| 摘要 |
The present invention concerns fast acting control systems which are usable to tune electronic devices such as an RF signal receiver. This invention speeds tuning by fast charging the integrator cap when large frequency errors exist. In this situation, which exists during a channel change, the phase locked loop (PLL) integrated circuit (IC) is sinking or sourcing current to charge (or discharge) C1. The voltage developed across R1/C2 turns on transistor Q1 or Q2 which can then sink or source much more current to C1 thus removing the slew rate limit to the tuning voltage. Once the loop is locked, the small correction current required by the PLL to compensate for temperature drift is not enough to turn on Q1 or Q2 and the loop behaves as if Q1 and Q2 were not present. This is significant because if the values of R1, C1, C2 or the maximum current from the PLL IC were changed, the steady state performance would also change, resulting in poor stability or spectral purity.
|
