In Phil's radio thread I mentioned wanting to use a Propeller to control the oscillator for a ne602 based shortwave radio. The goal is to improve the stability over the colpitts oscillator which drifts. Ideally I could create a DDS VFO and have a full coverage receiver, but I'll settle for a simple feedback circuit to improve stability in the short term.
Attached is an image of the oscillator portion of the schematic. This should be familiar to anyone who's seen ne602 based radios, as they all use roughly this design. I attached leads to +9 volts, ground, ne602 pin 7, and the top of the diode D1. My plan is to first measure the existing oscillator frequency on pin 7, then worry about controlling it via PWM and a transistor.
My first concern is the voltage at pin 7 as I don't want to fry the Propeller. The ne602 is powered by 5 volts, but up to 9 volts is reverse biased across the diode. So I don't know what voltage is present at pin 7. I used a DMM on AC and DC voltage, and it seems close to zero with either measure.
Does this mean I can connect it directly to a Propeller pin, or should I use a resistor just in case?