Band I to Band II audio RX / converter (yes, audio)

[Alex728]

This might seem like a bit of an odd concept, but I vaguely remember such devices being a popular project for ham bands in the era before multiband Japanese rigs were common.

I only want it for RX - 48.30 MHZ wide FM stereo to be precise. The reason being is I live near enough the ICR-FM main Band II transmitter that I should be able to monitor the studio to transmitter link frequency (as a way of checking whether any issues exist on this link as well as the studio and band II off air reception).

My scanner doesn't do WFM or I would have used it. I am not even sure if you can get WFM stereo on many scanners at this odd frequency as apparently nowhere else in Europe or the World uses Band I still for this purpose.

is it possible to build a converter that would output (at very low power) to a Band II frequency (complete with the stereo signal) so that I could monitor it on a cheap ASDA FM/DAB radio?

[G6Tanuki]

Yes it's quite simple: you need a local oscillator whose frequency - when mixed with the frequency you want - generates a frequency that you have a receiver for.

Example: mix together a 40MHz local oscillator with your 48.3MHz signal and receive it on a radio tuned to 88.3MHz.

I could do this with a single-transistor crystal overtone osc to generate the 40MHz, and something like the MD108 balanced hot-carrier-diode mixer. Even with no RF amplification it should be quite sensitive feeding into a decent-spec hifi tuner.

[Alex728]

thanks - I found a 40 MHZ crystal (third overtone) for 26p in Farnell - must brush up on my circuit design, been a long time since I hand built anything RF-related... (I admit I am only recently just getting back into electronics construction) after a long spell just dealing with computers and other ready built kit...

[G6Tanuki]

If you do go the double-balanced-mixer approach:

The 'front-end' is quite simple - MD108-type mixers have typically a 50-ohm impedance so if it's being fed from a dipole antenna you need a tuned-circuit, with an antenna-coupling winding which has the same number of turns as the output coupling-winding to the MD108.

These balanced-mixers need to be driven with quite a bit of local-oscillator power - enough to saturate the mixer so it is essentially being fed with a square-wave so it's turned hard-on/hard-off and working as a switch rather than linear. I recall doing this sort of thing using a 2N3053 transistor as the LO, and needing to fit it with a heatsink!

If you're a pedant you'll fit 'idler' loads on the output, sinking both the local-oscilaltor and signal-frequencies into 50-ohm termination resistors so the receiver you're using downstream doesn't see these signals. Most people didn't bother - it's only really significant if you're one of us amateur-types looking for 0.1microvolt signals.

[Herald1360]

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