The invention relates to a superheterodyne circuit characterized by comprising a first mixer converting a received signal into the first intermediate frequency, a second mixer converting the first intermediate frequency into a second intermediate frequency, and a product detection stage demodulating the second intermediate frequency to an audio frequency, by having a constitution wherein the local oscillation of the first mixer is input from a VCO for which a PLL circuit is used, wherein the local oscillation of the second mixer is an output obtained by mixing the frequency of a second oscillator with that of a third oscillator in a fourth mixer, wherein the second oscillator combined with the frequency of the VFO in the sixth mixer, alone or in further combination with the frequency of the output of a BFO input via to the product detection stage is the frequency of the third oscillator or an output obtained by mixing the frequency of the third oscillator with a frequency obtained through the frequency division or multiplication of the frequency of the first oscillator in a fifth mixer, and wherein the output of the second oscillator is input to the fourth mixer and, at the same time, a frequency obtained by mixing said output with the output from a VFO in a sixth mixer is directly input to a third mixer of the PLL circuit, or an output obtained by mixing said frequency with the frequency of the first oscillator or with a frequency obtained through the multiplication or frequency division of the frequency of the first oscillator in a seventh mixer is input to said third mixer, and by conducting a variation of the total bandwidth of an intermediate-frequency stage and a shift of the center frequency by setting a reception band determined by the relationship between the set frequency-division ratio of a programmable frequency divider in the PLL circuit and the frequency of the first oscillator input a seventh mixer or a frequency obtained through the multiplication or frequency division of said frequency, by tuning a received frequency determined by the frequency of the VFO, and by varying the frequency of the second or third oscillator or by simultaneously varying the frequencies of the second and third oscillators.
The filter coefficients of a digital band pass elliptic filter can be approximated to a linear function of the center frequency of the pass band width. The filter coefficients of a band pass digital filter having a predetermined center frequency and the slope of this linear function are stored in first and second memories. A central processing unit (CPU) obtains the difference between a desired center frequency and a predetermined center frequency. A multiplier multiplies the slope stored in the second memory by the difference obtained by the CPU. An adder adds each of the filter coefficients stored in the first memory to an associated one of the products output from the multiplier. The adding results become the filter coefficients for obtaining the desired center frequency. Those filter coefficients are set in a digital filter.
A system is disclosed for transmitting and receiving signals. The system includes the use of a frequency plan table a system for creating the frequency plan table. The frequency plan table relates carrier frequency channels to the operation of a synthesizer and a plurality of programmable frequency dividers in a locked loop. In a transmitter, a first programmable frequency divider accepts a reference signal and produces a comparison signal. A mixer accepts the reference signal and a transmission signal and produces a loop signal. A second programmable frequency divider accepts the loop signal and produces a loop signal having a divided intermediate frequency signal. A phase detector compares the comparison signal and the loop signal having a divided intermediate frequency and produces an output that controls a variable controlled oscillator. The variable controlled oscillator produces a modulated transmission signal.
In a double conversion tuner for a television receiver including a first local oscillator and a first mixer for converting the RF signal corresponding to a selected channel to a first IF signal and a second local oscillator and a second mixer for converting the first IF signal to a second IF signal, control apparatus is coupled to one of the local oscillators for offsetting its frequency and correspondingly the carriers of the first IF signal relative to the passband of a first IF filter coupled between the two mixers. In this manner the amplitude or phase of the carriers of the first IF signal can be controlled. An AFT signal is coupled to the other one of the local oscillators to compensate for the frequency offset of the carriers of the first IF signal in order to tune the carriers of the second IF signal to their nominal frequencies.
The filter coefficients of a digital band pass elliptic filter can be approximated to a linear function of the center frequency of the pass band width. The filter coefficients of a band pass digital filter having a predetermined center frequency and the slope of this linear function are stored in first and second memories. A central processing unit (CPU) obtains the difference between a desired center frequency and a predetermined center frequency. A multiplier multiplies the slope stored in the second memory by the difference obtained by the CPU. An adder adds each of the filter coefficients stored in the first memory to an associated one of the products output from the multiplier. The adding results become the filter coefficients for obtaining the desired center frequency. Those filter coefficients are set in a digital filter.
A method of reducing terrestrial interference is provided for a TVRO earth station receiver comprising a super-heterodyne circuit including a voltage-controlled oscillator (VCO), a mixer for combining incoming first IF signals with the output of the VCO to reduce the frequency of the first IF signals to a second IF range having a predetermined nominal center frequency, a pass band filter for filtering the second IF signals and having a pass band centered on the nominal second IF center frequency, and a microprocessor for controlling the input voltage to the VCO and for automatically setting the controlling input voltage at prescribed levels in response to a "channel select" command signal, so that the VCO produces an output frequency which centers the received signal for a selected channel on the nominal second IF center frequency. The method includes offsetting the controlling input voltage from the prescribed level for a selected channel to offset the actual second IF center frequency relative to the center of the pass band of the filter, in a direction that removes at least a portion of the terrestrial interference from the pass band.
