Amateur radio is the only hobby that offers its licensed operators the chance to legally design, build, and operate high power radio transceivers connected to unlimited antenna arrays for the purpose of communicating anywhere in the world. The most complicated part of this communication system is the single-sideband (SSB) high frequency (HF) transceiver. In reality, due to the proliferation of low-cost amateur equipment, there only exists a very small group of die-hards who actually design, build from scratch, and operate their own SSB transceivers. I am one of those die-hards, and in this post I will show you how to get started.
To understand how an SSB transceiver works we must first review the basic architectures of radio receivers. My favorite way of abstracting radio architecture is to consider everything at the block diagram level: filters, amplifiers, multipliers (or mixers as we call them), and assume all blocks are impedance matched.
The earliest radio architecture was known as tuned radio frequency (TRF), which became widely adopted in the mid to late 1920s for use in consumer receivers. The signal chain consisted of an antenna to collect the radio signal which was fed into four stages of filtering interspersed with three stages of amplification. The output of the last tunable filter was fed into an envelope detector (a diode) where the demodulated audio was amplified and played out of a loudspeaker. To tune in a station you would simply tune each of the filters to the desired frequency. Later models mechanically coupled the variable capacitors of each filter section together so that the user would only need to turn one knob to tune in a station.
The problem with TRF architectures was that multiple stages of tuned filters were expensive. To address this problem Edwin Armstrong combined the use of a low-cost un-tuned filter and frequency multiplication to create what was known as the superheterodyne architecture.
Edwin Armstrong realized the value of frequency multiplication. When two sinusoidal waveforms, one at frf and the second at flo, were multiplied together the result was the sum and difference of these two frequencies, F_if = F_rf – F_lo and F_if = F_rf + F_lo.
In RF design, we refer to multipliers as frequency mixers. In a superheterodyne receiver the desired RF signal is multiplied down to an intermediate frequency (IF) by use of a mixer and a variable frequency oscillator (VFO or Local Oscillator) where there exists a multi-stage filter to select the signal to pass onto an envelope detector. In other words, one of the two products from the mixer must equal the center frequency of the IF filter. To change the frequency at which the radio is receiving all you have to do is change the frequency of the VFO.
The figure below shows a block diagram of a table-top AM radio from the late 40’s where the radio tunes in stations at the frequency frf = frequency of OSC1 – 455 kHz. Changing the frequency of OSC1 changes the frequency at which the radio is tuned to frf.
Rather than demodulating the radio signal with an envelope detector a SSB receiver down-converts the IF one more time using a second frequency mixer to the audio frequency range (this second mixer is sometimes known as the product detector). The result is amplified and fed out of a loud speaker.
What is being heard on an SSB receiver is actually the radio frequency spectrum multiplied down to the audio frequency spectrum so we can listen to it. You are listening to the actual radio waves.
In this case the IF filter’s bandwidth is between 1.8 and 2.5 kHz, matched to the bandwidth of human speech. The IF filter’s center frequency and the 2nd local oscillator determine what side band is selected, either upper sideband (USB) or lower sideband (LSB). USB and LSB refers to shifting the human voice to just above and below the carrier frequency respectively.
An SSB transmitter is simply the SSB receiver in reverse. Filters and modern double-balanced frequency mixers work in both directions. Amplifiers are wired in such a way to relays or PiN didoes so they can be reversed in transmit mode. When you transmit SSB your voice is upconverted to the radio frequency spectrum, amplified, and radiated out of the antenna.
20m SSB Transceiver
The first SSB transceiver I developed was for 20m, which is arguably the most fun HF band. The marine net is on 14.300. Lots of DX during the daytime hours. From the shoreline in Connecticut I can routinely work Western Europe and deep into Russia with only 40 watts and a half-wave dipole antenna.
The block diagram of this transceiver is exactly as shown above. A lot of detail, schematics and additional info can be found here. This radio is representative of the vast majority of SSB transceiver architectures.
I have abstracted radio circuitry to the block diagram level. Once blocks are understood then a design can be made at the high level. After the block diagram is in place then circuits, ICs, or modules can be selected to fill in the blocks. Some circuits are borrowed from books or scaled from a design in a book. A great source for 50 ohm ICs and modules is mini-circuits. Others require the synthesis of a custom ladder network, which will be the case for the RF front-end filters. These are great resources for finding, borrowing, or synthesizing those circuits or entire radio architectures even:
There is so much more to basic RF design and the only way to really learn is to start doing it. Borrow as many circuits as you can from others. Cobble together your radio. You will get better at every aspect of design after each radio you build. Jump right in! There’s nothing like the satisfaction of making a long distance contact on a transceiver you built yourself. I look forward to communicating with some of you on the air soon!
My cousin, Juliet Hurley, MBA, MSF, MAC for type editing this post.
Gregory L. Charvat only operates radio equipment he builds from scratch, is the author of Small and Short-Range Radar Systems, co-founder of Hyperfine Research Inc., Butterfly Network Inc. (both of which are 4catalyzer companies), visiting research scientist at Camera Culture Group Massachusetts Institute of Technology Media Lab, editor of the Gregory L. Charvat Series on Practical Approaches to Electrical Engineering, and guest commentator on CNN, CBS, Sky News, and others. He was a technical staff member at MIT Lincoln Laboratory where his work on through-wall radar won best paper at the 2010 MSS Tri-Services Radar Symposium and is an MIT Office of the Provost 2011 research highlight. He has taught short radar courses at MIT where his Build a Small Radar course was the top-ranked MIT professional education course in 2011 and has become widely adopted by other universities, laboratories, and private organizations. Starting at an Early Age, Greg developed numerous radar systems, rail SAR imaging sensors, phased-array radar systems; holds several patents; and has developed many other sensors and radio and audio equipment. He has authored numerous publications and has received press for his work. Greg earned a Ph.D in electrical engineering in 2007, an MSEE in 2003, and a BSEE in 2002 from Michigan State University, and is a senior member of the IEEE where he serves on the steering committee for the 2010, 2013, and 2016 IEEE International Symposium on Phased Array Systems and Technology and chaired the IEEE AP-S Boston Chapter from 2010-2011