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 <[email protected]> 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: [email protected] [mailto:[email protected]] 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 <[email protected]> > 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 > > > > > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] To unsubscribe, go to > > https://www.febo.com/cgi-bin/ mailman/listinfo/time-nuts and follow > > the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > --- > This email is free from viruses and malware because avast! Antivirus > protection is active. > http://www.avast.com > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
