Design RF Remote Control Transmitter To Highest Quality

For typical keyfob driven by SAW resonator, the high quality usually means optimised for best frequency stability, fast startup time and ghost-less working, which will greatly affect actual working distance.

The first point, best frequency stability, is achieved when local LC network’s actual working frequency is a little more than desired frequency, and drifts SAW resonator higher a bit at around 10kHz, in this way, SAW resonator achieves it’s best frequency stability, also we can benefit a tighter frequency drift when temperature changes, since SAW resonator’s working frequency tends to drift down a little when temperature is too low or too high.

Fast startup time is achieved, when tuned LC network’s frequency is close to SAW resonator’s frequency, also we can add a small resistor between RF transistor’s base & GND, in this way, we have the following benefits.

A lower free wheeling start up oscillator frequency fits better to the final free wheeling oscillator frequency in steady state mode. If ghosting still appears, ghosting rests not so long and at a lower power level.

Wideband noise power generated by the free wheeling start up oscillator frequency provides more energy at the SAWR resonance frequency because both frequencies are closer now. More energy at SAWR resonance frequency during start up phase speeds the generation of the standing wave in the SAWR cavity. The SAWR becomes the master faster in the oscillator loop.

To further avoid ghosting effect, we also needs to tune components according to following manner.

Tighten the LC tank by increasing CC-E and reducing LCollector

Increase CE-GND, but this has an effect on output power especially for the emitter OUT version, so the CE-GND value can’t be too high.

Increase CE-GND / CC-E relation, this value usually should be between 3 to 5.

For more information regarding RF remote control tuning, please refer to EPCOS website, which can be found here