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.
