Simple Complicated radio - Generating the LO with PLL

A short analysis of the HAM Radio HF bands led me to the idea of under sampling the incoming signal and eliminate the mixer altogether here is my rationale

The HAM Radio bands are quite narrow in comparison to my ADC Nyquist band (0-Fs/2)

Checking the datasheet of LTC2314-14 it seems that is suited for under sampling because it has good full power bandwidth, so in theory is should be feasible to use aliasing in our favor and make the band limited signal to fall in our Nyquist band. This technique will work only for band limited signals (we have a band filter anyway) and is eliminating the need of the mixer. Actually the ADC is working like a mixer anyway (sampling).

Basically the architecture is changing from an IF receiver to direct RF (zero IF) receiver.

Simplified Front End

Generating the LO inside the FPGA with DDS ? why bother we might not even use a mixer?

The whole idea is to generate the LOs to down convert the desired amateur bands into the Nyquist band of the ADC (the ADC will sample with maximum of Fc = 4.375 MHz so we have about 2MHz available)

We are not going to use the mixer for tuning into a specific station in the band will try to sample the whole band in one go (this might be a bad idea but will make amends later)

We are focusing on

40m Band (7.0 MHz - 7.2 MHz)

20m Band (14.0 MHz - 14.350 MHz)

15m Band (21.0 MHz - 21.450 MHz)

maybe in the future (will see)

10m Band (28.0 MHz - 29.700 MHz)

What if we are not using a DDS ? we are not gong to tune (a specific station) some fixed frequencies are probably sufficient and are all related to 7 MHz, So i decided to use the PLL inside the spartan7 to generate the fixed LOs and I parked the fancy idea of DDS (which requires a good DAC which I don't have) - simple is always better.

At this point I'm not even sure if I'm going to use these LO signals if I'm deciding to use the ADC in down sampling mode, I guess it all comes down to experimenting with the ADC first.

More in the following days, is 2026 in some countries already.

73 de EI3HWB!

Happy Ney Year!