AN/PRT-4 manual
 

I have managed to find the depot repair manual for the AN/PRT-4
handheld transmitter, which I have whined about here in the past.
These transmitters easily cover the 6M band, and they are very
cheap and plentiful on the surplus market.

But now that I read the manual I realize why they are such lousy
performers. It appears one of the requirements was that the same
crystals be used on both the transmitter and receiver.

SO, the transmitter has a local oscillator, with the crystal, operating
10.7 MC away from the final frequency. Then it has a second local
oscillator operating at 10.7 MC, with crystal control AND with a varactor
diode to skew the tuning for modulation. The outputs of these two
oscillators are mixed together to form a product that is on frequency.

BUT! It doesn't stop there! The waveform that results is lousy, so
there is a PLL with a fairly narrow operating range, which locks to the
modulated signal, and the PLL shakes back and forth with modulation. The
output of THAT goes to a wideband amplifier and then to the antenna.

Needles to say the audio distortion is higher than it ought to be, and
the parts count is too.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."