A hackaday article discussed hacking a cheap ($5) doppler radio module of the type used to detect a person approaching an automatic door. So I spent $6 on Ebay and ordered an HB100 module and will see how this goes.
The pjrc article says he used an "x660 opamp circuit (OPA209A) and 0.1 uF cap directly into the Teensy 3.2 pin 16 (A2)".Tom's Electronics pages / tom@mmto.org