Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 72

Mixers are not the only way to create new frequencies at the output; frequency multipliers are also used to generate high‐frequency signals, particularly when creating mm‐wave sources. Frequency multipliers produce harmonics by changing a sine‐wave input signal into non‐linear wave. The basic doubler is a half‐wave or full‐wave rectifier, such as a diode bridge. A pair of back‐to‐back diodes turns a sine wave into a square wave, which is rich in odd harmonic content. This is essentially the same as a limiter.

The key figure of merits of a multiplier is the conversion loss from fundamental drive to the desired harmonic. Other important characteristics are fundamental feed‐through and higher‐order harmonics.

Limiters have the key characteristic of maximum output power; that is the power at which they limit. Also important is the onset of limiting and the compression point. Ideal limiters are linear until the onset of limiting, and then they effectively clip the output voltage above that level.

Other multiplier types are step‐recovery diodes, and non‐linear transmission lines that, when driven with a sine wave, effectively “snap” on to produce a sharp edge. Depending upon the design, the on‐time can be short, which produces an output rich in harmonics. Some digital circuits can also be used to create narrow pulses from a sine‐wave input as a pulse generator. Such a pulse will also be rich in harmonics.

One aspect of a multiplier that is not easily discerned is the group delay through it. That is because for some change in input frequency one will see a multiplied change in output frequency. FM that passes through a doubler will have the same rate as the original FM, but twice the deviation. For this reason, doublers or multipliers are seldom used in the signal or communication path of RF or microwave systems but can be used in the base carrier paths and in the LOs of many systems.