Diamond TX Series - Setting the Transmitter for Manual Mode (DT-TX Series)

120 9 November 2001 Transmitters
Setting Up AGC AGC mode provides the best C/N performance while
maintaining distortions within the transmitter’s specifications.
The 200-TX, 700-TX, and 800C-TX transmitters offer
factory-set AGC. The 800E-TX and 800G-TX transmitters
offer both factory-set and customer-adjustable AGC.
If signal levels in the return
path change frequently due to
ingress or scheduled addition
or deletion of channels, the
AGC will adjust the RF drive
level, causing the RF level out
of the receiver to change. To
prevent this condition, operate
the transmitter in manual
mode.
In AGC mode, a broadband AGC detector monitors the RF
drive level to the transmitter and automatically directs a PIN
attenuator to adjust the RF drive level (up to ± 3 dB) to
match a fixed reference level. This reference level is set so
that the transmitter achieves the optimum depth of
modulation.
The broadband AGC detector sees the entire RF bandwidth
and adjusts based on the total RF power present in the RF
signal. The total RF power is a function of the number of
analog channels loaded and whether the RF carriers are
modulated or CW.
If you use channel loading different from that identified in the
transmitter’s specifications, note that the AGC center point
and the ± 3 dB window around it will change.
Determining the RF Input Level for AGC
RF input level variations that
are within the AGC window
are automatically adjusted by
AGC circuitry.
The required RF input level, and the ± 3 dB AGC window
centered around it, changes with the number of channels
loaded as shown in the following formula:
input level window = -10•log (# of channels/rated channel load)
Channel
Loading
TX Series Nominal
Input
Level
AGC Window
128
800C 13 dBmV +10 to +16 dBmV
800G 17 dBmV +14 to +20 dBmV
77
700 / 800C 15 dBmV +12 to +18 dBmV
800E / 800G 19 dBmV +16 to +22 dBmV
40
700 / 800C 18 dBmV +15 to +21 dBmV
800E / 800G 22 dBmV +19 to +25 dBmV
20
200 18 dBmV +15 to +21 dBmV