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User Manual
Rev. 1.0- 29/11/05
PTRL NV/3 & RXRL NV/3
Next is another, higher-frequency low-pass filter that removes multiplex signal
harmonics. Again, this filter is a high-precision active circuit built around IC5 and IC6.
Another section (c) of IC6 provides for phase equalisation on the filtered signal. The
four analogue switches comprised in IC7 let you select the mono signal or the stereo
coder signal. They also achieve a slight variation of coder gain when toggling between
the mono position (100% of audio modulation) and the stereo position (90% audio,
10% pilot frequency). Two jumpers let you select the left channel (L), the right channel
(R) or the mean value of the summed channels (L+R) for mono operation, while the
output level remains unchanged as the combinations vary. Both jumpers are present
at the L and R positions to mix the right and left channels in the "MONOL+R"
transmission mode (factory setting).
Circuits IC10 and IC11 provide the time base of the coder; IC9 synthesises the pilot
frequency filtered and amplified by IC5a. A separate output with a 1Vpp signal is
provided at connector J2 to enable carrier synchronisation on an external RDS
generator (if fitted). However, this output is not connected externally to the transmitters
of series PTRLxx5, as these incorporate a modulation monitor output the serves the
same purpose.
8.1.4 Modulated Oscillator
Like the oscillator discussed previously, this oscillator is made up of a PLL circuit: in
this instance, synthesisation step is 100kHz throughout the preset operation band.
The VCO operates in the 600 to 900 MHz frequency range with very low noise. It
uses a FET transistor (TR2) as an oscillator modulated by varactor diodes D1 and
D2. Again, typical modulation distortion is less than 0.03% with an excellent S/D
ratio
The RF signal is separated and amplified by a sequence of amplifier microcircuits
IC1÷IC3 that also provide a feedback signal for PLL. IC3 drives the TR3 transistor,
that acts as a frequency doubler or tripler, as required by the final operating range. A
subsequent band-pass filter centred around output frequency removes any residual
spurious content from the signal.
Programmable synthesiser IC4 is very similar to that of the lower frequency oscillator,
just like its temperature compensated time base TCXO1 operating at 12.8MHz.
IC5 acts as an error amplifier and loop filter, whereas the lock detector is built around
IC6b.
8.1.5 Power Amplifier, Circulator and Output Filter
This module comprises three amplifier stages arranged in a sequence. A variable
pin diode attenuator (D1, D2) and a fixed resistive attenuator are provided at the
input end to achieve gain control and improve adaptation.
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