A wireless doorchime supplier required a new receiver module to reduce the production cost of a range of products. A significant cost saving was obtained with improved performance and fewer alignment adjustments in production.

A new, much cheaper receiver IC had already been evaluated in a paper-based feasibility study by WRL Consultancy. Although the new IC was a simpler design and lower specification than the existing IC, it was determined from the study that an acceptable performance level could be obtained with some improvements on the manufacturer's suggested external circuit. This provisional conclusion was subject to the evaluation of the performance of actual IC samples in a test circuit.

The improvements were to consist of an external local oscillator, external preamplifer, improved bandpass filter and improved IF filter, which could be financed by part of the cost saving on the IC. However, the evaluation tests showed that some of the provisional assumptions based on available data alone needed to be revised.

Some innovative measures were identified and implemented to address this problem, including:

• optimising the preamp noise figure and gain at low cost;
• a novel coupling network to improve the LO drive level fed to the mixer;
• a low loss, low cost SAW filter sourced in the Far East;
• a novel IF filter concept to optimise selectivity and accommodate component tolerances.

The resulting receiver module design proved to have somewhat greater sensitivity and immunity to interference than its predecessor whilst keeping its projected cost saving virtually intact.

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