[GreenKeys] CV-2460

[Jim Haynes]

This was unfamiliar to me, so I skimmed through the manual.  For those
who may be interested -

FSK modem for two specific conditions:
wide shift  1575 and 2425 Hz (850 Hz shift centered on 2000 Hz)
narrow shift 500 and 700 Hz  (200 Hz shift centered on 600 Hz)

There is no tuning indicator, so apparently it is intended for use only
with modern frequency-synthesized equipment which preserves accurate
frequency translation.  Would be very difficult to use if the incoming
signal could be at frequencies different from the design frequencies
and you have to tune the receiver or receiver BFO to get them right.
Or you need to know the incoming frequencies exactly so you can set
the transceiver dial frequency correctly.

Can be used half-duplex with half-duplex radio (just one frequency)
or half-duplex with full-duplex radio (two radios on two different
frequencies but with just one telprinter) or full-duplex using one
modem, two radios and one teleprinter and one tape reader.

For testing the audio output can be looped back to the audio input.

Does not require a send-receive switch.  Instead it turns on the
transmitter when the operator starts typing, turns it off when the
operator stops.

Inputs and outputs are a choice of current loop or low voltage polar.

Principle is limiter-discriminator.  The discriminator is quite an unusual 
design, at least to me.  The input goes through a bandpass filter chosen 
by which shift is selected.  Then is limited, and the limiter output goes 
through a phase shift network which shifts one frequency by 45 degrees and 
the other by 135 degrees.  (different shifter for each frequency pair) 
The un-phase-shifted signal and the phase shifted signal are applied to 
the inputs of a product detector.  Since they are square waves the 
productor detector is a transistor operating digitally, which is to say it 
produces pulses from the overlap of the shifted and non-shifted input 
pulses.  The average of the output of the product detector is therefore a 
lower or higher voltage according to mark or space frequency input. 
Averaging is done by a low-pass filter.  Output of the filter is then 
applied to a threshold circuit which produces a digital output for mark or 
space.

Jim W6JVE

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man