Q.
Why do you typically recommend using a 10uF series capacitor at the baseband output of an IQ demodulator? For Direct Conversion or Zero-IF architectures the link should be dc-coupled. So why use AC coupling here?
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A.
When designing Direct Conversion Receivers, designers face two key challenges:
1) Eliminate DC offsets that are created when the LO-RF leakage term mixes with the LO. This creates a DC term at the baseband outputs which plagues our desired signal that is centered around DC.
2) Match the output common mode voltages from the Demodulator to that of the Analog-to-Digital Converter. Having a series AC coupling capacitor easily addresses these issues.
Let’s take the first case stated above. The differential dc offsets on the demodulator outputs are in the order of a few millivolts. When we use an AC coupling capacitor, it places a high-pass corner frequency in the frequency response that is approximately:
F (high pass) = 1/( 2*Pi*R*C), where R is the load impedance that the demodulator is driving and C is the AC coupling capacitance.
If we provide a large enough AC coupling capacitor so that the introduced high pass corner is very low compared to the bandwidth of the desired signal, then we can successfully eliminate the DC offsets that plague our desired signal and thus enhance the Signal to Noise Ratio. For instance, suppose we have a 10MHz wide WiMAX signal that drives the RF input of the demodulator. With a 10 MHz BW signal, 10 μF ac coupling capacitors with a 500Ω differential load results in a high-pass corner frequency of ~64 Hz, which absorbs an insignificant amount of modulated signal energy from the baseband signal. By using ac-coupling capacitors at the baseband outputs, the dc offset effects, which can limit dynamic range at low input power levels, can be eliminated.
Also the AC coupling capacitors resolve any common mode voltage difference when matching up two parts that have significantly different common modes.