Why is DC offset from LO feedthrough less problematic in the second mixer stage of a dual conversion receiver?

Question

A dual conversion receiver has two frequency translation stages. The first mixer translates from RF to IF; the second (IF) mixer translates from IF to baseband.

This architecture is supposed to help avoid the problem of DC offset, caused by LO leakage to the mixer input followed by self-mixing of the leakage down to baseband.

Direct conversion receivers, which have only one frequency translation stage, suffer from DC offset because the LO leakage energy is immediately translated to baseband. The dual conversion architecture takes care of the LO leakage from the first mixer by ensuring that it appears at an offset of IF from the signal of interest.

But the LO leakage of the IF mixer, if any, is translated directly to baseband when it self-mixes. Why isn't the LO leakage from the second (IF) mixer equally as problematic as the LO leakage in the single mixer of a direct conversion receiver?

I've read one answer in this book:

DC offset due to self mixing is introduced when the IF LO signal leaks to the input of the IF mixer and then self mixes to produce a DC output in the analog baseband. This on-chip IF leakage is more deterministic than its direct conversion counterpart in which the LO can leak off-chip to the antenna and be amplified by the low noise amplifier (LNA) before self mixing occurs. Unlike a direct conversion receiver, RF LO self mixing in a dual conversion architecture creates a DC offset at IF and does not introduce any impairment in the analog baseband.

but it's not clear to me why the two mixers are different in this respect. Why does the first (or only) mixer suffer from off-chip leakage, but the second mixer does not?

Answer 1

Why does the first (or only) mixer suffer from off-chip leakage, but the second mixer does not?

A hopefully simple example - say you need 80 dB of total gain in the reciever and this is split in two halves - the first 40 dB is between antenna and the first mixer and the second 40 dB of gain is before the final mixer: -

If LO1 leaks a small signal into the antenna, the output of mixer1 will contain a DC error and a frequency that is twice the LO frequency. OK so far?

Next, consider what outputs from the 1st mixer when a proper signal arrives - it will neither be DC nor twice the 1st LO frequency so those errors produced by leakage are easily filtered out leaving a desired signal that is exactly what you want.

The second stage will produce a leakage LO error of course - this cannot be avoided but, compared to a direct conversion receiver, the leakage is 40 dB down (in this simple example).

You could point out that the 2nd LO can get right back to the antenna and be amplified just as in a direct conversion receiver. However, you have to choose the 2nd LO so that any sum or difference frequencies you might get when it mixes with the first LO don't land in the desired band of frequencies due to a proper receiver input.

For instance, if your desired signal is 1000 MHz with a bandwidth of 10 MHz and your first local oscillator was 970 MHz you would get a desired mixed output of 25 MHz to 35 MHz. The 2nd LO would of course be 30 MHz and if 30 MHz fed back to the antenna it would mix to produce undesired frequencies of 970 MHz and 1030 MHz from the 1st mixer. Neither of these are anything like the desired band of frequencies so they are easily got rid of.

Answer 2

The first mixer is fed by an LNA with a lot of gain making it more sensitive not only to the signal you're trying to receive, but also to signals you aren't trying to receive, including internal leakage. The dual conversion method uses IFs that are far enough away in frequency from the signal you're interested in that they are easy to filter out.

Say you want to receive 100 MHz at baseband. In a direct conversion recover, you need a 100 MHz LO. If the signal our of your LO leaks into the antenna, it's indistinguishable from the signal you're interested in. However, a dual conversion system might use frequencies of 70 MHz and 30 MHz. If either of these leak into the antenna, it's trivial to filter out.

Basically, it's no so much about the mixer itself as what the mixer is connected to.