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sBITX – An open source SDR that you can hack

Here how to convert your existing homebrew radio into a full-fledged SDR for less than
$100.

The idea of hybrid architecture was first implemented in this form by Bob Larkin in his
seminal work with DSP-10 transceiver. The details are in the ARRL book Experimental
Methods in RF Design and its accompanying CD. An open secret about the EMRFD is that
it is also an excellent introduction to Digital Methods and Bob is a great teacher. I spent a
rainy day in Portland with Bob discussing radio and life and that evening we were joined
by Jeff Damm (WA7MLH) for an early dinner. This project was born that evening.

Here are the headlines:
● Based on Raspberry Pi inside your radio
● Open source, plain C code that is easy to read, understand and change
● Adaptable to any superheterodyne radio that you may already have built
● Based on hybrid architecture
● Uses the simpler Fast Fourier Transforms in place of RF phasing systems

This article is divided into three parts:

  1. Our Game Plan
    We will set out how we are going to build an SDR leveraging our skills with
    conventional superhet analog radios
  2. The Software
    This describes the very core of the software, line by line. Transmit and Receive is
    broken down to make it easy to understand.
  3. SDR for the uBITX
    We now adapt a standard QRP superhet (the uBITX) to become a full featured SDR.
    Our Game Plan
    There are many ways to build SDRs, (See the compact and informative presentation by

Howard White, KY6LA at https://nparc.ca/wp-content/uploads/2019/06/Four-
Generations-of-SDR.V3.0.pdf)

There are two standard approaches to building a software defined radio today:

  1. Direct Digital Conversion : The radio directly digitizes the RF right at the front-end.
    For an HF radio, it can generate 60 million samples per second. Handling data at such
    speed needs specialized circuitry built using FPGAs (Field programmable gate arrays) in
    addition to a PC.
    To build this kind of radio, you need multilayer boards, FPGA programming skills and also
    some money. This is clearly not what we want to do.
  2. Phasing Radio : Two identical (called In-phase and Quadrature) direct conversion
    radios are used, both operated by the same local oscillator. However, the local oscillator
    drive to the Quadrature radio is delayed by exactly 90 degrees. The baseband audio from
    both the channels is fed to the two stereo inputs of an audio codec and the SDR software is
    run on a moderately fast CPU at audio frequencies.
    The phasing radio has two main challenges. First, it needs a very precise 0 and 90 degree
    RF phase difference between the two channels. Second it also needs exactly similar audio
    gain and audio phase delays in both the channels. Even a 0.01 degree phase difference can
    degrade the opposite sideband suppression horribly. A nominal ladder filter made from 6
    microprocessor grade crystals could do a far more efficient job than the most complex SDR
    front-ends. Every capacitor in the radio’s signal chain can change the phase and amplitude
    balance before it gets into the PC. Maintaining this over a large set of frequencies is a
    challenge especially if you are as sloppy a builder as I am.
    Our approach is a hybrid one. On receive, using superhet architecture, we will bring out a
    25 KHz slice of RF spectrum down to a low IF centered around 25 KHz (extending from
    12.5 KHz to 37.5 KHz). A 24-bit audio codec running at 96000 samples/second will bring
    this digitized audio into the Raspberry Pi.
    To transmit, we will generate the SSB/CW/FM/AM signal in software for a carrier
    centered around 25 KHz. We will upconvert this signal to the RF frequency of choice.

Engineering is the art of negotiation with science and economy. Our compromise is on
limiting the maximum width of the waterfall to 25 Khz. Though this is not a major
compromise, the experience shows that this waterfall is enough for the most. Rob
Sherwood, NC0B, (known for his list of high performing radios) says that he prefers a 10
KHz wide spectrum when operating contests.
With this limitation in place, we can build an SDR that will run circles around those that
cost thousands of dollars. This architecture was first implemented by Bob Larkin, W7PUA
in his now famous DSP-10 transceiver it was copied by many commercial transceiver
including the Elecraft K3 line, the FTDx-101D, etc. The links to his amazing series of
articles are at the ARRL’s SDR page on http://www.arrl.org/software-defined-radio.
The DSP-10 is almost a quarter century old design now and the sbitx is a tribute and a
reboot of the original.

At the moment the sBITX is still in the development stage, information is spread between the video after the break, it’s accompanying PDF, the GitHub repo, and a thread on the BITX20 group.

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