Circuit Description; Rf Section - RF Technology Eclipse Series Operation And Maintenance Manual

5.1 RF Section

5 CIRCUIT DESCRIPTION

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TecHelp/ Service Monitor 2000 can be supplied by your dealer, distributor or by contacting
RF Technology direct.
5 Circuit Description
The following description 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 R.F. bandwidth prior to the R.F. amplifier
transistor Q1. The output impedance of FL1 is matched to the input of Q1 by C177, C178
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.
MX1 is a high level double balanced diode ring mixer with excellent intermodulation
performance. It has a conversion loss of approximately 7dB. The gain between the receiver
input and the mixer input is approximately 10dB so that the total gain between the antenna
input and the I.F. input is 3-4dB.
Monolithic amplifiers MA1, MA2 and transistor Q5 amplify the VCO output to the necessary
L.O. level for MX1 approximately +13dBm.
The network C8, C9, L1-3 and R6 passes the 45MHz I.F. frequency to the I.F. amplifier and
terminates the R.F. and L.O. frequency components.
5.2 I.F. Section
The first I.F. amplifier uses two parallel connected JFET transistors Q2 and Q3 to obtain 8-
10dB gain. The two transistors provide improved dynamic range and input matching over a
single transistor.
A two pole 45MHz crystal filter XF1 is used between the first and second I.F. amplifiers.
The second I.F. amplifier Q4 provides additional gain of 6-10dB. A two pole crystal filter is
nd
used between Q4 and the 2 oscillator mixer. These two crystal filters provide some adjacent
channel rejection and all of the second I.F. image frequency rejection.
U1 is a monolithic oscillator and mixer I.C. It converts the 45MHz I.F. signal down to
455KHz. The second oscillator frequency or 45.455MHz is controlled by crystal Y1. The
nd
455KHz output of the second mixer is fed through the ceramic filter CF1 to the 2 I.F.
amplifier transistor Q27. Q27 provides an additional 15dB gain ahead of the limiter and
discriminator I.C. U3.
CF1 provides additional adjacent channel selectivity for 25KHz versions and all of the
adjacent channe l selectivity for 12.5KHz versions. CF1 and termination resistors R15 and
R24 are the only component differences between the 12.5 and 25KHz versions.
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RF Technology R800 Page 9