The SP5659 is a single chip frequency synthesiser designedfor tuning systems up to 2·7GHz.The RF preamplifier drives a divide-by two prescaler whichcan be disabled for applications up to 2GHz, allowing directinterfacing with the programmable divider, resulting in a stepsize equal to the comparison frequency. For applications upto 2·7GHz the divide-by two is enabled to give a step size oftwice the comparison frequency.The comparison frequency is obtained either from an on-chip crystal controlled oscillator or from an external source.The oscillator frequency FREF or the comparison frequencyFCOMP may be switched to the REF/COMP output; this featureis ideally suited to providing the reference frequency for asecond synthesiser such as in a double conversion tuner (seeFig. 5).The synthesiser is controlled via an I2C bus and respondsto one of four programmable addresses which are selected byapplying a specific voltage to the Address input. This featureenables two or more synthesisers to be used in a system.The SP5659 contains four switching ports, P0-P3 and a5-level ADC, the output of which can be read via the I2C bus.The SP5659 also contains a varactor line disable andcharge pump disable facility.