Column: Embedded design


A contractor’s tales of radio horror


By Myk Dormer, Engineer and Director, Smallwireless Limited H


ere I would like to share a series of short anecdotes from my professional life as both a contract and a consultant RF engineer.


Many years ago I wrote a similar piece, recounting my experiences as an engineer in more conventional employment. Perhaps surprisingly, the nature of the incidents hasn’t changed that much, except that now I am paid by the hour to fix design problems.


What’s missing? A client manufactures a pre-production batch of an already proven design: a small ISM band transmitter based around a QFN package PLL chip. T e new units don’t work at all, but the client insists that the build is 100% true to the prototype. In testing I fi nally discover that the units


function if considerable mechanical force is applied to the main IC, and when that chip is removed, the complete absence of solder on the centre ground-pad is revealed. It transpires that, while the PCB design


and bill of materials are correct, the solder paste stencil has a glaringly visible manufacturing fault.


Low power? A client complains that, in production, a new 500mW transmitter design is only producing 475mW. T is is only about -0.3dB low, and near enough to the loss of the one meter long cable between their test jig and power meter. Yet, the client maintains that this is a fundamental circuit design problem.


It takes six months for the tedious admin


processes to allow the change to be implemented; in the meantime, multiple prototype and


pre-production units are destroyed, each costing over ten thousand pounds


An expensive delay? A large and complex radio system for a military application manifests a cascade failure mode. In a nutshell, if we applied a mismatch to


the TX output then the PIN diode antenna switch circuits failed catastrophically, due to the diode driver circuit being overloaded by rectifi ed RF energy. Aſt er a few days of diagnostics, the fault


was fully understood and a simple cure found: we up-rate the driver circuits by simply replacing some bipolar switching transistors with higher current and voltage rated versions, and under previously fatal mismatch conditions the unit just got comfortably warm. T e horror story begins then. Although


the new transistor is in the same package as the original and from the same product family of the same manufacturer, it takes six months for the tedious admin processes to allow the change to be implemented. In the meantime, multiple prototype and pre-production units are destroyed, each costing over ten thousand pounds.


What has changed? A new design goes into pre-production, and circuitry that had functioned perfectly


in prototyping now fails spectacularly. Vital circuits seem to be off -tune, matching networks have the wrong impedance ratios, VCOs stall, and power amplifi ers fail to produce their designed output levels. Assembly error is quickly discounted and, in the end, I swallow my pride (and my profi t) and retune the mysteriously defi cient circuits, assuming some sort of transcription fl aw or human error on my part in the design. T en the client starts reporting similar


faults on older products, which have been made in considerable numbers in the past, but are suddenly throwing up identical problems. Aſt er many late nights and much torn-


out hair, it turns out that the customer has a new PCB supplier, and the new boards have very thin insulating layers (50µm) between the outer and inner copper, resulting in massively increased parasitic capacitances.


Drift? T e customer buys a complete design – a small RF-chip based transceiver for use in a nano-satellite – and successfully “tiles” the layout onto their motherboard. T ey then undertake a series of tests,


www.electronicsworld.co.uk February 2026 15


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