SA602A

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

Product data sheet

Rev. 3 — 27 May 2014

6 of 19

NXP Semiconductors

SA602A

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 at least 200 mV (peak-to-peak).

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

radio. As shown, an overtone mode of operation is utilized. Capacitor C3 and inductor L1

suppress oscillation at the crystal fundamental frequency. In the fundamental mode, the

suppression network is omitted.

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.

When operated above 100 MHz, the oscillator may not start if the Q of the tank is too low.

A 22 k

 resistor from OSC_E (pin 7) to ground increases the DC bias current of the

oscillator transistor. This improves the AC operating characteristic of the transistor and

should help the oscillator to start. A 22 k

 resistor does not upset the other DC biasing

internal to the device, but smaller resistance values should be avoided.

a. Colpitts crystal

oscillator

(overtone mode)

b. Colpitts L/C tank

oscillator

c. Hartley L/C tank

oscillator

Fig 6.

Oscillator circuits

SA602A

aaa-013237

C1

C3

C2

XTAL

L1

SA602A

aaa-013238

SA602A

aaa-013239