F800 CIRCUIT DESCRIPTION
14 (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
Receiver
The receiver is a double super heterodyne with 1st IF at 21,4 MHz (F804 and F805) or
10,7 MHz (F802) and the 2nd IF at 455 kHz. The 1st local oscillator signal is obtained
from the frequency generator. The 2nd local oscillator signal is generated by a crystal
oscillator in the IF circuits. The front end is varactor tuned.
Front End and 1st IF
The signal from the antenna switch is connected to the front end of the receiver. The
front end consists of one amplifier stage and four or five band pass filters. The number
of filters depends on the frequency band. The BP filter chain is tuned by capacitance
diodes in each filter. The tuning voltage is obtained from the CPU board via a buffer
stage on the radio board. The tuning is done in 16 steps, each step with a specific DC
level connected to the capacitance diodes. Each tuning step will give the front end
filters a band width of 3,2 MHz. In fact the 16 different tuning steps are not equal in
size, the most narrow is approximately 1,7 MHz and the step with the highest band
width is approximately 3,7 MHz. The gain in the front end is approximately 10 - 15 dB
and the output is also limited.
+
-
RF In
RF Tuning Voltage
Fig I: RF front end F802/4
After the front end the RF signal is connected to the 1st mixer. In the mixer the RF
signal is mixed with the 1st local oscillator signal from the VCO. To achieve faster
locking of the frequency generator, the frequency range ( 50 MHz ) of the VCO is
divided into four sub bands, each sub band approximately 10 - 15 MHz. To further
speed up the scanning procedure only two VCO sub band are used as a local
oscillator in the receiver. The local oscillator signal is below a certain frequency which
is lower then the RF signal. For RF signals higher then a certain frequency the local
oscillator signal is higher then the RF signal. The following example is valid for F804:
RF signal > 154,6 MHz the local oscillator is RF signal - 21,4 MHz
RF signal < 154,6 MHz the local oscillator is RF 21,4 MHz
