Nautel XR12, Troubleshooting Manual, Page: 99 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-28 Page 2-31
RF drive buffer PWB ( NAPE77A /01)
See electrical schematic Figure SD-30.
The 0 - 15 V RF carrier frequency signals generated by the RF synthesizer PWBs are applied to the
RF Drive A (J1) and B (J2) inputs. The active RF Drive signal, as selected by relay K1, is applied to
MOSFET drivers U2A through U5A . The buffer amplified RF Drive signals are applied to the RF
power modules. An identical test signal is provided on the
RF DRIVE (TEST)
output (J5).
A sample of the active RF Drive input is peak detected by diode CR1, resistor R5 and capacitor C1
and applied to the inverting input of operational amplifier U1A. During normal operation, this dc
voltage is greater than the threshold voltage (4.2 V) applied to U1A’s non-inverting input. U1A’s
output is 0 V, which turns off transistor Q1. The
RF Drive Fail output (J3-8) is open collector (non-
alarm state). If the RF drive signal becomes too low or disappears, the voltage applied to U1A’s
inverting inputs is below the 4.2 V on the non-inverting input. U1A’s output switches to 15 V, which
turns on transistor Q1. The
RF Drive Fail output (J3-8) switches to a current-sink-to-ground (alarm
state). This alarm signal is applied to the control/display PWB via the distribution PWB and exciter
interface PWB. A
SENSITIVITY
jumper (E3) is provided to change the level required to cause an RF
Drive Fail alarm. If the transmitter is configured for IBOC, this jumper should be set to
LOW
(shorting pins 2 and 3).
The IBOC phase input, if used, is applied to the RF Phase In inputs [J14 -5 (+) and J14-4 (-)]. The
signal is applied through RF transformer T1 and inverters U12A and U12 D, where it is connected to
both
RF Drive A and B signal paths. Shorting jumpers E1 and E2 must be set in the
EXT
position
(shorting pins 2 and 3) for IBOC use.