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, so I wondered if it would be possible to modify one of these to tune in RF in the WWVB and LF-RFID (125kHz) neighborhoods.
Before pulling anything apaprt I thought I'd check the math see if it was even reasonable. I assume these units are standard superhet designs with a local oscillator and a 455kHz IF section, such as the circuit 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). The capacitance 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 for this capacitor would be between 620uH and 660uH; choosing 640uH and working 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) or
2079kHz kHz, a range of 2.1. Calculating from the low end the associated inductor (LO tank coil) would need to be 428uH yielding an 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 of 500uH, my LO calculations imply that the LO will be as much as 200kHz "off" from the ideal mixing frequency of (RF+455kHz) at either end of the dial; since radios appear (in general) to work, I'll have to assume there's a problem with my LO math.But ignoring that for now (famous last words ), that says that if I stick a 75000uH choke in series with the radio's ferrite loopstick, giving a total inductance of 75640uH, the _RF_ section will (theoretically) have a tuning range of 49-149kHz. _IF_ I can get a LO that tunes in sync with this, that is, 504-604kHz, then the IF section will accept and amplify any AM signal from 49 to 149kHz, envelope-detect it, and feed the result through the receiver's amplifier.
I can see a couple of problems, even if I figure out why my LO math doesn't work out:
1) My desired LO range is 1:1.2, well below what the LO section of the tuning capacitor is designed for (1:2.4). However, it looks like slapping a 100pF disk cap in parallel with the LO section will reduce its range to around 1:1.2, and I'd need to find (or wind) a new tank coil anyway. 2) By putting a (relatively huge) RF choke in series with the loopstick antenna, I'm creating a fairly unbalanced "tapped inductor". I have no idea what problems might result from this. 3) I'll be limited to broadcast-alike (10kHz AM) or similar signals unless I want to add a BFO. Since curiosity is what's prompting me, I can live with this restriction. 4) Something Completely Different that I haven't even considered.Has anyone tried this? I looked around the 'web a bit, and did find some "upconverter kits" which appear to turn a superhet receiver into a super-superhet receiver , but no one attempting to directly modify an AM receiver.
I have a couple of other items with higher priority (e.g. my "Federal and State 2008 Final Tax Exam") I have to get out of the way before I'll have time to solder anything, but I would like to hear from anyone who caes to comment (especially on my math ).
Thanks...
Frank McKenney
-- All scientific men were formerly accused of practicing magic. And no wonder, for each said to himself, "I have carried human intelligence as far as it will go, and yet So-and-so has gone further than I. Ergo, he has taken to sorcery." -- Baron de Montesquieu
-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)