CXA3556N Datasheet, PDF (11/13 Pages) Sony Corporation – IF Down Converter for Digital Broadcast

CXA3556N
Description of Functions
The CXA3556N is an IF down converter IC for digital broadcasts. This IC contains a mixer circuit, a local
oscillator circuit and an AGC circuit that standardizes IF signals in response to levels of input signals.
It can also operate in a mode other than down converter (linear amplifier mode) and making possible use in a
wide variety of digital broadcast tuners. (Control using Pin 23 bias voltage.)
1. AGC Circuit
This is a variable gain amplifier circuit that standardizes output in response to levels of input signals. This
circuit has especially excellent low distortion and noise characteristics even when the input level is large. It
also has a variable width of 56dB as the gain control range with excellent linearity of the gain control curve.
2. Local Oscillator Circuit
This circuit has a structure that uses a crystal oscillator and oscillates at 3rd overtone frequency. The circuit
sets the parallel frequency of the parallel LC resonator between Pins 7 and 8 close to a tertiary overtone
frequency (FVCO). This oscillator circuit can also use an LC resonance circuit in place of the crystal oscillator
to form an oscillator circuit.
3. Output Amplifier Circuit
Adding a resistor between Pins 16 and 17 allows the gain to be changed in proportion to that resistance.
(Refer to characteristic graph Fig. 5 Gain vs. resistance between E1 and E2.)
The output stage of this amplifier is also designed to be 500Ω load/1Vp-p.
4. Mixer Circuit
This circuit outputs the frequency difference between the signal input to IF IN and the local oscillator signal.
5. RF AGC Circuit
This circuit has high image quality in both strong and weak electric fields. In particular, it controls the RF IC
gain existing in the first stage of the CXA3556N to delay the characteristics related to S/N to the maximum
limits. The circuit controls the RF IC gain using RF AGC voltage (Pin 3) in order that signals input to IF IN
(Pins 1, 2) grow larger in weak electric fields. The RF AGC voltage at Pin 3 is obtained from results of a
comparison between the AGC control voltage (Pin 24 input voltage) that detects output signals and the
voltage at Pin 18 that can be freely set using the resistance value. When the control voltage is low, the RF IC
gain decreases and when the control voltage is high, the RF IC gain increases.
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