Larcan FM-250, Техническое руководство

Страница: 36 / 86

FM-250 FM BROADCASTING EXCITER
Issue Date: 21 Jan 2003
The adjustment to control the deviation flatness is provided by R53. This is used to provide
constant deviation by adjusting the dc conditions to Varactor diodes, D21 and D22, when
the synthesizer frequency is changed.
2.
CONTROL MODULE (980508)
a) PSU and Internal Monitoring
The Control module has two dc power inputs. The dc (+48V battery) input is connected to
PL8 pin 3 , while the rectified and smoothed (a/c supply) input is connected to PL8 pin 1 .
The nominal voltage in this case is +28V.
Each of these inputs is monitored for indication purposes via R88 and R89 , which drive
external Leds on the Exciter front panel. Led D42 indicates voltage applied.
The two, dc voltages are combined via D36 and D37 (providing isolation), to fuse F1
(protection). The voltage is now distributed to various points as follows:
•
IC7 (Switch mode Psu). This provides +21.5V output and has Undervoltage startup
protection via TR9 and overvoltage protection (nom52V) via IC6/a and TR8.
Overvoltage trip level s et via R102.
•
IC9 (Switch mode Psu). This provides -12V output from the +12V supply.
•
IC8 (Linear regulator). This provides +12V output from the 21.5V supply.
•
PL4 (pins 5 and 6). This provides the dc output to the Stereo coder module (option).
•
PL5 (pin 4). This provides the +21.5V dc output to the Synthesizer module.
•
PL6 (pins 24 and 25). This provides the +21.5V dc output to the Display module.
•
IC6/b (-12V Psu sensing). This provides Psu fault indication via TR10/RL6.
b)
RF Psu Monitoring
PL2, ( pins 3 and 4) provide a differential current monitor for the RF PA power supply via
IC4/a. This configuration gives good common mode rejection and has an adjustment (R69)
for zeroing the common mode signal. The output is now amplified by IC4/b to give a voltage
level proportional to the RF PA Current. (I.e. 1 volt = 1 amp). Level adjustment is via R72
(I Cal). The voltage is then output to PL6 pin 16.
PL2 (pin 1) provides the voltage sensing for the RF PA power supply, and IC5/a detects for
low voltage against the +12V reference. The sense output is then distributed to PL6 pin 26.
c) Forward RF detection and Alarm
The Forward RF monitoring input is via SK1 (SMB), which provides a RF signal (nominally
0dBm), proportional to the RF forward power of the Exciter. A dc output proportional to
the input power is achieved via
D1, D2 and IC1/a (The diodes provide temperature
compensation). Adjustments are provided for Level (R8) and Offset (R6). The Offset
voltage is adjusted for 0.8V nominal with no input signal at SK1. This is done to allow the
use of linear metering of the RF Forward power in the Display module. The nominal adjusted
dc level is then
+5V/OdBm.