Philips Semiconductors

Product specification

SA612A

Double-balanced mixer and oscillator

1997 Nov 07

5

Besides excellent low power performance well into VHF, the

SA612A is designed to be flexible. The input, output, and oscillator

ports can support a variety of configurations provided the designer

understands certain constraints, which will be explained here.

The RF inputs (Pins 1 and 2) are biased internally. They are

symmetrical. The equivalent AC input impedance is approximately

1.5k || 3pF through 50MHz. Pins 1 and 2 can be used

interchangeably, but they should not be DC biased externally. Figure

5 shows three typical input configurations.

The mixer outputs (Pins 4 and 5) are also internally biased. Each

output is connected to the internal positive supply by a 1.5k

Ω

resistor. This permits direct output termination yet allows for

balanced output as well. Figure 6 shows three single-ended output

configurations and a balanced output.

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 L.O. is beyond

oscillation limits, or the system calls for an external L.O., the

external signal can be injected at Pin 6 through a DC blocking

maximum.

Figure 7 shows several proven oscillator circuits. Figure 7a is

appropriate for cordless phones/cellular radio. In this circuit a third

overtone parallel-mode crystal with approximately 5pF load

capacitance should be specified. Capacitor C3 and inductor L1 act

as a fundamental trap. In fundamental mode oscillation the trap is

omitted.

Figure 8 shows a Colpitts varacter 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 circuits 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

assume correct system operation.

INPUT

612A

12

612A

12

612A

12

a. Single-Ended Tuned Input

b. Balanced Input (For Attenuation

of Second-Order Products)

c. Single-Ended Untuned Input

SR00103

Figure 5. Input Configuration