15 years ago
remote control radio transmitters and have found that there is an
enormous radiated emission from the AVR at the second harmonic of the
16 MHz crystal. According to my spectrum analyzer the 32 MHz emission
is 40 dB at 25 cm, while the output of the transmiter at 1 m with the
antenna down is only 45 dB.
This noise is reducing the effective range of the remote control from
nearly 1 km to less than 100 m before the receiver can no longer hear
the transmitter. I've verified that the AVR alone is causing the
problem; on a breadboard with just the microcontroller, decoupling caps
and crystal the noise levels are identical. There is no radiated noise
at 72 MHz, so it works fine with the US transmitters.
I've tried a variety of replacement chips, chokes, caps and shields to
no avail. I even tried a Mega128 in TQFP packaging and found a
similar amount of noise. Does anyone have any further ideas?