User's Manual

8 THEORY OF OPERATION
8.2 DTX-165 RF BOARD
8.2.1 RECEIVER
RF amplifier and Bandpass Filters
The incoming RF signal from the antenna connector passes backwards through the transmitter harmonic
filter and to a diode switch. The diode switch, CR101, CR103, and CR201 route the RF signal to a
lumped-element bandpass filter and then an amplifier Q101. The amplifier is followed by another lumped-
element filter. The lumped element filters are designed to pass all signals within the 215-245 MHz band
without much attenuation, but to attenuate out-of-band signals.
1
st
Mixer, 1
st
IF filters, and 1
st
IF Amplifier
The output of the front-end stage is passed to the first mixer U101. This device converts the RF signal to
the first IF frequency of 43.65 MHz. L107, R108, and C132 match the mixer output to the 1
st
IF filter
Y101.
The first IF filter, Y101, is at the first IF frequency of 43.65 MHz and has a passband of +/-6 kHz. This is
followed by an identical filter, Y102. The output of the 1
st
IF filters is amplified by Q105 and then routed
to the IF IC U102.
2
nd
IF IC
U102 is an integrated FM IF IC which contains a mixer, high gain limiting IF amplifier, FM discriminator
(detector), and other support circuitry. The mixer in U102 converts the RF signal at the first IF to the
second IF of 450 kHz. The output of the mixer exits the IC and is filtered by the second IF filter, FL101.
The output of the filter re-enters the IC and drives the high gain limiting amplifier. Because the
discriminator inside U102 is sensitive to both 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 Y103.
2
nd
Local Oscillator
The two mixers in this radio act to produce an output signal whose frequency is equal to the difference
between the frequency present at the RF input port and the frequency at the local oscillator port. 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 – 0.45) is used. This signal is created by tripling
the output of the radio’s 14.4 MHz master reference oscillator, Y401. Transistor Q106 acts as a frequency
tripler. Its associated components are used to bias the transistor at an harmonic rich bias point and to filter
the output such that only the third harmonic remains for use as the 2
nd
local oscillator.
VCO and Synthesizer
The frequency synthesizer is responsible for generating the carrier in transmit and the first local oscillator
in receive. A voltage-controlled oscillator (VCO) is an oscillator whose frequency can be controlled by an
external signal. The receiver synthesizer, almost wholly contained within U401, divides the VCO
frequency by digital dividers and compares the result with an accurate reference. An error signal,
proportional to the frequency error is created which is routed to the frequency control input of the VCO.
This action locks the VCO to a frequency which is equal to the reference frequency multiplied by the
divider number. To set the VCO frequency, different divider numbers can be programmed into the
synthesizer. In most synthesizer designs, the divider must be an integer, which forces the reference
frequency to be equal to the synthesizer step size. This synthesizer, however, uses a fractional-N technique
to generate smaller step sizes. The reference frequency is derived by digitally dividing the frequency of the
14.4 MHz master oscillator. When locked, the VCO attains the same relative frequency stability as that of
the master oscillator.
The output of the VCO is amplified to a level of about 0 dBm by Q401 and Q402. Q603 with R602 and
C604 act as a very low noise power supply filter for the VCO.