Software defined radio or SDR is the most exciting frontier in the field of radio, transferring as it does all signal functions from the analogue to the digital domain. Radios using SDR techniques can be surprisingly straightforward and easy to understand, and [Ray Ring]’s little SDR receiver manages to combine this with the novel use of an audio DSP rather than a computer to perform its SDR functions.
The front end is a conventional enough direct conversion design with an Si5531 clock generator providing I and Q phase-shifted local oscillator signals to a TS3A5017 analogue switch used as a mixer. An unexpected presence is an LTC6252 op-amp as an RF amplifier, but the special part comes after the I and Q baseband signals have been filtered. The SDR part of this receiver is an audio DSP, but it’s one that might not be an immediate choice. The Spin Semiconductor FV-1 is a dedicated digital reverb chip for musical effects boxes, but it comes with the feature that its internal DSP core can access custom code from an external ROM. [Ray] has written his own code for demodulation of AM, USB, and LSB signals rather than musical effects, and used the device’s left and right audio channels to process I and Q quadrature signals. The use of a single purpose chip to do something its designers never intended gives it the essence of a good hack, and we’re mightily impressed at his spotting the potential for an SDR in a musical effect. Hear it in action in the video below the break.
Meanwhile if the operation of a receiver such as this one is a mystery to you, we published a handy primer back in 2017.
Thanks [Ziew] for the tip.


SI5351 Jenny. Caught my attention as I’m about to order another and some Si4432’s for the tinySA/VNA project.
Neat to read too. Literally have this link in an tab I almost forgot about: https://www.electrodragon.com/product/si4432-wireless-transceiver-r2-433m1km-range/
These can be hacked by removing the low pass filter and bypassing for wider frequency range use. Here’s a nice write up: https://groups.io/g/HBTE/message/846
Cool project. BTW the website of the author (circuit salad) worth the time to visit as there are many other cool stuff too.
Tha analog switch part of the design goes back to the Softrock SDR’s which used the si570 oscillator, and sent audio to a PC soundcard.
The audio DSP is a remarkable feature, replacing the entire PC the Softrock needed for decoding the audio signal.
do you think it would be possible to add fm support to the dsp?
I was looking at my Mini-3 Bat Detector since I’m adding a BNC connection so I can use for electromagnetic wave reception too (goes up to 160kHz) and not just the pressure waves with the transducer… and am reminded like I found before that after a certain date there are lot’s of SDR chips for typical AM/FM radio since I see the Mini-3 design is using an AM tuner chip TEA5551A and Kieth Maries was kind enough to provide a schematic and detail a little where “It is a somewhat unusual heterodyne detector, because the tuned oscillator (VC1 and T4; 295 – 445 kHz) operates at lower frequencies than the fixed local oscillator (QA1 and T1; 455 kHz).” “The U30 used a special capacitance microphone, and operated up to 200 kHz.” I’m guessing the older S-25 used the same special capacitance microphone.
An neat article can be making or hacking something into a good wideband ultrasonic microphone/transducer.
I’ve been reading about the si570 also since thinking about making a Specan spectrum analyzer as a project and at the least have been wanting to test a si570 and really more-so Si569, Si564, Si549 and ADF4351. Only the ADF4351 are on cost effective modules I’ve found so far.
Neat SDR radio build for sure and circuit salad has some great posts for sure.
@bob: “do you think it would be possible to add fm support to the dsp?”
Or were you thinking something more complex logic related on the chip via firmware?
sdr’s were always known to be flexible in modes but i should have phrased the question differently. did the author leave out fm due to lack of interest in bands above 20mhz or due to there being limitations in the circuit.
“An unexpected presence is an LTC6252 op-amp as an RF amplifier”
The LTC6252 is an RF amplifier with 400Mhz bandwidth.
I chose that op amp because I had some on hand…it works fine but there are some even better choices…that I will offer as alternates on my blog