Modding an AM BCB radio for VLF reception

Frnak McKenney · 2008-12-29T03:00:04+00:00

I'd like to listen in on NIST's 60kHz time broadcasts on WWVB"directly", but I like building things, and besides, I'm too chea...er, "thrifty" to go out and buy a VLF-specific receiver.Down in the basement (a.k.a. The Gorge of Eternal Electronic Peril)I have a couple of AM/FM clock-radios I've accumulated (yard sale?local thrift shop? I forget). Unlike today's Walkman-alike radios,these tend to have large, discrete, and easy-to-reach components, soI wondered if it would be possible to modify one of these to tune inRF in the WWVB and LF-RFID (125kHz) neighborhoods.Before pulling anything apaprt I thought I'd check the math see ifit was even reasonable. I assume these units are standard superhetdesigns with a local oscillator and a 455kHz IF section, such as thecircuit at: AM BCB: 540-1600 kHz Tuning capacitor: 140- 15 pF (RF) 60- 10 pF (LO) 0- 12 pF (trimmers)From Ye Olde Traditional Formula: F = 1/(2 * %pi * sqrt(L * C) )it follows that, for a fixed L, F varies with 1/sqrt(C). Thecapacitance of the RF gang of the tuning capacitor has a 10:1 range,so it can control a frequency range of 1:sqrt(10), or 1:3.2.Similarly, the LO section provides a frequency range of 1:sqrt(6),or 1:2.4.Working this backwards, it looks like an antenna loop designed forthis capacitor would be between 620uH and 660uH; choosing 640uH andworking forward the RF section would tune from 532 to 1624 kHz.This means the LO will run from (532+455) or 987kHz to (1624+455) or2079kHz kHz, a range of 2.1. Calculating from the low end theassociated inductor (LO tank coil) would need to be 428uH yieldingan LO range of 993-2433kHz; working from the high end we get 586uH,an LO range of 849-2079kHz. Even if I choose a "middle" value of500uH, my LO calculations imply that the LO will be as much as200kHz "off" from the ideal mixing frequency of (RF+455kHz) ateither end of the dial; since radios appear (in general) to work,I'll have to...