DDC experiments with Airspy
Robert Nickels <w9ran@...>
Hi Fellow Airspies!
As some of you may know, I've been working with Youssef on the "Spyverter" that will add full HF coverage to the Airspy, but I'd like to share the results from a different approach.
Using experimental code, I've successfully used the direct ADC input to the Airspy for direct reception of HF from 0 through 10 Mhz (while the normal code rolls-off the upper and lower 500 Khz, this version does not). The HF antenna is transformer isolated and diode clamps are provided to protect the ADC input but I wouldn't feel comfortable using this method without additional filtering and isolation to protect the ADC.
Here's a screen shot of the bottom 10 Mhz of the HF spectrum:
I'd post a video shortly, but I think you'll agree this is pretty impressive! The ability to see such a large chunk of the HF spectrum at once is something new for most of us, and is made possible by the unique architecture of Airspy and its driver for SDR#. Tuning is done by "point and click" but with zoom it's possible to get dead-on, and the noise floor runs about -80dB. Even with a good all-band vertical antenna with large capture area, HF signals remain within the dynamic range of the ADC without saturation or the need for a preamp. Decimation can be used to reduce the sample rate, but the center frequency remains fixed at 5 Mhz.
Since the input transformer is a broadband balun, this is truly an HF receiver that uses no tuned circuits! The downside to this approach of course is aliases and the likelihood of hearing unwanted signals even into the VHF range by means of undersampling. That's why this is intended as informational and not an endorsement of the direct sampling method without addressing those issues.
Thus, direct digital downconversion (DDC) with 10 Mhz bandwidth without the use of FPGA or specialized RF silicon - just a microcontroller and some highly optimized software on a PC.
73, Bob W9RAN