Philips Semiconductors

Product specification

SA602A

Double-balanced mixer and oscillator

1997 Nov 07

4

The oscillator is capable of sustaining oscillation beyond 200MHz 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 Pin 6 through a DC blocking capacitor.

Figure 7 shows several proven oscillator circuits. Figure 7a 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 8 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 which will

assure correct system operation.

When operated above 100MHz, the oscillator may not start if the Q

of the tank is too low. A 22k

Ω resistor from Pin 7 to ground will

increase 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 22k

Ω resistor will not upset the

other DC biasing internal to the device, but smaller resistance

values should be avoided.

87

65

4

3

2

1

OUTPUT

150pF

330pF

120pF

1.5 to

100nF

220pF

INPUT

47pF

22pF

1nF

100nF

10nF

VCC

6.8

µF

5.5

µH

0.209 to 0.283

µH

44.2

µH

10pF

34.545MHz THIRD OVERTONE CRYSTAL

602A

0.5 to 1.3

µH

SR00070

Figure 3. Test Configuration