In theory, any good balanced mixer can do the job. The AD831 is good, but is not specially tailored to use as a quadrature demodulator, so it is no better than, say, a MiniCircuits SBL-1 or some other good (ie. balanced) active or passive mixer.
There are some critical problems in this type of demodulator that call for a better mixer. For example, one key problem is that if you build your own demodulator, it is likely that you will get LO leakage into your mixer RF inputs. When this happens, you get undesired DC output from your mixers and this DC gets in the way when you want to apply baseband gain to the output. Another typical difficulty is in getting an accurate 90 degree phase shift in the LO over a broad range of frequencies, and related to this is the problem of maintaining good amplitude and phase balance between the two channels. To solve these problems, the experts integrate both mixers along with a 90 degree phase shifter onto one IC. By doing this, they insure good matching of the mixers, so that amplitude balance of LO and RF to both mixers and IF output level matching from both mixers is good. They also integrate a clever LO circuit that includes a divide by 2 stage and a digital phase shifter. In this case, the LO injection to the IC is at twice the desired LO frequency. So, when your layout allows signal to leak from the 2xLO input pin to the RF input pin (which is hard to avoid), it has a much smaller affect, which effectively eliminates the DC problem. To show you an example of this kind of IC, take a look at the AD8348 (**broken link removed**.
The block diagram shows how there are two mixers in the one IC, and all inputs and outputs are buffered. Also, the LO circuit includes a divide by 2 and a good phase shifter. This IC, although it does not match your desired frequency of 20 MHz, is an example of what you should be looking for if you want the best performance.
Here is another example of this kind of architecture:
https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1011,C1725,P1711,D3850
Having said all this, it is quite possible that your selected frequency is so low that the problems that I mention above are fairly minor. With good pcb layout, you might be able to use a discrete or less integrated design and make it work. I hope you know how to design a pcb for best balance and isolation at 20MHz.
I did a search at Digikey for Mixer IC and then realized that most mixer ICs don't appear to have IF outputs that have bandwidth down to DC, so they wouldn't work well as your demodulator. So, I stand corrected, that we have to be a bit more choosy than I first thought. Your AD831 does have this feature of IF output down to DC.
Others that might work include a range of mixers from Analog Devices including AD8343 and AD8342 although these might be harder to use due to the IF output circuit configuration. I still favor the SBL-1+ from Minicircuits.