Ramsey Electronics FZ-146 Specifications download pdf (Page 81)

FX-146 • 81

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 FX146

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.T

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.

The second internal mixer is 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 2 Meter band simplex channel, 146.52 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 146.52 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

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Ramsey Electronics FZ-146 Specifications Page 81