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F800 CIRCUIT DESCRIPTION
9 (25)
Uppgjord (även faktaansvarig om annan) -
Prepared (also subject responsible if other)
Nr
-
No.
KL/ECS/S/LT Magnus Lindahl
ECS/S/LT-96:5044
Dokansv/Godk -
Doc respons/Approved
Kontr
-
Checked
Datum
-
Date
Rev
File
KI/ECS/S/LTC
1996-06-06
A
965044.DOC
VCO and prescaler
The main oscillator is a voltage controlled oscillator, VCO, of LC type with a FET
transistor as the active part. The tuning of the VCO is controlled by altering the
capacitance in the resonance circuit, which is done by a number of capacitance
diodes. The resonance circuit is implemented with stripline components as well as
traditional SMD components. The total frequency range for the VCO is approximately
50 MHz. To achieve faster locking and improve scanning times, the frequency range
is divided into four sub bands. The impedance of the resonance circuit is changed by
two transistor switches, controlled by the CPU- board. The two transistor switches are
used in combination and thus four sub bands are created.
The VCO also has a modulation input used for frequency modulation. The modulation
signal is amplified and filtered before it is connected to the resonance circuit in the
VCO. The modulation is done by changing the capacitance in resonance circuit by one
capacitance diode.
The RF signal from the VCO is passed through a buffer amplifier stage to make the
VCO less sensitive for changes in load on the output. From the buffer stage the RF
signal is distributed to the prescaler as well as to two amplifiers, one for TX carrier and
one for RX local oscillator. Both the TX amplifier and the RX amplifier are active as
soon as the unit is switched on. The RF signals are switched by tunnel diodes in the
transmitter and in the receiver.
The correct output RF frequency from the VCO is adjusted in the reference oscillator.
No additional tuning is required.
+
DC from LOOP filter
Modulation (FM)
Band selection
To Amp.
Fig D: Principal diagram for VCO