TX Techniques TX FM1, Technical Manual

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TX-FM1/S/R/A Technical Manual www.txtechniques.co.uk
1. Introduction
1.1 General
The TX-FM1/S/R/A is a 30W VHF FM Broadcast Exciter Transmitter with integral
stereo encoder, RDS encoder and multiband audio processor (with ALC and
clipping).
At the heart of the TX-FM1/S/R/A is a high quality modulator, which employs
proprietary 'Complimentary Varicap Compensation' for linear characteristics and
constant modulation levels across the entire FM band. Carrier frequency is selectable
via internal rotary dial switches, with a trimmer provided for output power adjustment
from 10 to 30W. The efficient MOSFET RF power amplifier includes a 7-pole
Chebyshev output filter and uses high power rated metal clad mica capacitors and
large air-wound coils for exceptional ruggedness.
Traditional slow acting needle meters have been replaced by the latest analogue to
digital measurement technology, with menu driven LCD screens for monitoring and
adjustments.
SWR cut back, and both audible and visual alarms activate in the event of
mismatches in the antenna system.
Careful design and layout of internal circuitry results in low board space, no wire
jumper or 'zero ohm' links. Every connection to every PCB is pluggable, allowing fast
serving. The use of 'through-hole' components means components can be replaced
easily by engineers using traditional methods.
The TX-FM1/S/R/A is designed to operate continually for many years without
maintenance. There are no cooling fans to replace and the carefully specified, quality
components ensure ultimate reliability.
1.2 RDS Encoder
The RDS Encoder broadcasts most RDS data types (excluding EON information),
and includes dynamic switching of the TA flag using DTMF signalling via programme
audio.
1.3 Stereo Encoder
The Stereo Encoder is part of the above RDS option board but is available without
RDS by means of reduced component population. The pilot tone and subcarrier are
generated from a single clock source for perfect phase correlation, and a high quality
DSSC modulation technique is employed for high quality results. Stereo operation
can be defeated using the LCD menu control. Composite clipping is applied to Coder
output.
1.4 Audio Processor
The 5-band audio processor provides optimum loudness, within tight deviation limits
and, together with the ALC section, ensures a constant level during both short term
and long term fluctuations. A ‘noise gate’ prevents noise on landline feeds or STL's
rising during long pauses, by returning the ALC section to 0dB gain.