they produce the desired facsimile signals at the
controls of the facsimile transceiver are used in
input of the facsimile transceiver. The received
their normal manner to select either positive or
negative transmission as required. In either case,
carrier swings over a total of 800 cycles and normal
adjust the system so that a minimum amplitude
heterodyning action of the beat-frequency oscil-
signal output from the facsimile transceiver
lator in the receiver produces a receiver output
produces the lowest output frequency from
which varies from 2,300 to 1,500 cps. The low-
exciter Unit O-5B/FR and a maximum amplitude
pass filter in Converter CV-2C/TX changes the
signal output from the facsimile transceiver
2,300-cps signal into a minimum amplitude output
produces the highest output frequency from
signal and the 1,500-cps signal into a maximum
amplitude signal. Received carrier frequencies
Exciter Unit O-5B/FR. The frequency and fre-
quency deviation at the output of Exciter Unit
between the low extremity (7,499.6 kc) and the
O5B/FR will be multiplied by the radio trans-
high extremity (7,500.4 kc) produce amplitudes in
mitter frequency multipliers, but the transmitter
between minimum and maximum, which the re-
output frequency must be kept to a maximum
ceiving facsimile transceiver prints as intermediate
frequency swing of 800 cps. This requires that
shades of gray between black and white. If the
the transmitter output frequency be 400 cps below
receiver hf oscillator output is set to the low side
the assigned carrier frequency when a minimum
of the received carrier, the received picture tones
facsimile signal is sent, and 400 cps above the
will be reversed. A positive transmission will be
assigned carrier frequency when a maximum
received as a negative, and vice versa.
facsimile signal is sent. The actual output
Frequency of
Amplitude of re-
Frequency of
Frequency of
frequency of the transmitter will vary between
covered facsmile
receiver
receiver
receiver hf
output
signal
carrier
these two extremes from instant to instant in
accordance with the brightness or darkness of the
Minimum.
2,300 cps
7,501.9
kc
7,499.6
kc
transmitted copy being scanned on the facsimile
Maximum.
1,500
cps
7,501.9
kc
7,500.4 kc
transceiver drum. The following chart shows the
actual transmitter output frequencies for min-
(1) When using receivers that do not use
mum and maximum facsimile signals (when the
crystal-controlled conversion, front-end
assigned rf carrier is 7,500 kilocycles (kc)), when
insertion of a highly stable heterodyning
either positive or negative facsimile transmissions
signal is a necessity for satisfactory re-
are being made:
sults. The constant frequency output of
Frequency Meter BC-221 is ideal as a
Assigned carrier
Amplitude of fac-
Tranmitter
output
substitute for crystal-controlled hf oscil-
frequency
simile signal
lator injection. The following procedure
sets Frequency Meter BC-221 to the high
7,500
kc
Minimum
7,499.6 kc.
side of the received carrier:
7,500
kc
Maximum
7,500.4 kc.
(a) Tune in the facsimile signal and zero
beat the frequency meter to this signal.
(b) Reduce the receiver rf gain control for
must be adjusted so that the highest received fre-
a reading of approximately 1 on the
quency is interpreted as a maximum amplitude
receiver S-meter.
signal at the receiving facsimile transceiver, and
(c) Couple the output of Frequency Meter
the lowest received frequency is interpreted as a
BC-221 to the receiver at the receiver
minimum amplitude signal at the receiving fac-
antenna terminal.
simile transceiver. To obtain this desired rela-
(d) Without disturbing the receiver rf gain
tionship, the output of the receiver hf oscillator
control, adjust the injection level by
must be on the hf side of the received rf signal.
adjusting the coupling to the receiver
This will produce an af beat note (2,300 cps) at
for a full-scale reading on the receiver
the receiver output when the received rf signal is
S-meter (direct coupling, although not
at its lowest frequency, and a low-frequency beat
recommended, may be necessary).
note (1,500 cps) at the output of the receiver when
Note. The exact level will vary slightly
the received rf signal is at its highest frequency.
for different receivers and can be determined
The received carrier frequencies are the same as
best by the overall performance of the
those assumed for the transmitting station, and
system.
62
