Larcan Inc DTT250M ユーザーズマニュアル
GENERAL
DESCRIPTION
TSM 20-275D rev 0: Jul 1, 2010
1
DTT250M
DTT250M
– 250W Digital Television Transmitter
INTRODUCTION
This manual describes the LARCAN model DTT250M VHF Digital Television Transmitter.
LARCAN all‐solid‐state 250W VHF transmitters are designed to operate conservatively at 250W average
DTV power with superb performance, reliability and operating economy.
The transmitter and exciter or translator chassis are packaged in a single 19" cabinet. The simplicity of
design, the deployment of all modular and other subassemblies, and the use of standard readily
available components, enhances serviceability.
Important transmitter parameters are monitored, and can be displayed on the meter built into the
amplifier. Additionally, all meter readings are made available as DC signals for telemetry by remote
control systems. The DTT250M, like all other LARCAN transmitting equipment, is suitable for
automatic or remote‐control operation.
AMPLIFIER CHAIN
The RF output of the exciter is fed to a conservatively designed broadband solid‐state amplifier. This
amplifier requires no tuning or adjustment within its band of operation. Simplicity of operation,
reduced maintenance costs and increased reliability are a few of the major benefits derived from this
modular amplifier. This module is operated well below its maximum ratings.
The amplifier chain consists of three stages of amplification.
The preamplifier stage is a high gain, broadband, thin‐film integrated circuit amplifier operating class A.
The IPA stage consists of a pair of push‐pull FETs in a single case, operating in class AB as a linear
amplifier. This amplifier uses the identical dual FET device that is used by the PA module.
The final amplifier stage consists of six push‐pull FET amplifiers that operate in class AB, in three groups
of two in quadrature, and are combined in quadrature and then in a 3‐way combiner. The amplifier
module is rated for 250 watts average ATSC output. The module is provided with soft‐start, VSWR
protection, and a monitor port.
The amplifier output is fed to the bandpass filter and the directional coupler, which provides a small
sample of forward and reflected output power for AGC and VSWR supervisory functions. The
transmitter output then passes to the antenna system.
The amplifier output is fed to the bandpass filter and the directional coupler, which provides a small
sample of forward and reflected output power for AGC and VSWR supervisory functions. The
transmitter output then passes to the antenna system.