Direct conversion receivers are popular among ham radio operators and others who build radios. Suppose you want to listen to a signal at 7.1 MHz. With a direct conversion receiver, you’d tune a local oscillator to 7.1 MHz, and mix it with the incoming signal. The resulting sum and differences of the input frequencies will include the audio of an AM signal on the desired frequency.
[kk9jef] decided to build a receiver like this for the 40 meter band (around 7 MHz). He started with an earlier design and replaced its analog VFO with his Si5351 DDS VFO he’d built previously (and uses the ubiquitous Arduino). One interesting twist: the original design uses a Polyakov or Russian detector that requires a “half frequency” local oscillator. That is, to tune to 7.0 MHz, you actually tune to 3.5 MHz. This is sometimes known as a subharmonic mixer. The advantage is that a lower frequency oscillator is easier to make stable. The designer of the original receiver, [ke3ij], has an interesting write up on his tests to reveal why this scheme works.
The original design was for the 3.5 MHz band (and used a 1.25 MHz local oscillator). [kk9jef] redesigned the input filter to accommodate the new operating band. The Arduino software driving the local oscillator also reads an encoder and drives an LCD. It is a simple matter to make it read the actual operating frequency.
Of course, there probably isn’t much advantage to running the Si5351 at a lower frequency. It should do just fine anywhere it is able to oscillate. We’ve covered variable frequency oscillators using this chip before, and our own [Bil Herd] explained how these kinds of chips operate internally in the video below, if you’d like to know.