Elecraft W1 - Manual (Page 11)

11
Circuit Description
Refer to the schematic diagrams on pages 12 and 13 . Transformer T1 is a directional coupler sampling the forward and
reflected power passing through J1 and J2. The forward RF power sample is terminated by resistor R1 and the reflected
power is terminated by resistor R2. The coupler has about 20 dB of isolation, so at 140 watts either resistor may dissipate
nearly 2 watts. For that reason 3 watt resistors are used and spaced slightly above the board for good air circulation.
A special RF detector circuit is used that has good accuracy over a much wider range of powers than that used in most power
meters and SWR bridges. Diode D1 rectifies the forward power sample while D2 rectifies the reverse power sample. They
are shunt rectifiers whose anodes are grounded by capacitor C3, so they develop a positive dc voltage output. To enhance the
low power sensitivity, both diodes are forward biased from the +5V supply through resistor R11 and the 324K 1% resistors
R3 and R4. About 1 microampere flows through the diodes even when no signal is present, overcoming the lack of sensitivity
and linearity that normally is found when a diode is operated at very low voltages. However, diodes operated this way are
very sensitive to ambient conditions, especially temperature. Diode D3 is included as a compensating diode. No RF is applied
to D3, just the d-c bias from the +5V supply so its current is not affected by the RF applied to the coupler. The voltage D3
produces across R5 is subtracted from the voltages produced by D1 and D2 to compensate for any changes in the outputs of
D1 and D2 caused by environmental conditions.
Operational amplifiers U1 and U2 sample the voltages and send the outputs of the diodes to controller U4 (sheet 2) which
drives the Forward Power and SWR LEDs. The Forward Power and SWR LEDs are illuminated alternately by transistors Q3
and Q4. Signals from U4 turn on Q3 and Q4 alternately at a rate too fast to see by eye.
At high power levels RF detector diodes D1 and D2 would produce dc voltages too high for U4, so U4 provides a SCALE
output to switch in fixed attenuators for D1, D2 and D3 by turning on Q1, Q2 and Q5 to ground R12, R13 and R20. U4 also
drives LEDs D26, D27 or D28 to indicate that the Forward Power indicated is either 1.4 watts, 14 watts or 140 watts full
scale.
Potentiometer R17 provides an adjustable bias for U4 that calibrates the Forward Power display.
Transistors Q6 and Q7 condition the signals being provided to and received from the serial data port J5.
Voltage regulator U3 provides a stable 5V output from either an internal 9V battery or an external 12V source. Diodes D4
and D5 isolate the two supply sources so the external supply won’t force current into the battery and the battery won’t be
drained if a load is left attached to J4.