Page 46 of 101
DWG ID: 181-0101-00A
Date: 2003-09-26
narrow enough to filter out the spurious responses of the first frequency converter, while
wide enough to support a performance bandwidth of 26 MHz.
12.1.1.2
1
st
Frequency Converter, 1
st
IF Filters, and 1
st
IF Amplifier
IC101 is an active double-balanced frequency converter, which converts the incoming RF
signal to the first intermediate frequency (IF) of 43.65 MHz. This frequency converter
has a gain of 0 dB and a noise figure of 10 dB. Its differential output is matched to the
first IF filter, YF101, by L111, L112, C130, and C140. An IF amplifier based around
Q102 is used to provide gain. Its output drives another IF filter section, YF102, which is
identical to YF101. These two filters serve the double function of filtering out the
spurious responses of the second frequency converter and, with the second IF filter, of
removing signals at the adjacent and further removed channels.
12.1.1.3
2
nd
IF IC
The output of YF102 drives the mixer internal to IC102. IC102 is a FM IF IC which
contains a frequency converter, high gain limiting IF amplifier, FM discriminator
(detector) and other support circuitry. The frequency converter in IC102 converts the RF
signal at the first IF to the second IF of 450 kHz. The output of the frequency converter
exits the IC and is filtered by the second IF filter, YF103. The output of the filter reenters
the IC and drives the high gain, limiting amplifier. Because the discriminator inside
IC102 is sensitive to amplitude and frequency modulation components, a limiter must
precede it to remove any amplitude modulation. The output of the limiter amplifier
drives the discriminator. The resonator for the discriminator is YF104.
12.1.1.4
Receiver Audio and Carrier Detection
The recovered audio from IC102-9 is filtered and dc shifted by IC103A and associated
components.
Carrier detection is based upon the absolute RF signal level at IC102’s input. IC102 has
circuitry that develops a dc current, which is proportional to the input RF signal level.
Passing this current through a resistor (R123) creates a voltage, which varies from about
0.5 V at no signal input to about 3 V with –70 dBm at the antenna connector.
12.1.1.5
2
nd
Local Oscillator
To convert signals at the first IF frequency of 43.65 MHz to that of the second IF at a
frequency of 450 kHz, a local oscillator signal at a frequency of 43.2 MHz (43.65 MHz –
0.45 MHz) is used. Tripling the output of the radio’s 14.4-MHz master reference
oscillator, Y101, creates this signal. Transistor Q111 acts as a frequency tripler. Its
associated components are used to bias the transistor at a harmonic rich bias point and to
filter the output such that only the third harmonic remains for use as the 2
nd
local
oscillator.