Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 112

WJ-8718-19/FE HF RECEIVER
CIRCUIT DESCRIPTION
3.4.7
1ST CONVERTER (FE-A1A2A3) (270901)
The 1st Converter mounts on the Frequency Extender, FE-Al, Assembly
(see paragraph 3.3.2). Figure 3-14 is a detailed block diagram of the 1st Converter
which should be referred to in the following circuit description. Figure 6-10, 1st
Converter Schematic Diagram, may be referred to for greater component level detail, if
desired.
VHF signals (30-100 MHz) from the Preselector, FE-A1A1A3, are routed
through bandpass filter FL1 to the input of Ul. FL1 has a 50 ohm characteristics and
filters output signals above 100 MHz and below 30 MHz with an overall loss of -2 dB.
Ul amplifies the signals by +15 dB and sends the signals to voltage controlled
attenuator U2. U2 has a nominal loss of -2 dB when the dc voltage at its control
input is > +7 Vdc and a loss of > -40 dB when the control input voltage is 0 Vdc. U2
output drives the input to mixer U5.
The 1st LO signal, 190-260 MHz, from the VCO, FE-A1A1A2, is amplified
by +15 dB by buffer U3 and drives mixer U5. U5 mixes the 1st LO with the VHF RF
signals and produces the 29.5 MHz 2nd IF output. The 2nd IF output is routed to RF
Switch, FE-A2.
AGC control voltage from the IF/Demodulator Section is applied to level
changer U6. Under no signal input conditions, the AGC voltage is 0 Vdc, and the U6
output which drives the control input of U2 is +7.5 Vdc. This gives an overall loss in
U2 of -2 dB. Under maximum signal input conditions, the AGC voltage is +4 Vdc, and
the U6 output is +1.5 Vdc. This gives an overall loss in U2 of -32 dB.
RF
MIXER,
U5
FL1
AMP
Ul
ATTN
U2
•f
<>
AMP
U3
IF
IN
AGC
\
OUT
1
i
AGC AMP
U6
IN
1ST LO
IN
LO AMP
U3
Figure 3-14. 1st Converter, FE-A1A2A3, Block Diagram
3-29