Elecraft K1 - Manual (Page 61)

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LECRAFT
Q9 isolates the VFO from noise pickup on the frequency counter
line. U5 and U6 double-regulate the VFO supply voltage for
improved stability. Since U5 is a low-dropout 8-volt regulator, the
K1's supply voltage can drop to as low as about 8.5 V without
affecting the VFO frequency. U6 is a 6.0-volt regulator with a very
tight tolerance of 2.5%.
Premixer U7 mixes the VFO with one of the crystals on the Filter
board.
Receiver (Sheet 1)
U1 is a double-balanced receive mixer, which provides an excellent
noise figure at low current drain. If input levels are too high, an
attenuator can be switched in via K1. RFC9 (across K1) suppresses
attenuator on/off switching noise that might otherwise be audible.
Emitter-follower Q3 provides some power gain while providing a
low driving impedance for the crystal filter (X1-X4). The crystal
filter has variable bandwidth, by virtue of varactor diodes D6-D8.
T2 steps up the crystal filter output to match the 1500-ohm input
impedance of the product detector/BFO, U2. The BFO is mixed
with the I.F. to produce an audio signal.
U3 amplifies the audio signal enough to drive the audio-derived
AGC detector, D2. R21 and C18 remove high-frequency hiss.
D2 is DC-biased just below the level needed to turn on darlington
transistor Q2. When a signal is present, Q2 will turn on in
proportion to the signal amplitude, pulling the AGC line down from
its nominal 1.2 V to as low as 0.6 V. The AGC line is connected to
the input bias pins of both U1 and U2, so that the gain of both
mixers is reduced as signal strength increases. R1 limits the gain
reduction at U1, so most of the AGC action is due to U2, which
minimizes the chance of front-end overload when AGC is activated.
Only signals in the passband of the crystal filter can cause AGC in
any case, so the AGC line stays at about 1.2 V most of the time.
Q10 and Q11 are shunt and series mute devices, respectively. Both
are required due to the large amount of audio gain in the receiver.
U4 amplifies the received audio signal and sidetone to headphone or
speaker level. The AF GAIN control is located on the front panel,
but is electrically connected between Q11 and U4 at the AF1 and
AF2 points.
In receive mode, the AGC signal is routed onto the RF/SMTR line,
which is then sampled by an A-to-D input on the MCU (U1, front
panel). Q1 is used to connect or disconnect the AGC signal from
the RF/SMTR line.
Transmitter (Sheet 2)
To vary the power output, JFET Q5 and PIN diode D18 control
the amount of premix signal input to the transmit mixer, U8. The
DC control voltage at the drain of Q5 is set by the MCU (U1, front
panel). ALC (automatic level control) is implemented in firmware.
During keying, the ALC determines what DC voltage is needed to
get to the requested power level, as well as what voltage
corresponds to zero output. As you key the transmitter, the control
voltage alternates between these two levels so that that the keying
waveform can be effectively shaped by R10 and C59.
Video amplifier U9 buffers and amplifies the TX mixer output. Q14
keeps U9 fully turned off during receive. The low-level transmit
signal is passed through the RF band-pass filter on the Filter board
via J7.
D9-D13 form a fast, high-isolation T-R switch that allows the
appropriate RF band-pass filter to be shared between receive and
transmit. On transmit the signal is routed through D9 to the driver
transistor, Q6. On receive, D10, 11, and 12 are turned on,
bypassing the transmit stages.
Q7 is the class-C final amplifier. It operates at a nominal collector
impedance of 12.5 ohms, which is stepped up to 50 ohms by T4.
Q7's output is filtered by one of the low-pass filters on the Filter
board, with signals routed through J8. RF detector diode D15
samples some of the RF output voltage to provide a power output
indication. If the KAT1 antenna tuner is installed, its SWR
bridge/wattmeter output takes over from D15, via pin 5 of J8.