RX3361
Functional Description
The RX3361 IF receiver IC incorporates a mixer, crystal-based local oscillator, IF ampli-
fier, quadrature FM demodulator, audio filter, and a squelch circuit and is capable of
demodulating FM input signals.
Mixer
The mixer provides frequency downconversion from the 1st IF input on pin 16 down to
the 455 KHz 2nd IF output on pin 3. The external 2nd IF filter should be chosen so that
the specified output impedance on the mixer output port is an acceptable termination for
the filter.
Crystal Oscillator
A single dcdc coupled transistor is available as a two port gain element (pins 1 & 2) to
implement a crystal oscillator. The device is biased on-chip to a constant dc current.
IF Amplifier
The IF Limiter amplifies and limits the pin 5 input from the external 2nd IF filter. The
output is a square wave that drives the following demodulator stage. Two off-chip decou-
pling capacitors to Vcc are required on pins 6 & 7 to remove dc content from the limiter
input.
FM Demodulator
The demodulator provides an audio output generated by multiplying the input IF from the
limiter with a quadrature version of the input. The quadrature version is generated by the
combination of an on-chip 10pF series capacitor with an off-chip shunt LCR resonator
connected between pin 8 and Vcc. Biasing for the input transistor on pin 8 comes from the
off-chip resonator. The shape of the demodulator S-curve is determined by the shunt
resistance on this LCR resonator. An off-chip RC low pass filter should be used on pin 9
for removal of the 2nd IF components.
Filter Amplifier
The filter amplifier provides an inverting gain element between pins 10 & 11 for the
implementation of an audio filter. Typically, a bandpass Delyiannis-Friend active filter is
built by adding externally at least two resistors and two capacitors to this amplifier. An
external dc path must be provided through the feedback network across these pins to set
the dc operating point. The design equations are as follows:
R2
R1 ⋅ R2
R1 – R2
1
Q
⎛
⎞
⎠
--------
2Ao
------------- ----------------
--------------------
R1
=
R2
=
⋅
R3
=
(2Q)2
⎝
π(fo ⋅ C)
where:
fo is the desired center frequency, Q is the quality factor, Ao is the voltage gain at band cen-
Page 5
July 2005
For:Natertech
