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CXA3125N

Description of Operation

(See the Electrical Characteristics Measurement Circuit.)

VHF oscillator circuit

This circuit is a differential amplifier type oscillator circuit. Pin 11 is the output and Pin 9 is the input.

Oscillation is performed by connecting an LC resonance circuit including a varicap to Pin 11 via coupled

capacitance, inputting to Pin 9 with feedback capacitance, and applying positive feedback.

Note that if the capacitance across Pins 9 and 11 is too large, positive feedback may be applied via a

parasitic capacitance causing undesired stray oscillation.

VHF mixer circuit

The mixer circuit employs a double symmetrical mixer with little local oscillation signal leakage. The RF

signal is input to Pins 6 and 7. During normal use, the RF signal is input to one pin while the other pin is

connected to GND. The RF signal is converted to IF frequency by the signal supplied from the oscillator and

then output to Pins 2 and 3. Pins 2 and 3 are open collectors, so power must be supplied externally.

this time must be DC 4.0 V or more.

UHF oscillator circuit

This oscillator circuit is designed so that two collector ground type Colpitts oscillators perform differential

oscillation operation via an LC resonance circuit including a varicap. Feedback capacitance is built into IC,

and an LC resonance circuit including a varicap is connected between Pins 12 and 14 via coupled

capacitance.

UHF mixer circuit

This circuit employs a double symmetrical mixer like the VHF mixer circuit. The RF signal is input to Pins 7

and 8. There is a symmetrical input at the differential from both edges of the secondary coil of the pre-stage

double-tuned circuits, or an asymmetrical input to Pin 7 with a capacitor connected at Pin 8 to GND.

Otherwise, the conditions and usage are the same as those for the VHF mixer circuit.

IF amplifier circuit

The signals frequency converted by the mixer are output from Pins 2 and 3, and at the same time are AC

coupled inside the IC and input to the IF amplifier. Single-tuned filters are connected to Pins 2 and 3 in order

to improve the interference characteristics of the IF amplifier.

The signal amplified by the IF amplifier is output with symmetrical or asymmetrical output format. Selecting

symmetrical or asymmetrical is performed at Pin 10. Asymmetrical output when Pin 10 is for open state;

symmetrical output when connected to GND. During symmetrical output, SAW filter direct connection is

possible and during asymmetrical output, output stage drive capability is increased to drive 75

Ω load.

During asymmetrical output, output is performed from Pin 16, and during symmetrical output, output is

performed from Pins 15 and 16. The output impedance is approximately 35

Ω for symmetrical output; 30 Ω

for asymmetrical output. When asymmetrical output is selected, connect Pin 10 to GND through capacitor.

U/V switch circuit

UHF operation is chosen by applying voltage of 3 V or more to Pin 13 VHF operation for 0 V or open.