Ramsey Electronics FZ-146 Specifications download pdf (Page 21)

FX-146 • 21

Stage CR: Integrated FM Receiver

The MC13135 is a complete FM narrowband receiver from antenna input

(pin 22) to audio 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.

Our design injects the PLL controlled VCO output through C35 to pin 1

rather than using U1's internal local oscillator circuit. The VCO input to pin 1

is mixed with the RF input from the antenna circuitry.

The first mixer amplifies the signal and converts this RF input to 21.4 MHz.

This IF signal is applied 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. We used a precision

ceramic 455 KHz filter with 6 poles for a 2nd IF filtering scheme designed to

solve the adjacent- frequency swamping effect experienced with many

handhelds costing much more.

The receiver has good "hysteresis" characteristics, the ability to hold the

squelch open once it has been broken by a marginal signal, even if the

signal becomes weaker. The squelch is activated by signal strength, not by

noise.

R13 permits squelch adjustment. Finally, the carrier detect circuitry affords

the same COR ("Carrier Operated Relay") action as needed in any repeater,

which is why the output of pin 16 is also available on the PC board,

designated "COR."

Stage DR:

Antenna Input and RF Preamplifier: At Antenna jack J3, C71, L12 and C72

form a LOW pass filter, The filtered signals are coupled through C47 to be

amplified by Q3, NE021, favored for its high gain and low noise (15 db gain,

1 db noise).

Front-end components C30, L5, C28, L2, C31, L6 form a BANDPASS filter,

which sets both upper and lower limits on the RF passing from Q3 to Q2 for

further amplification and coupling via C17 to U1, pin 22.

The PIN diodes, D2, D6 and D7, perform all RF T-R functions. PIN diodes

can pass RF energy either way when turned on by DC voltage and also

block RF from the other direction when not powered by DC. During Receive,

D6 is "on" and permits RF to flow from the antenna through C47 to the

amplifier stage just discussed. Because any DC device needs a ground

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Ramsey Electronics FZ-146 Specifications Page 21