2
6
1
SECTION II
CIRCUIT DESCRIPTION
2.1
MODULATOR:
EMCEE EM1 / Scientific Atlanta 6340 / RF Comm. 2000
t
A1
IF OUT
8dBm peak visual typical
The modulator processes baseband video and audio signals to provide a composite IF output
composed of a visual carrier at 45.75MHz with 5M75C3F modulation, and an aural carrier at
41.25MHz with 250KF3E modulation. Any of these modulators can be used to drive the EMEX1
2 Watt Exciter drawer (A2). The optional video sense circuit provides an indication of whether the
modulator is receiving a baseband video signal. The output of the video sense is either a logic
high (
+5Vdc) with video present or a logic low (
0Vdc) when video is not present. This logic
signal is used by the Exciter Control/Interface Board (A2PC1) to place the transmitter in standby
if the video signal is lost.
2.1a
Video Detector Switch:
Burst VDS
t
A9
For those modulators which do not contain a video sense alarm, the Burst Electronics Video
Detector Switch (Model VDS) will be installed in the transmitter cabinet. The VDS detects the
presence of composite video connected to its A INPUT directly from the VIDEO connector mounted
on the Video/Audio/Remote Panel (A8J2). If a loss of video is detected, then the unit switches to
the B INPUT, lights its red LED and energizes an internal TALLY relay. Through this relay, a
ground is sent to the Exciter Control/Interface circuit via the transmitter’s Control/Monitor board
(A5PC1) placing the transmitter in a nonradiating condition.
2.2
EMEX1 2 WATT UHF EXCITER:
Interconnection Diagram 40404001/Rev 55
t
A2
Composite IF IN (J1)
8dBm peak visual
21dBm average aural
RF OUT (J2 & J3)
+18dBm typical peak visual
+5dBm typical average aural
The EMEX1 Exciter drawer converts the composite IF signal from the modulator to the desired
UHF frequency while amplifying this RF signal to the appropriate output level. The Exciter supplies
approximately 200mW of peak visual power to drive the 500W UHF Power Amplifier drawer (A3).
Power control, automatic gain control (AGC) and heterodyne conversion are performed by the
IF/Converter module (PC1). Utilizing the composite IF signal, the Linearity Corrector furnishes
performance improvements for the transmitter’s ICPM, differential phase and gain, sync amplitude
