Automatic Tuning of Quality Factors for Vhf Cmos Filters

To correct the frequency response of an integrated filter for process and temperature variations, tuning of the cut-off frequency [l] (f-tuning) is generally applied. Several filters are also provided with automatic tuning of the quality factors (Q-tuning) [2]. Combined fand Q-tuning can be applied with either a master VCF [2] (Voltage Controlled Filter). or a master VCO [3] (Voltage Controlled Oscillator). In fig. 1 the method using a master VCO is illustrated. Consider first the Q-tuning loop. The Q-loop controls the amplitude of the VCO in such a way that it will oscillate with a constant amplitude. If the Q of the VCO is infinite, then the VCO will oscillate harmonically with a constant amplitude (the oles are exactly on the j w axis of the complex planey. If the voltage used for tuning the Q of the two integrators in the master VCO is copied to the (matched) integrators in the slave filter, the quality factors of the filter will automatically be correct. The amplitude of the VCO signal is uncritical as long as the integrators in the VCO operate in their linear region. The f-control loop is a well known phase locked loop (PLL) which locks the oscillating frequency to an external reference frequency. The voltage used for tuning the frequency of the VCO is copied to the slave filter. The combination of fand Q-tuning is possible if the fand Q-control loo s are independent. This is difficult in practice. I? the Q-tuning loop is much faster than the ftuning loop, the ftuning loop will be quasi static and then the fand Q-loops become practically independent. For VHF filters the Q-loop must be fast enough to tune the VCO which oscillates at least at the cut-off frequency of the filter, which can be up to 110 MHz [SI. The Q-tuning loop must therefore be very fast. This paper describes a Q-tuning technique without a physical loop, so that it is very fast and therefore suitable for VHF.