Theory of operation:
The receiver:
The core of the receiver is comprised of two SA612 active mixers, with a 5 crystal ladder filter between them for selectivity. An analog switch
routes the VFO and BFO to the appropriate mixer as needed for either receive or transmit.
During receive, the first mixer, U2, combines the input signal with the Local Oscillator (VFO) to produce an IF frequency of 9 MHz. An emitter
follower, Q9, buffers the output of the mixer to provide the crystal filter with a resistive load. This help reduce ripples in the filter response as
seen on the output side.
The output of the crystal filter is also terminated with a resistive load and drives the input of the second mixer, U1 which is acting as the
product detector. A 9 MHz crystal oscillator provides the BFO frequency which mixes the 9 MHz IF down to audio.
The output of the product detector, U1, is differentially coupled to a LM386 audio amp, U3. This provides an additional 6 dB of gain over
single ended coupling and helps eliminate any common mode signals on the output of the mixer. The '386 provides a voltage gain of 20 with
a minimum of external parts. A “P” channel J-fet is connected across the input pins to provide AGC action, more on this later.
The output of the first audio gain stage (U3) drives a pair of non-inverting amplifiers (U7) through a 100 K resistor. One of the amplifiers has
a modest voltage gain of 4.5, while the other has a much higher gain of 100. The output of the low gain stage (U7a) is the receiver audio.
The high gain stage is used to drive the AGC circuits, which is comprised of Q18, Q16 and Q14. The way this works is Q16 and Q14 are
normally turned off by a positive voltage on their gates when there is no input signal, allowing the full amplification of the audio signal by U3
and U7a. When the audio signal on the output of U7b starts to exceed 500 mV, Q18 starts to turn on. That reduces the gate voltage on Q14
and Q16, allowing them to start turning on. Q14 shunts the signal between the input pins of U3 while Q16 shunts the signal to ground, using
the 100K resistor R31 as a dropping resistor element.
The gain of U7b is such that AGC action keeps the output signal on U7a to be no more than about 50 mV peak. R44, a 1 meg ohm resistor
and C68, a 4.7 ufd cap sets the AGC time constant. R43, a 1 K resistor between the time constant and the controlling transistor Q8 slows
down the attack time. Without the limiting resistor, the AGC can overshoot, causing a loss of audio until C68 can charge up again.
Originally, only the Q16 shunt to ground was used for AGC. While this was enough to limit the signal to 50 mV, very strong signals would
cause U3 to saturate and clip, causing a distorted signal. Adding Q14 to reduce the input signal to U3 eliminated this problem.
The audio signal from U7a is routed through one of the analog switches in U4, the 74HC4053, to provide muting during transmit. The output
of the switch drives the volume control and then the final audio power amp, U8 which is also a LM386. By putting the AGC action before the
volume control, audio level is consistent and is only varied by the volume control.
The Transmitter:
A small microprocessor, U5, controls the transmit / receive switching. Not only does it simplify T/R control and timing, it allowed adding the
Tune mode and CW mode features which would have been more complex to do in a strictly analog fashion.
When the PTT input goes low, (grounded), the following sequence of events happen:
1.
The audio is muted
2.
The VFO and BFO oscillators are switched between the two mixers. The product detector (U1) is now the transmit mixer while the
Receiver input mixer (U2) is now the balanced modulator.
3.
The transmitter amplifiers are enabled by turning Q3 on via Q6. This also actives the QSK switch Q1, disconnecting the antenna to
T3, the input tuned circuit. Q7 is also turned on, shorting the output side of T3 so that any transmit signal which might leak past Q1
is shorted to ground before it can upset the operation of the mixer, which is now being used as the balanced modulator.
4.
The Microphone buffer transistor, Q10, is turned on by releasing the ground on the base via Q13.
Now, when you speak into the microphone, the audio is mixed with the 9 MHz BFO signal, which produces a double sideband signal, 9 MHz
+/- the audio frequency, on the mixer output. The crystal filter removes the upper side band (9 MHz + the audio frequencies), leaving only the
desired lower sideband signal.
The resulting 9 MHz LSB signal is then mixed in U1 with the VFO to produce the desired operating frequency in the 75 Meter band. The
desired mixer product is selected by the tuned circuit T4 and then amplified by Q8 and Q4 to a suitable level to drive the power output
MOSFET Q16 (which should have been labeled Q19).
Tune and CW mode:
In order to produce a single frequency output from the transmitter, an appropriate 600 Hz tone is injected into the microphone circuit. The
tone is generated by the TINY13A microprocessor and is of course a square wave. The square wave is filtered through a low pass filter
comprised of R33, C46, L3 and C47 to remove harmonics and generate a nice sin wave to modulate the transmitter. C48 across L3 blocks
the second harmonic, which would otherwise take a second filter section to effectively remove. When the tone is being generated, Q13 is
turned on which turns off the microphone so ambient noise isn't picked up by the mic. Q11 is also turned on, connecting the tone to the
modulation level control.
17
Manual revised 20160229
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