RFRXD0420 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

RFRXD0420

UHF ASK/FSK/FM Receiver

Microchip Technology 

rfRXD0420/0920

employed in this design because it can handle a larger

dynamic range while consuming less power with simple

circuitry than AGC circuitry.

The internal resistance of the 2IFIN pin is approximately

2.2 kΩ. In order to terminate ceramic IF filters whose

output impedance is 330 Ω, a 390 Ω resistor can be

paralleled to the 2IFIN and FBC2 pins.

FBC1 (Pin 12) and FBC2 (Pin 13) are connected to

external feedback capacitors.

The IF Limiting Amplifier pins are illustrated in

Figures 2-7 and 2-8.

FIGURE 2-7: BLOCK DIAGRAM OF IF

LIMITING AMPLIFIER INPUT

PINS

VDD

2IFIN

11

VSS

VDD

FBC2

13

VSS

2.2 kΩ

2.2 kΩ

200 µA

Vss

VDD

FBC1

12

VSS

FIGURE 2-8: BLOCK DIAGRAM OF IF

LIMITING AMPLIFIER OUTPUT

PIN

VDD

VDD

2IFOUT

15

VSS

40 µA

VSS

2.5.2

RECEIVED SIGNAL STRENGTH

INDICATOR (RSSI)

The RSSI signal is proportional to the log of the signal

at 2IFIN. The 2IFIN input RSSI range is approximately

40 µV to 160 mV. The slope of the RSSI output is

approximately 26 mV/dB of RF signal.

The RSSI output has an internal 36 kΩ resister to Vss

fed by a current source. This resistor converts the

RSSI current to voltage.

For Amplitude Shift Keying (ASK) demodulation, RSSI

is compared to a reference voltage (static or dynamic).

Post detector filtering is easily implemented by

connecting a capacitor to ground from the RSSI pin

effectively creating an RC filter with the internal 36 kΩ

resistor.

For FSK and FM demodulation, the RSSI represents

the received signal strength of the incoming RF signal.

The RSSI pin is illustrated in Figure 2-9.

FIGURE 2-9: BLOCK DIAGRAM OF RSSI

PIN

VDD

RSSI

21

VSS

50 Ω

I (Pi)

36 kΩ

VSS

2.6 Demodulator

The demodulator (DEMOD) section consists of a phase

detector (MIXER2) and amplifier creating a quadrature

detector (also known as a phase coincidence detector)

to demodulate the IF signal in FSK and FM modulation

applications. The quadrature detector provides all the

IF functions required for FSK and FM demodulation

with only a few external parts.

The in-phase signal comes directly from the output of

the IF limiting amplifier to MIXER2. The quadrature

signal is created by an external tuned circuit from the

output of the IF limiting amplifier (2IFOUT, Pin 15) AC-

coupled to the MIXER2 DEMIN (Pin 16) input. The input

impedance of the DEMIN pin is approximately 47 kΩ.

The external tuned circuit can be constructed from sim-

ple inductor-capacitor (LC) components but will require

one of the elements to be tunable. A no-tune solution

can be constructed with a ceramic discriminator.

The output voltage of the DEMOD amplifier (DEMout+

and DEMout-, Pins 23 and 24) depends on the peak

deviation of the FSK or FM signal and the Q of the

external tuned circuit. DEMout+ and DEMout- are high

impedance outputs with only a 20 µA current capability.

The Demodulator pins are illustrated in Figures 2-10

and 2-11.

FIGURE 2-10: BLOCK DIAGRAM OF

DEMODULATOR INPUT PIN

VDD

DEM IN

16

VDD

VDD

47 kΩ

VSS

 2003 Microchip Technology Inc.

Preliminary

DS70090A-page 7

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