The superregenerative RF receiver modules used in most of the wireless projects have both a digital and linear (or analog) output. The linear/analog output can be used (along with a microcontroller) as an RSSI (Received Signal Strength Indicator) to tune the receiver module to the desired frequency. The modules themselves have a tuning slug and are capable of tuning over a fairly broad frequency range. Each of the preprogrammed PICs can be configured to output an RSSI value as the last byte(s) of the serial output stream.

Start with a transmitter located near the receiver, transmitting continuously, using either no receiving antenna or a simple 1/4 wave wire. Tune the receiver for a maximum RSSI value. Move the transmitter to a point 25-30 feet distant and repeat. The second tuning should be with the normal receiving antenna. The first tuning will get you close and the second will fine tune it.

The 'scope screenshot below shows why a microcontroller is needed. The top trace is the linear/analog pin while the lower trace is the digital pin. There's no way to distinguish between the noise on the left of the screen from the signal on the right without the microcontroller watching for the lead-in pulse that delineates the beginning of a signal. A voltmeter on the linear/analog pin will read a nearly identical average level for both noise and signal. Because the module has AGC (the effects of which can be seen in the slope of the start pulse), the absolute level is not a reliable indicator of signal strength. The microcontroller lets us time ADC readings for a pulse and the following space with the difference being an excellent indicator of signal strength.

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