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FM-BCB 50 mHz Downconverter

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Rusty52

New Member
Several of my friends are into restoring and driving historic military vehicles. They often outfit their vehicles with period radio gear, commonly using wideband FM in the 30-70 mHz range.

I thought they'd appreciate a way to receive broadcast band FM radio in their trucks without having to drag along a (non-authentic) boombox.

I'd like to build a downconverter to translate FM-BCB to the mil frequecy range. The small, battery operated circuit would contain an 88-108 mHz filter/RF pre-amp, 50 mHz oscillator, mixer circuit, and 30-60 mHz output filter/amp. Inserted in the receive antenna line, all the operator would have to do is subtract "50" from the desired FM BCB radio frequency and set his military radio to that. OK, it's not stereo or even high fi, but it's better than listening to the tarp flap while winging down the road in your M715 truck!

I'm a digital guy, and I am hoping some RF guru could come up with a quick schematic with LC values. I figured I'd make a few for some friends, but, who knows, there may be a market out there?

Thanks,

Rusty
 

JimB

Super Moderator
Most Helpful Member
An interesting concept, however I think there is a fundamental problem.

The deviation of BC band FM is 75khz, to recieve this without distortion requires a receiver with an IF bandwidth of about 160khz. (Carson's rule, Google it).

I suspect that the old military radios in the vehicles have an IF bandwidth of 50khz or less.
This will result in severe distortion to the received signal.
OK I know it is a fairly noisy old truck, but my best guess is that even a tone deaf guy like me would not want to listen to such a thing for very long.

JimB
 

kchriste

New Member
Forum Supporter
Not to mention the fact that the 50Mhz LO frequency falls right smack dab in the middle of your "tunable RF" frequency range of the mil radio.
Is there room inside the old radio to put a PCB gutted from a modern portable radio? That may be the better way to go. Adapting the mechanical tuning mechanism would be the toughest part of the project.
 
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Rusty52

New Member
50 mHz Downconverter

JimB,

Yeah, I thought of that, but I remember lighting up my old RT-524, tuning across the 52 mHz band and receiving the audio portion of conventional TV channels. I can't imagine it was being transmitted as NBFM. In fact, I recall hearing somewhere that the deviation on those old rigs was greater than usual to provide better "talk power" under battle conditions and better reception of slightly off freq. field radios.
 

Rusty52

New Member
KCHRISTE,

Drat; you hit the nail right on the head! Well, I guess that drives a stake through the project. I did put a "good time radio" in a gutted R-442, but I thought this would be a cleaner solution.

Thanks to all for your input!

Rusty
 

Nigel Goodwin

Super Moderator
Most Helpful Member
KCHRISTE,

Drat; you hit the nail right on the head! Well, I guess that drives a stake through the project. I did put a "good time radio" in a gutted R-442, but I thought this would be a cleaner solution.

Don't forget, conversion like this is sum or difference - use an oscillator on the high side of the FM band., keeping the LO no where near the radios frequency range.

However, like others, I suspect it won't be a wideband receiver, only NBFM (of various types).
 
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