Operating instructions

Page 10 RF Technology R350/R500
5 CIRCUIT DESCRIPTION 5.1 RF Section
5 Circuit Description
The following descriptions should be read as an aid to understanding the block and
schematic diagrams at the rear of this manual.
5.1 RF Section
A two section helical filter FL1 is used to limit the RF band width prior to the RF
amplifier transistor Q1. The output impedance of FL1 is matched to the input of Q1 by
C165, C1 and a microstrip line on the printed circuit board. Q1 is a very low noise
device with good intermodulation performance.
A four section filter consisting of FL2 and FL3 is used between Q1 and the mixer
MX1. This filter provides additional image and spurious frequency rejection.
The gain between the receiver input and the mixer input is approximately 10dB.
Monolithic amplifiers MA1, MA2 and transistor Q5 amplify the VCO output to the
necessary LO level for MX1.
MX1 is a high dynamic range double balanced mixer. The LO input level to MX1 is
approximately +13dBm the mixer conversion gain is approximately -7dB.
The network C7, C9, L1, L3 and R7 passes the IF frequency of 45 MHz and
terminates the RF and LO components.
The total RF section gain from J1 to the IF input is approximately 3dB.
5.2 IF Section
The first IF amplifier uses two parallel connected JFET transistors Q2 and Q3 to
obtain 12-15dB gain. The two transistors provide improved dynamic range and input
matching over a single transistor.
A two pole 45 MHz crystal filter XF1 is used between the first and second IF
amplifiers. The second IF amplifier Q4 provides additional gain of 6-10dB. A two
pole crystal filter is used between Q4 and the 2nd oscillator mixer. These two crystal
filters provide some adjacent channel rejection and all of the second IF image
frequency rejection.
U1 is a monolithic oscillator and mixer IC It converts the 45 MHz IF signal down to
455 kHz. The second oscillator frequency or 45.455 MHz is controlled by crystal Y1.
The 455 kHz output of the second mixer is fed through a ceramic filter CF1 to the
limiter and discriminator IC U3. CF1 provides additional adjacent channel selectivity
for 25 kHz versions and all of the adjacent channel selectivity for 12.5 kHz versions.