Operating instructions

RF Technology R70/R71 and R150 Page 11
4 FREQUENCY PROGRAMMING
4 Frequency Programming
Channel and tone frequency programming is most easily acomplished with RF
Technology TecHelp/Service Monitor 2000 software. This software can be run on
an IBMcompatible/Windows PC and provides a number of additional useful
facillities.
TecHelp/Service Monitor 2000 allows setting of the adaptive noise squelch
threshold, provides a simple means of calibrating the signal strength output and
minimum signal alarm.
TecHelp/Service Monitor 2000 can be supplied by your dealer, distributor or by
contacting RF Technology direct.
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 pole voltage tuned filter (D13, D14, L35-39) is used to limit the RF band
width prior to the RF amplifier transistor Q1. The tuning voltage is supplied by the
frequency synthesizer through voltage follower U28D. The circuit values are chosen
so that the centre frequency tracks the VCO frequency.
RF amplifier transistor Q1 is followed by a second two pole voltage tuned filter
(D15, D16, L28-32) which provides additional image and spurious frequency
rejection. The filter output is connected to the RF input port of the mixer MX1.
MX1 is a high level double balanced diode ring mixer with excellent inter-
modulation performance. It has a conversion loss of approximately 6 dB. The gain
between the receiver input and the mixer input is approximately 10 dB so that the
total gain between the antenna input and the IF input 3-4dB.
Monolithic amplifiers MA1, MA2 and transistor Q5 amplify the VCO output to the
necessary LO level for MX1 approximately +13dBm.
The network C8, C9 L1-3 and R6 passes the IF frequency of 45 MHz and terminates
the RF and LO components.
5.2 IF Section
The first IF amplifier uses two parallel connected JFET transistors Q2 and Q3 to
obtain 8-10 dB gain. The two transistors provide improved dynamic range and input
matching over a single transistor.