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FRG-7 (was: Re: [Toronto] Freebie)



To be fair to those who don't necessarily read their email
every day, I'll make a random drawing of the names I've
received at 2000 UTC Friday. For reasons of cost and
practicality I'll weight the draw in favour of local
responses, but everybody is in with a chance. I wish
I had a dozen to give away - this is a receiver that many
people think very highly of!

Technical digression:

The FRG-7 uses an interesting system (a Barlow-Wadley Loop)
for mixing to a VHF first IF, then down to an MF second IF
for the final processing. Here's how it works...

The first LO tunes 55.5 to 84.5 MHz. This mixes DC-30 MHz
in 1 MHz chunks to 54.5 to 55.5 MHz.

The output of the 1st LO is also mixed with a comb of harmonics
generated by a 1 MHz crystal oscillator. The receiver uses
a selective amplifier to choose the comb + LO product that is
at 52.5 MHz, mixing to the 2 to 3 MHz second IF. The
processing from there involves an LO that tunes 2455 to
3455 kHz, for a final IF of 455 kHz.

The key to the receiver stability (outstanding, by 1970s
standards) is the fact that the up- and down-conversion
frequencies are generated from the same potentially
drifty oscillator, but convert in opposite directions.
The drift thus cancels.

The 1 MHz steps in the tuning are selected and controlled
by monitoring the presence (or absence) of a mixing product
at the output of the selective 52.5 MHz amplifier. This
drives the LED on the front panel, familiar to FRG-7 owners.
-- 
Laura Halliday VA3LDH    "Laisse le vent tout emporter..."
Grid: FN03gs                - Foly/Viennet
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