Get critical with beginner radio; Design and Construct a Single Sideband Transceiver from scratch Half 1
Ham radio is the only hobby that offers its licensed operators the ability to legally design, build, and operate high performance radio transceivers connected to unlimited antenna arrays to communicate anywhere in the world. The most complicated part of this communication system is the single sideband radio frequency transceiver (SSB). In reality, with the proliferation of inexpensive amateur equipment, there is only a very small group of die-hard equipment that design, develop and operate their own SSB transceivers from scratch. I’m one of those die-hard people, and in this post, I’m going to show you how to get started.
To understand how an SSB transceiver works, we first need to review the basic architectures of radio receivers. My preferred way of abstracting the radio architecture is to look at everything at the block diagram level: filters, amplifiers, multipliers (or mixers, as we call them) and assume that all the blocks match the impedance.
The earliest radio architecture was known as Tuned Radio Frequency (TRF), which became widespread for use in consumer receivers in the mid to late 1920s. The signal chain consisted of an antenna for collecting the radio signal, which was fed into four filter stages that were interspersed with three amplification stages. The output of the last tunable filter was fed into an envelope detector (a diode), where the demodulated audio was amplified and played through a loudspeaker. To tune into a station, you simply tune each filter to the frequency you want. Later models mechanically coupled the variable capacitors of each filter section together so that the user only had to turn a knob to tune in a transmitter.
Block diagram, schematic and photos of a typical TRF radio.
The problem with TRF architectures was that multiple stages of matched filters were expensive. To address this problem, Edwin Armstrong combined the use of an inexpensive, untuned filter and frequency multiplication to create what is known as an overlay architecture.
Edwin Armstrong saw 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.
How a frequency mixer works.
In RF design, we refer to multipliers as frequency mixers. In a heterodyne receiver, the desired RF signal is multiplied to an intermediate frequency (IF) using a mixer and a variable frequency oscillator (VFO or local oscillator) with a multi-stage filter to select the signal to be routed to a signal should be the envelope detector. In other words, one of the two products from the mixer must correspond to the center frequency of the IF filter. To change the frequency that the radio receives, all you need to do is change the frequency of the VFO.
The following figure shows a block diagram of a table AM radio from the late 1940s, on which the radio tunes into stations with the frequency frf = frequency of OSC1 – 455 kHz. Changing the frequency of OSC1 changes the frequency at which the radio is tuned to frf.
Block diagram, schematic, and photos of a typical superheterodyne broadcast receiver.
Instead of demodulating the radio signal with an envelope detector, an SSB receiver converts the IF again into the audio frequency range with a second frequency mixer (this second mixer is sometimes referred to as a product detector). The result is amplified and fed from a loudspeaker.
What can be heard on an SSB receiver is actually the radio frequency spectrum which is multiplied by the audio frequency spectrum so that we can hear it. You hear the actual radio waves.
In this case the bandwidth of the IF filter is between 1.8 and 2.5 kHz, adapted to the bandwidth of human speech. The center frequency of the IF filter and the 2nd local oscillator determine which sideband is selected, either the upper sideband (USB) or the lower sideband (LSB). USB and LSB refer to the shift of the human voice to just above and below the carrier frequency, respectively.
Blog diagram of a 20m SSB receiver and representative diagrams of the signals at each phase of the receiver.
An SSB transmitter is simply the reverse SSB receiver. Filters and modern double-balanced frequency mixers work in both directions. Amplifiers are wired with relays or PiN-Didoes so that they can be reversed in transmit mode. When you broadcast SSB, your voice is converted up into the high frequency spectrum, amplified and emitted by the antenna.
Block diagram of a 20m SSB transmitter.
20 m SSB transceiver
The first SSB transceiver I designed was for 20m, which is arguably the most fun RF band. The marine net is at 14.300. Lots of DX during the daytime hours. Off the coast in Connecticut, I can routinely work with only 40 watts and a half-wave dipole antenna in Western Europe and deep in Russia.
A 20m long SSB transceiver that was built from scratch.
The block diagram of this transceiver is exactly as shown above. Many details, circuit diagrams and additional information can be found here. This radio is representative of the vast majority of SSB transceiver architectures.
I’ve abstracted radio circuits to the block diagram level. Once the blocks are understood, a high-level design can be made. After the block diagram has been created, circuits, ICs, or modules can be selected to fill the blocks. Some circuits are borrowed from books or scaled from a draft in a book. A great source for 50 ohm ICs and modules are miniature circuits. Others require the synthesis of a custom ladder network, which will be the case with the RF front-end filters. These are great resources to find, borrow, or synthesize these circuits or entire radio architectures, yourself:
Solid state design for radio amateurs
Joseph J. Carr’s Secrets of RF Circuit Design
Chris Boweick RF circuit design
Rohdes communication receiver
There’s a lot more to basic RF design, and the only way to really learn is to get started. Borrow as many circuits from others as you can. Cobblestone your radio. After every radio you build, you get better at every aspect of the design. Jump right in! There is nothing like the satisfaction of making remote contact through a transceiver you have built yourself. I look forward to communicating with some of you in the air soon!
My cousin, Juliet Hurley, MBA, MSF, MAC for type editing this post.
Gregory L. Charvat only operates radios that he has built from scratch. He is the author of Small and Short-Range Radar Systems, co-founder of Hyperfine Research Inc., Butterfly Network Inc. (both companies of 4catalyzer) and visiting scholar at the media lab of the Camera Culture Group of the Massachusetts Institute of Technology, editor of the Gregory L. Charvat- Practical Approach to Electrical Engineering series and guest commentator on CNN, CBS, Sky News and others. He was a technical assistant at MIT Lincoln Laboratory, where his work on transit radar was recognized as Best Paper at the 2010 MSS Tri-Services Radar Symposium, and is an MIT Provost 2011 research highlight office. He taught short radar courses at MIT, where his build is a 2011 Small Radar course was MIT’s high-level vocational training course and is widely used by other universities, laboratories, and private organizations. At a young age, Greg developed a variety of radar systems, SAR imaging sensors for rails, and phased array radar systems. holds multiple patents; and has developed many other sensors, as well as radio and audio equipment. He has written numerous publications and received press for his work. Greg received his PhD in Electrical Engineering in 2007, MSEE in 2003, and BSEE in 2002 from Michigan State University, and is a senior member of the IEEE where he served on the Steering Committee for the 2010, 2013, and 2016 International IEEE Symposium on Phased Array Systems and Technology and Chairman of the IEEE AP-S Boston Chapter from 2010 to 2011