Larcan DTT250M, General Description Manual

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VHF OUTPUT RF METERING & AGC CIRCUIT BOARD
PUB96-33 rev 1: May 16, 1997
33-3
RF Output Metering 20B1299G3
kout.
‐ adjust R18.
,
ached, do not
‐ adjust R20. This adjustment is done in precisely the same way as in step a) above.
l Power"), then adjust R29 for a telemetry
utput reading of 3.0 VDC measured at U4 pin 7 or J3 ‐ 6.
d power meter upper scale
ads 25. This now corresponds to an actual reflected power of 1/40 (2.5%).
s
orresponding to 10% power level will then be 2.0 VDC. Most remote control systems can
after one or two occurrences. The third occurrence within a predetermined time (C20, R51) should
cause loc
If enough VSWR events within a short time, or one sustained occurrence, causes U5 to produce three
pulses in rapid succession, C20 acquires a sufficient charge thru R51 to raise the voltage of pin 5 of
comparator U6 higher than its reference voltage on pin 6, then Q5 will be driven HIGH which energizes
relay K2, thus locking out the transmitter.
When +12V regulated power is taken from the companion exciter, regulator VR1 is not needed nor used.
RF Metering Board Test and Calibration:
Forward Power Meter Calibration ‐ Zero Adjust
With no RF input connected, measure the DC voltage at U1 ‐ 7 (or TP1) and adjust R18 until the output
voltage at U1 ‐ 7 (TP1) drops to a minimum, approximately 10 to 20 mVDC. A DC coupled scope will
make the adjustment easier to see; the objective is to place the U1 output as near the op ‐ amp ground
rail as possible without the op ‐ amp going into saturation. Turning the pot farther will decrease the
sensitivity of the system for small signals. Once this minimum voltage has been reached, do not
re
Reflect Power Meter Calibration ‐ Zero Adjust
With no RF input connected, measure the DC voltage at U2 ‐ 7 (or TP2) and adjust R20 for a minimum
which should be approximately 20 mVDC. Once this minimum voltage has been re
re
Forward Power calibration and Telemetry
Turn off the AGC and set the exciter RF output for the transmitter to run at rated peak sync (or at
maximum licensed power, whichever is less; call this "Ful
o
Reflected Power calibration and VSWR Trip Settings
With the transmitter still at "full power", disconnect the RF input cable from J1 and connect it instead to
the reflected power input J2 via a 16 dB pad. Adjust R30 so that the reflecte
re
Adjust R47 so that the telemetry output at U4 pin 1 reads 1.5 VDC. This is the voltage that correspond
to 2.5% power. Full scale 10% power will be 3.0 VDC. We are reading a front ‐ panel meter whose
scale is calibrated to a square law so it displays power, and we want the corresponding voltage. In the
event that the circuit cannot deliver 1.5 VDC, go for 1.0 VDC instead. The telemetry voltage
c
accommodate
this.