![background image](http://html2.mh-extra.com/html/kantronics/talon-udc-series/talon-udc-series_user-manual_3412850049.webp)
Page 49 of 101
DWG ID: 181-0101-00A
Date: 2003-09-26
are narrow enough to filter out the spurious responses of the first frequency converter,
while wide enough to support a performance bandwidth of 20 MHz.
12.2.1.2
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 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
L107, L108, C128, and C137. 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.
12.2.1.3
2
nd
IF IC
The output of YF102 drives the frequency converter 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. In addition,
the noise-based carrier detection system available with this product requires that the RF
signal at the discriminator stay constant in amplitude as the RF input signal level varies.
The output of the limiter amplifier drives the discriminator. The resonator for the
discriminator is YF104.
12.2.1.4
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 (R115) 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.
12.2.1.5
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 Q112 acts as a frequency tripler. Its
associated components are used to bias the transistor at a harmonic rich bias point and to