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...

Dec 29, 2008 44 Replies

Rate this thread:

Frnak McKenney 17 years ago

Just

--snip--

Ben,

Thanks for joining in.

Hm. Yes, I have a 2MHz signal generator around here somewhere I could use with a SS VOM.

Thanks. That's the one Dale pointed me at, and I think it's the best way to get started with VLF. Once I've wandered around the "terrain" a bit I'll have a much better idea of what I'll want to do next.

Frank

-- "...each new generation born is in effect an invasion of civilization by little barbarians, who must be civilized before it is too late." -- Thomas Sowell

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

Vote

JosephKK 17 years ago

Actually for inductors made that way maybe an N220 or N330 capacitor would be more appropriate.

Vote

Jan Panteltje 17 years ago

On a sunny day (Thu, 01 Jan 2009 20:11:12 -0800) it happened JosephKK wrote in :

Copper coil resistance will drift over temp, changing Q, Not sure about the effect of temperature on the ferrite, is that what you are referring to?

Vote

JosephKK 17 years ago

In particular i was referring to coils wound on forms, often with "tuning slugs", that have typical inductance tempco of + 220 ppm or +

330 ppm. A lot of old stuff from the tube days is like that.

Vote

Jan Panteltje 17 years ago

On a sunny day (Sat, 03 Jan 2009 11:37:14 -0800) it happened JosephKK wrote in :

Yes, I noticed that temp coefficient when looking up ring core ferrites :-)

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required