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Determining the FM Bandwidth of a Wideband Varactor Tuned VCO

Determining the FM Bandwidth of a Wideband Varactor Tuned VCO

The most basic characteristic of a Voltage Controlled Oscillator (VCO) is the manner in which the oscillator changes frequency as a function of voltage applied to its tuning port. As the state of the art has evolved, the demands placed on VCO’s to change frequency in a more agile fashion have increased. VCO’s used in phase lock loops are required to acquire phase lock more rapidly. Spread spectrum communications systems are demanding VCO’s that can provide direct FSK modulation at faster rates. Frequency hopping radios are changing frequency at ever increasing rates. The common thread in all these applications is a VCO characteristic, Frequency Modulation Bandwidth (FM BW). Every VCO will have a specified static frequency versus tuning voltage response as well as a tuning sensitivity over the allowed tuning voltage range. The tuning sensitivity curve versus tuning voltage should theoretically be the first derivative of the tuning voltage response versus tuning voltage.

A straight forward method has been presented to measure the FM Bandwidth of varactor tuned VCO’s. This method is not impacted by the normal varying tuning sensitivity of a practical wide bandwidth VCO and does not suffer from measurement sensitivity problems or measurement bandwidth limitations for VCO’s that have FM bandwidths in excess of 10 MHz. A simple downconverter was also presented using a second of the VCO’s under test as an LO, in order to measure VCO’s that have a frequency range above the measurement bandwidth of the DSO. Measurements were performed on a sample of Hittite Microwave’s MMIC VCO’s and the data has been presented. All of Hittite Microwave’s MMIC VCO’s were shown to have wide FM bandwidths and are well suited for the state of the art designs such as fast acquisition PLL’s and wideband FSK signal generation that will demand this type of performance.

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