FREDERICK ELECTRONICS lSOOB1500B, Instruction Manual

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The 2.5 kHz audio signal at the collector of Ql is coupled
thru Tl to the bases of active detector Q2-Q3. The resultant
rectified negative-going detector pulses are routed thru AGC
switch S3 to the base of Q4.
If RF GAIN control R4 is set at min­
imum gain (maximum negative), negative voltage is coupled through
CRl to increase current flow in Q4.
This action tends to reduce
the current in QS, causing a corresponding increase in the Q6
current and a decrease in the Q7 current.
The overall effect of
the operation is to reduce Receiver gain by feeding back a positive
voltage to the cathode of mixer stage Vl and to IF amplifiers Zl
and Z2.
During normal signal reception the RF GAIN control is ro­
tated to some higher gain position (slider moves towards ground).
As a result, less negative voltage is coupled thru CRl and the
detected audio signal assumes control of the circuit.
Current in
Q4 thru Q7 will thus vary in accordance with the detected sig­
nal, causing more or less Receiver gain. Strong signals increase
the negative feedback; thereby, reducing Receiver gain. Weak
signals decrease the feedback to produce the opposite effect.
The slow and fast positions of the AGC switch permit the
operator to choose the most favorable operating conditions for a
given receiving condition.
Slow AGC is normally desirable for
receiving FSK signals, since a slow release time introduces the
proper amount of delay to suppress noise during momentary signal
fadeouts.
The slow release circuit in the Receiver consists of
capacitor C8 and resistors Rl4-Rl5. Release time is approximately
7.5 seconds.
Fast AGC is desirable for receiving FSK signals during rapid
fades, since Receiver sensitivity recovers quickly enough to follow
the changing signal.
The fast release circuit consists of capaci­
tor C7 and resistors Rl4-Rl5. Release time is approximately 1.1
seconds.
The external signal input to the AGC circuit consists of
mark and space tones from an external FSK Demodulator such as the
FEC Model 1200. The mark tone is connected across pins 8 and 6 of
rear panel Molex connector J21; the space tone is connected across
pins 7 and 6 of the same connector. These tones are coupled through
their respective transformers (T2 and T3) and connected to separate
amplifiers:
Ql7 for the mark and Ql8 for the space. The tone out­
puts are summed by amplifier Ql4 and the resultant collector sig­
nal is coupled through T4 to active detector Ql5-Ql6.
The detected
output· is then routed through the remote position of the AGC switch
and applied to the base of Q4. From this point on, circuit opera­
tion is identical to that of the internal AGC.
External AGC is controlled by potentiometer R3. Adjustment
of R3 varies the amount of degenerative feedback applied to Ql4.
Maximum gain is obtained with the slider of R3 at ground; minimum
gain is obtained with the slider at the other extreme.
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