User Manual

Contents:
Sec Topic Page
1 RF Metering Board Description.................................34-1
2 RF Metering Board Test and Calibration........................34-3
List of Figures:
Fig Title Drawing Reference
1 Metering Board Assembly..................................20B1235G7
2 Metering Board Schematic................................20B1245sh2
1. RF Metering Board 20B1235G7: Figures 1 and 2.
The function of this board is to monitor the forward & reflected power. Except
for their functions and input names, metering boards have identical RF detectors.
For this reason, Detector #1 for "Forward" will be described, and #2 for
"Reflected" will be referenced by its component numbers inside parentheses ( ).
a) RF Detectors:
The #1 Forward (#2 Reflected) RF power sample is applied to J1 (J2) and is
terminated by R2 (R4). A small amount of forward bias is applied to CR1 (CR2)
via R1 and R5 (R3, R6) to overcome the threshold voltage of the diode and enhance
its detection linearity at low signal levels. The opposing connection of CR1
(CR2) diode junction and Q1 (Q2) emitter-base junction provides temperature
compensation.
Q1 (Q2) buffer amplifier provides a low impedance source to drive the trap C3,
C4, and L1 (C5, C6, L2), through R9 (R10). This trap is broadly resonant to 4.3
MHz, and significantly attenuates 3.58 MHz NTSC color subcarrier as well as any
4.5 MHz intercarrier that may be generated in CR1 or CR2 due to the presence of
visual and aural RF signals together in the system. Removal of these subcarrier
components before the signal is peak detected, enables the circuit to be
responsive to sync peak power only (for visual) or just CW (aural) power, and
relatively immune to undesired carriers.
CR3 (CR4) is a peak detector with a time constant set by C7 and R11 (C8, R12).
The signal from this peak detector is fed to op-amp U1 (U2) pin 5 and . The gain
of this stage is 0.5x (0.5x), and its output on pin 7 feeds the meter which is
located on the front panel of the amplifier.
U1 (U2) output pin 7 zero-offset voltage is controlled by R18 (R20). This pot
should be set with no RF input, so that while you watch the voltage on TP1 (TP2)
as you are setting the pot, you will observe the decrease of the voltage towards
zero. When it ceases decreasing, stop adjusting. Expect about 50 mV offset
Comment:
The footer date stamp
must agree with the revision
date.