Information reprinted from Signetics Data Sheet
NE/SA602 Double-Balanced Mixer and Oscillator
"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 capacitor. External L.O. should be at least 200mVp.p."
"When operated above 100MHZ, the oscillator may not start if the Q of the tank is too low. A 22k 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. 22k will not upset the other DC biasing internal to the device, but smaller resistance values should be avoided."
Signetics Technical Bulletin AN1982
"...the Q of the tank will affect the upper frequency limits of oscillation: the higher the Q the higher the frequency. The NE602 is fabricated with a 6GHZ process, but the emitter resistor from Pin 7 to ground is nominally 20k. With 0.25mA typical bias current, 200MHZ oscillation can be achieved with high Q and appropriate feedback.
The feedback, of course, depends on the Q of the tank, it is generally accepted that a minimum amount of feedback should be used, so even if the choice is entirely empirical, a good trade-off between starting characteristics, distortion, and frequency stability can be quickly determined.
LC Tank Circuits
LC tanks present a little greater challenge for the designer. If the Q is too low, the oscillator won't start. A trick which will help if all else fails is to shunt Pin 7 to ground with a 22k resistor. In actual applications this has been effective to 200MHZ with high Q ceramic capacitors and a tank inductor of 0.08uH and a Q of 90. Smaller resistor value will upset DC bias because of inadequate base bias at the input of the oscillator. An external bias resistor could be added from Vcc to Pin 6, but this will introduce power supply noise to the frequency spectrum.
The Harley configuration offers simplicity. With a variable capacitor tuning the tank, the Hartley will tune a very large range since all of the capacitance is variable. Please note that the inductor must be coupled to Pin 7 with a low impedance capacitor. The Colpitts oscillator will exhibit a smaller tuning range since the fixed feedback capacitors limit variable capacitance range; however, the Colpitts has good frequency stability with proper components.
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