SA602A oscillator problems

Discussion in 'The Projects Forum' started by vladm, Mar 16, 2013.

  1. vladm

    Thread Starter New Member

    Mar 16, 2013
    Hello all;

    I am building a direct-conversion receiver for 3-4 MHz, which uses the SA602A chip for frequency conversion down to audio.
    The SA602A is a double-balanced mixer IC with inbuilt oscillator circuitry.

    I followed advice from the datasheet:
    and this page:

    and designed a voltage-tuned oscillator (see attachments).

    To test the circuit, I connected pin 1 to ground via a 10k resistor and pin 2 to ground via a capacitor and connected pin 4 to an oscilloscope. The result was a sinewave of about 50mV p-p, variable between 2.8 and 3.8 MHz according to the tuning voltage (see attachments). Connecting the probe to pins 7 and 6 also had the same result, with a smaller amplitude.

    To preform the next test i set up the circuit as shown in the attached diagram and inputted a 1MHz signal from a signal generator (1MHz is the highest the sig gen could go, so with the oscillator set to 3MHz the output should show frequencies of 2 and 4 MHz).

    Now here is the problem: disconnecting the 10k resistor from pin 1 stops the oscillator. The output pin just outputs the input signal, no heterodyning products seen whatsoever on the 'scope FFT display. Connecting the probe to pins 6&7 now shows no signal.

    The circuit is currently built on breadboard (see attachments), I have been told that it's generally a bad idea to use breadboard for prototyping RF circuits. The mixer circuit in question on the image is center, bottom is an RF filter and far right is an AF amplifier.

    I know of issues concerning the poor dynamic range of the SA602A, so I had set the voltage of the sig gen to be well within the dynamic range of the chip.

    Any help or suggestions appreciated.