Ramsey Electronics FX-440 Manual page 82

,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 MC3335 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 MC3335 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 MC3335 IC for the FX-series because of its
features especially suited for state-of-the-art FM voice and digital
communications requirements.
The MC3335 is a complete FM narrowband receiver from antenna input
(pins 1,20) to audio preamp output (pin 12). 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-440 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 18 and 19 to establish the basic
operating frequency. However, our design injects the output of the
transceiver's PLL-controlled VCO through T1 to pins 18 and 19. The
differential drive achieved by T1, similar to a "push-pull" circuit, gives
superior performance. The VCO output to pins 18 and 19 are mixed with
the RF input from the antenna circuitry to pins 1 and 20.
The first mixer amplifies the signal and converts this RF input to 21.4
MHz. For increased gain, this IF signal is amplified by Q1 before going to
the second internal mixer via pin 16, 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 4) requires filtering. A 6 pole ceramic
IF filter block is used for this purpose. Virtually all FM radio receivers now
make use of such filters due to their small size and that they never
require tuning.
Let's summarize the basic "double-conversion• principle for receivers,
using the national 440 MHz band simplex channel, 446. 00 MHz as our
working example: ,
1. An antenna could be connected directly to pin 20 of U1, and our
receiver-on-a-chip would indeed WORK The antenna would bring in our
Assm. 22 FX-440