Although the coherent demodulator using the analog VCO produces a good output signal when it is properly tuned, changing from one station to another is difficult, and the limited range of the VCO we are using severely restricts the number of stations we can receive. The DDS is easy to tune to any frequency from DC to about 60 MHz, which allows us to receive any AM station up to 30 MHz, taking into account the division by 2 in the I/Q mixer. Since we were able to make the DDS behave like a VCO in Exercise 8, it should be possible to replace the analog VCO with the DDS VCO and have a wide range, coherent AM receiver. Your Design Challenge is to do precisely that.
Because of the quantization in the A/D converter and the limited update rate of the software VCO, the phase locking will not be as robust as with the analog VCO, but you should be able to achieve satisfactory reception of strong signals. As a starting point, try scaling the A/D converter value with respect to the full tuning word to give a tuning range of about 3 kHz.