Pag. 47
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
Electronic Broadcast Equipment
THE SYNTHESISED OSCILLATOR
The frequency synthesizer is a classical phase-locked-loop circuit with 10-kHz step synthesis across the
entire FM band.
The very low-noise fundamental-frequency VCO consists of a FET-oscillator transistor TR5, modulated
by the varactor diode D4÷D7 which also set the operating frequency. The circuit is sensitivity compensated
vs. carrier frequency variation so that its modulation gain varies less then 0.5 dB across the entire operating
range.
Modulation distortion is typically lower than 0.03% with over 90 dB S/N ratio in mono mode within the
30 - 20,000 Hz band.
The RF signal is buffered and amplified by three cascaded transistors TR6 ÷ TR8, from which is derived
the feedback signal to the PLL and the drive signal for the output RF stage. This latter one is composed by
two small mosfet transistors TR9 and TR10 and delivers about 900 mW output level (+29 dBm) over the
full FM range. To correctly operate, TR9 and TR10 require a gate bias voltage, which is factory pre-set by
RT1.
The digital PLL circuit is entirely contained in IC2, whose frequency reference is derived from an highly
precise temperature compensated oscillator (TCXO1) running at a frequency of 12.8 MHz.
To correctly operate on the chosen frequency, IC2 must be serially programmed with complex data. This
task is performed by the transmitter CPU through 3 control lines.
IC1 also serves as a loop filtering from IC2 frequency comparator output to the varactor diodes and
lock detection. Note that bias voltage is removed from output transistors through TR4 and TR3 to turn-off
RF when the PLL is not locked on the right frequency. The control loop was designed to ensure that cross-
talk added to stereo-composite signal is below -55 dB at 30 Hz and is virtually not influent at just slightly
higher frequencies.
THE RF DRIVER AMPLIFIER
This stage is designed with one high gain mosfet RF power transistor, which has some 17 dB gain, in a
broadband design.
A resistive input attenuator (R2, R3, R4) enhances matching with the previous stage and contributes
to insulate the two stages. A broadband matching network feeds the transistor gate and and the other one
follows the transistor drain.
The available output power of this unit is well over 25W on the whole FM range, with 28V power supply.
To limit that power up to nearly 20W, a lower power supply voltage is applied, usually 20V.
