An automatic gain control circuit and method is provided that constantly outputs uniform output signal size regardless of size change of an input signal. The automatic gain control circuit uses a multiplier and a negative feedback system. The automatic gain control circuit includes an amplitude detector detecting an amplitude of an input signal, a comparator, a multiplier multiplying the amplitude detected in the amplitude detector and an output signal. The output signal is fed back to the multiplier. The comparator has an inverting terminal coupled to an output terminal of the multiplier across a first resistance and coupled to an input terminal receiving the input signal across a second resistance. A non-inverting terminal of the comparator is grounded. The automatic gain control circuit and in particular, the comparator, supplies the output signal of which size is determined by the ratio of the first resistance and the second resistance regardless of the size of the input signal.

A monostable multivibrator having a time constant that is approximately one-quarter of the period of a pulse signal supplied to its input is used for frequency doubling of a 50% keying ratio input pulse signal. The output signals of the multivibrator are low-pass filtered and supplied to inputs of a different amplifier operating as a difference integrator by virtue of a capacitance connected between its output and its inverting input. This output is utilized to modify the time constant when the period of the input pulse sequence becomes more than about four times the time constant of the multivibrator, thus keeping the keying ratio of the output signals at 50% over a wide range of variation of the frequency of the usually symmetrically input pulse sequence. The multivibrator is connected so as to be triggered by both rising and falling flanks of the input pulses. Both ECL and TTL embodiments are shown.

The present invention includes a variable gain amplifier to output two differential signals which are level shifted and compared through two comparators to drive a charge pump which produces either a discharge current or a charge current to provide feedback control to the variable gain amplifier.

An amplitude modulation apparatus for achieving an extremely deep extinction ratio (in excess of -90 dBc) at nanosecond rise times required by an SAW device comprises the cascaded combination of a controlled switching device (GaAsFET), capable of providing a medium degree (40 dB) of attenuation at nanosecond switching rates, and a double balanced mixer, both multiplier input ports of which are coupled to receive split outputs from the GaAsFET switch. Because of the nonlinearity of the transfer function of the mixer when driven by the same RF carrier input at both multiplier ports, the output of the mixer is in excess of -90 dBc. In an SAW-based signal processing system, the output of the mixer is coupled to RF drive input of the SAW device. With the throughput functionality of the GaAsFET switch being precisely controlled to supply an integral number of half RF carrier frequency cycles to the double balanced mixer, the resultant waveform that is applied to the SAW device drive input from the mixer contains no energy at the RF carrier that could otherwise introduce unwanted Fresnel distortion.

A means for obtaining an output signal which is the sum of the frequencies of two periodic input signals that may vary in amplitude and frequency over time. The apparatus, which provides means for realizing trigonometric functions of the form sin(.alpha.+.beta.)=2 sin .alpha. cos .beta.-sin(.alpha.-.beta.) or sin n.alpha.=2 sin(n-1).alpha. cos .alpha.-sin(n-2).alpha., comprises three basic circuit elements including one or more analog multipliers, one or more envelope detectors, and a subtracter. A method is disclosed for generating a series of even and odd harmonics of a single continuously varying input signal using a plurality of cascaded harmonic generator circuits.