Stage CR: Integrated FM RECEIVER (with Squelch Control)
This step is named "CR" so that the assembly step numbers do not resemble
designator numbers for capacitors.
To fully appreciate the marvel of U1, the MC13135 FM Receiver IC, one would
need to study the schematic diagram of any FM receiver more than 15-20
years old, including fully "solid-state" models. The MC13135 IC is truly a
"Receiver-on-a-Chip." The cluster of parts to assemble and understand
around U1 is minimal in comparison to what was previously required for a
quality FM receiver circuit
For years, Ramsey Electronics has employed a similar IC, the MC3359 as the
heart of our popular FM receivers for the 10, 6, 2 and 1.25 Meter bands. We
adopted the MC13135 IC for the FX-series because of its features especially
suited for state-of-the-art FM voice and digital communications requirements.
The MC13135 is a complete FM narrowband receiver from antenna input (pin
22) to audio preamp output (pin 17). The low voltage dual conversion design
results in low power drain, excellent sensitivity and good image rejection in
narrowband voice and data link applications. The FX-223 implementation of
this IC yields increased image rejection by using a 21.4 MHz first IF rather
than the traditional 10.7 MHz. A precision 2-pole crystal filter (FL1) is used for
the 21.4 MHz first IF.
The receiver IC is so complete that it includes an internal local oscillator
requiring only a crystal across pins 5 and 6 to establish the basic operating
frequency. Our design injects the output of the transceiver's PLL-controlled
VCO through C35.
As an option, a helical resonator module can be installed in the front end RF
amplifier area for excellent receiver performance in high RF environments.
The first mixer amplifies the signal and converts this RF input to 21.4 MHz.
This IF signal is sent to the second internal mixer via pin 18, where the 2nd IF
frequency of 455 KHz is achieved by mixing with the 21.855 MHz oscillator.
The oscillator circuit is internal to U1; the crystal is Y1, 21.855 MHz.
The 455 KHz second IF output (pin 7) requires filtering to provide good
adjacent channel rejection. A high performance 6 pole ceramic filter is used.
Filters such as this are used in virtually all FM radio transceivers. Let's
summarize the basic "double-conversion" principle for receivers, using the
national 220 MHz band simplex channel, 223.50 MHz as our working
example:
1. An antenna could be connected directly to pin 22 of U1, and our receiver-
on-a-chip would indeed work. The antenna would bring in our theoretical test
signal of 223.50 MHz as well as every other signal in the radio spectrum.
Since that's a bit much to expect the receiver to handle, we will build up that
network of filters, tuned circuits and RF preamplifiers that constitutes Stage
"DR" of this project. The purpose of the "DR" circuitry is to filter out or at least
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