JPS6458123A - Frequency synthesizer circuit - Google Patents

Abstract

PURPOSE:To reduce the response time at frequency switching of a frequency synthesizer circuit by adding a series connection circuit comprising two or three diodes to an inverse amplifier so as to optimize the input DC bias of a loop filter. CONSTITUTION:Transistors(TRs) Q1, Q2, diodes D1, D2 and a load resistor RL constitute an inverse amplifier with a low input current, high gain and nearly 2.5V of input bias voltage as features in the constitution of the loop filter 5. The diodes D1, D2 act like keeping the DC bias voltage at the input terminal of the loop filter to nearly 2.5V. A voltage difference between the output of a phase frequency comparator in switching the local oscillation frequency from a low into a high frequency and the operating point of the inverse amplifier is nearly 2.4V and the voltage difference in switching the local oscillation frequency from a high to a low frequency is nearly 2.6V conversely, then the both are nearly equal to each other, and the response time at the local oscillation frequency switching is made uniform in switching the frequency from high to low and vice versa thereby reducing the response time.