Regards
Paul
WB8TSL

On Tue, Sep 9, 2014 at 8:50 PM, paul swed <paulswedb at gmail.com> wrote:

> 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.
> http://www.qsl.net/ik2pii/lf/dcrx136.htm
>
> 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.
> Regards
> Paul
> WB8TSL
>
> On Wed, Aug 20, 2014 at 9:01 AM, Bill Riches <bill.riches at verizon.net>
> wrote:
>
>> 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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