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SA612A

Double-balanced mixer and oscillator

NXP Semiconductors. 

NXP Semiconductors

SA612A

Double-balanced mixer and oscillator

The oscillator can sustain oscillation beyond 200 MHz in crystal or tuned tank

configurations. The upper limit of operation is determined by tank ‘Q’ and required drive

levels. The higher the ‘Q’ of the tank or the smaller the required drive, the higher the

permissible oscillation frequency. If the required LO is beyond oscillation limits, or the

system calls for an external LO, the external signal can be injected at OSC_B (pin 6)

through a DC blocking capacitor. External LO should be 200 mV (peak-to-peak) minimum

up to 300 mV (peak-to-peak) maximum.

Figure 6 shows several proven oscillator circuits. Figure 6a is appropriate for cordless

phones or cellular radio. As shown, an overtone mode of operation is utilized. Capacitor

C3 and inductor L1 act as a fundamental trap. In fundamental mode oscillation, the trap is

omitted.

C2

L1 C3

XTAL

C1

SA612A

SA612A

SA612A

aaa-013381

aaa-013382

aaa-013383

a. Colpitts crystal

oscillator

(overtone mode)

Fig 6. Oscillator circuits

b. Colpitts L/C tank

oscillator

c. Hartley L/C tank

oscillator

Figure 7 shows a Colpitts varactor tuned tank oscillator suitable for synthesizer-controlled

applications. It is important to buffer the output of this circuit to assure that switching

spikes from the first counter or prescaler do not end up in the oscillator spectrum. The

dual-gate MOSFET provides optimum isolation with low current. The FET offers good

isolation, simplicity, and low current, while the bipolar transistors provide the simple

solution for non-critical applications. The resistive divider in the emitter-follower circuit

should be chosen to provide the minimum input signal that assures correct system

operation.

SA612A

Product data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 3 — 4 June 2014

© NXP Semiconductors N.V. 2014. All rights reserved.

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