Larcan FM-250, Technical Manual

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FM-250 FM BROADCASTING EXCITER
Issue Date: 21 Jan 2003
a) SYNTHESIZER AND MODULATOR (980507)
The digital synthesizer derives the final operating frequency from a 10MHz reference (TCXO)
using a PLL (Phase lock loop) and dividers. The frequency can be selected Locally (In 100kHz
steps via rotary BCD switches) or Remotely (via a external parallel control). The direct carrier
modulator uses special techniques to obtain very low distortion and high stereo channel
separation (>60dB). This level of performance is not normally found on competitive equipment,
especially at low audio frequencies.
The Synthesizer is equipped with four modulating inputs, three of which are external (BNC's)
and one internal (SMB) for use with optional Stereo coder.
The synthesizer is protected from Out of Lock and Out of Band operation by removing the RF
Output when either of these conditions exists.
b) RF PA MODULE (980510)
The RF output (Nom10dBm) from the synthesizer buffer is amplified by a three stage RF
power amplifier to obtain the final output power of >250W. The final device is operated in
push-pull mode to obtain high efficiency. All devices are operated well within their ratings.
Wideband techniques are used throughout the PA and a built in harmonic filter is used at the
output to suppress all harmonics. Directional couplers are used to sample the output for control
and monitoring purposes.
A temperature-sensing device is provided for thermal monitoring. This device is situated at the
final RF Stage to provide accurate temperature monitoring of the RF PA Heatsink.
Current and voltage sensing is provided for control and monitoring purposes along with the
ALC circuit, which controls and monitors the RF output power.
A Series regulator is used to ensure protection and utilizes three parallel devices for maximum
reliability.
c) CONTROL MODULE (980508)
The control Pcb provides the standard telemetry functions and interconnections for the Exciter.
The unit receives two dc inputs, one from the external battery supply and one from the rectified
a/c supply and has a ‘on board’ low power switching power supply. The various voltages are
distributed to the other modules via the Control Pcb.
Forward and Reflected RF Inputs are processed and distributed for ALC control and
monitoring purposes. RF PA current, Voltage and Temperature are also processed and their
voltages distributed.
Various voltage free contacts are provided for telemetry purposes and are Link selectable for
o/c or s/c condition.
d) POWER SUPPLY MODULE (980515)