High-performance HF transceiver design: A ham's perspective

The article that appeared in this magazine and in EDN last year about ham radio in the 21st century generated a lot of interest and a lot of questions about various aspects of the hobby. This article grew out of that interest, but is not strictly about ham radio. It is about the design tradeoffs that engineers make every day in designing all kinds of electronic equipment. HF transceivers are used as an illustrative example.

Radio design is a lot like other equipment design
The design of a modern high-performance transceiver for amateur radio is really not much different from the design of many other systems. For example, the volumes are modest, in the few thousands of units per year for a typical model. This precludes the use of ASICs, since most ASICs require much higher volumes to be economically viable. In addition, the moderate volumes don't often attract the product planners of major semiconductor houses.

This means that the design engineer cannot rely on simply marrying a few purpose-built application-specific chips together and call it a hardware design. Nor can he rely on a manufacturer to prepare a ready-made reference design, as is done in high-volume markets like PCs, cellphones, and tablets. Instead, lots of components must be chosen and made to work together to meet the system's performance goals. It takes creativity and use of all available technologies to get a new product into the market, meeting both the performance and cost targets.

Like most systems, compliance to some government regulatory standards is required. In the case of amateur radio transceivers, the government is mainly concerned with the transmitted signal's purity to prevent interference to other services. In the U.S., the applicable rule is 47CFR97.313(d) which states: "... the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission." Most amateur transceivers are designed for sale worldwide, and must comply with a variety of other regional standards (CE, for example) before they can be sold.

Beyond the government's regulations, the customers are very sensitive to performance - and will often pay a premium for it. The same could be said about many industrial systems, test and measurement instruments, and medical equipment. In low-performance systems, price becomes the dominant specification, and profits are hard to come by. On the other hand, a focus on high performance can yield good returns.

This article will focus primarily on the design of the receiver side of several modern high-performance amateur transceivers. There are other HF transceivers available, designed for commercial and government application where the customer can afford a higher price than an individual consumer. Delivering high performance while meeting a consumer price point adds additional challenges to the design task.

As with most systems, the first thing to do is define the worst-case system performance requirements. For high-performance receivers, the salient specifications are sensitivity (ability to hear weak signals), selectivity (ability to reject unwanted signals), and various ways of specifying the overall linearity of the signal chain. Linearity is important because any non-linear stages that receive multiple signals (or even one single large signal) will cause artifacts that are indistinguishable from real signals. And like audiophiles, connoisseurs of radio performance (radiophiles?) have certain test cases and on-the-air circumstances that they use to determine a radio's real performance.

Sensitivity is actually the easiest specification to accomplish. It is relatively easy to design a receiver for the HF spectrum (3-30 MHz) with sufficiently low noise figure that the system noise floor is set by atmospheric noise, not receiver noise. The generally-accepted test for an amateur receiver's Minimum Discernible Signal (MDS) is to determine the input RF level that raises the audio output in a narrow bandwidth (500 Hz or so) by 3 dB compared to no input signal. Most modern receivers exhibit MDS on the order of -135 dBm.

Amateur radio is one of the few licensed HF radio services that does not use specified discrete channels (the recently-assigned 60-meter band is the only amateur band that uses fixed channels). In any given band, stations are free to use any frequency that is not in use by another station. Signal spacing can be surprisingly small. Consider the spectrum photo in Figure 1. This shows seven separate CW Morse-code signals in a 2 kHz bandwidth (less than the width of one typical single-sideband voice signal).


Selectivity can be achieved by analog filtering (usually multi-pole crystal lattice filters), DSP, or a combination of the two. The optimum tradeoff of analog/digital filtering, and where in the signal path it is applied, is an ongoing cost/performance compromise that changes with each generation of radio design...just like any other systems.

Linearity is the difficult specification. It determines how well a radio performs in the presence of other nearby signals. Various test methods are used to determine a radio's performance, and even constructing a decent test setup for the desired test condition is challenging.

In a receiver with linearity problems, spurious signals can result if the input signals are large enough. For example, the signals at 1823.5 kHz (f1) and 1824.0 kHz (f2) might produce a 3^rd-order intermodulation product at 2f1 - f2 or 1823 kHz, well within the passband. No amount of filtering can eliminate this signal once it has been generated, since subsequent stages have no way of differentiating it from a real signal.

The problem is exacerbated when a large number of signals impinge on the front end of the receiver. In Figure 2, a 50-kHz slice of spectrum is shown during a popular operating event.