F800 CIRCUIT DESCRIPTION
11 (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
RF switch and RF amplifiers
The RF signal from the frequency generator is connected to the RF driver through the
RF switch. The RF switch is a diode switch that prevents leakage of RF signals from
the frequency generator through the transmitter in RX mode.
The RF driver stage is a single transistor amplifier and an LP filter. The regulation of
RF output power is done in the RF driver stage. The RF signal from the frequency
generator is amplified to an acceptable level for the final RF stage. The RF output
power is regulated by altering the amplification in the RF driver stage. The bias
voltage to the transistor stage is changed by a regulator in the transmitter control
block.
The final RF amplifier is an integrated module with two internal amplifier stages. The
amplification in the final stages is set to fixed ratio (in dB). To obtain maximal output
power both stages in the RF module are supplied with 12 V directly from the DC
source. Internally in the module there are filters to avoid oscillation etc. The output
from the final stage is passed through an LP filter before it is passed on to the
directional coupler etc.
RF PA
+10VT
Carrier
from VCO
+
Power control
LP
Filter
+12V
FIG F: Block diagram for RF PA
Power control loop and SWR
A directional coupler is used to measure the reverse RF power from the antenna and
also to monitor the RF output power. The RF samples are rectified and filtered and
used in the power control loop. The signal that represents the output power is
connected to a comparator in the RF power indicator. The output from the comparator
is connected to the CPU board and logical "1" indicates that the RF output power
exceeds 0,2 Watts. SWR or Standing Wave Ratio is a term used to describe the
relationship between the RF output power and the reversed RF power. Normally a
SWR at 2:1 is acceptable, this represents a reverse power of approximately 10 % of
the output power.
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