Elecraft XV Transverter Owner's Manual - Manual (Page 30)

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The control circuits may not release relays K8 and K9 fast enough to
prevent damage to mixer Z1 from a high-level 28 MHz I.F. signal, so
LEDs D10 and D11 are used to protect the mixer. LEDs D10 and D11 will
conduct and limit the I.F. signal to a safe level. LEDs are used because
their greater forward-conduction voltage avoids the need to use diodes in
series.
External PA Keying: When the transverter switches to transmit mode,
controller U1 sends a signal to Q4 that turns it on, providing a path to
ground at EXT PA KEY jack J5 to key an external power amplifier.
Power Monitor: D1 produces a DC voltage proportional to the RF power
output of U7. The voltage, Po, is sent to controller U1 on the front panel.
The controller drives D1 through D10 to indicate the RF power level from
1 through 30 watts. The LEDs below 20 watts are green. The 20-watt LED
is yellow, and the 25 and 30 watt LEDs are red to indicate that the
specified maximum power output is being exceeded.
The metering circuit is calibrated by potentiometer R10.
Display Mode: When the transverter is used with an Elecraft K2, the
behavior of the LEDs on the front panel follows the display mode selected
by the operator at the K2. These selections determine the LED brightness
and whether the power output LEDs illuminate in a line extending from
the left (bar mode) or if only one LED corresponding to the power output
illuminates (dot mode). The display mode information is reported to
controller U1 via the AuxBus. The controller changes the brightness by
varying the bias on Q2 and Q5 on the front panel PCB.