mixer noise and frequency band
mixers have three
frequency bands where noise is important:
1. Noise already present at the IF: The transistors and resistors in the circuit
will generate noise at the IF. Some of this noise will make it to the
output and corrupt the signal. For example, the collector resistors will
add noise directly at the output IF.
2. Noise at the RF and image frequency: Any noise present at the RF and
image frequency will also be mixed down to the IF. For instance, the
collector shot noise of Q1 at the RF and at the image frequency will
both appear at the IF at the output.
3. Noise at multiples of the LO frequencies: Any noise that is near a multiple
of the LO frequency can also be mixed down to the IF, just like the
noise at the RF.