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-boun...@febo.com [mailto:time-nuts-boun...@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 <csteinm...@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 > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/ mailman/listinfo/time-nuts and follow > the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
