SA602A_15 Datasheet, PDF (6/19 Pages) NXP Semiconductors – Double-balanced mixer and oscillator

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.
C2
L1 _C3
XTAL
C1
SA602A
SA602A
SA602A
aaa-013237
aaa-013238
aaa-013239
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.
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.
SA602A
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 3 — 27 May 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
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