When the power converter is in proximity to the PA in an RF transmitter, regardless of whether it is located at the top or the base of the antenna tower, the switching action of the converter is a potential source of RF interference. Many features of Lambda’s PAH450S series of power modules (see the Figure) provide safeguards against this.

For example, David Norton, V.P. of technical support for Lambda, states the sealed casting of the module effectively acts as a containment shield for radiated EMI generated by the converter that could be sensed and amplified by the RF PA. Conducted EMI is also a concern, with the chief sources being conducted EMI from the converter input back onto the supply bus, and voltage ripple on the converter’s output being coupled into the PA.

Norton states that to suppress conducted EMI, customers generally design their own input filters. One such customer is the Andrew Corporation, which uses Lambda converters to power their analog RF PAs in several applications.

Steve Harris, power analog engineer at Andrew, states that guidelines for conducted emissions standards generally stop at 30 MHz, while the typical frequency range for the PAs is between 800 MHz to 2 GHz. Therefore, these standards are mainly directed at guarding against potential interference with other RF equipment sharing the PA’s power bus. This precaution, Harris states, can be compared to the philosophy of warning pacemaker patients about the presence of a microwave oven: the risk may be slight, but the consequences can be drastic.

Figure. The compact size (half brick) of Lambda’s PAH450S dc-dc converter provides a high power density suitable for driving analog PAs in integrated modules for base-station applications.

Regarding this potential interference, as well as the potential interference from the converter’s output voltage ripple, both Norton and Harris state that while general guidelines are applied to prevent RF interference, this risk is also minimized by the fact that the RF and power converter frequency domains are significantly different. Therefore, with careful design practices, RF interference from the converter is not a major concern, though both men note that as converter frequencies increase, it may become a significant RF design issue.

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