Carrier buffer having current-controlled tracking filter for spurious signal suppression
First Claim
1. A carrier frequency generator for a communication device having a frequency synthesizer employing a current-controlled oscillator, which generates an output frequency in accordance with a control current therefor, said frequency synthesizer having a phase locked loop containing said current-controlled oscillator, said carrier generator being driven by said frequency synthesizer and including a tunable tracking filter stage, operational characteristics of which are controllable in accordance with said control current, such that said tunable filter stage has a cut-off frequency that is linearly proportional to said control current, whereby said frequency of the tracking filter effectively tracks said carrier frequency.
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Accused Products
Abstract
Spurious energy suppression for a data communication system is achieved without using a large order noise suppression filter, by means of a pre-mixer tracking filter incorporated into an emitter-coupled logic configured buffer of a carrier frequency generator, using a MOSFET-implemented current-controlled resistance component of a resistor-capacitor network and an associated current control stage. The MOSFET-implemented resistance components of the filter are controlled by the same control current that establishes the carrier generator'"'"'s output frequency. As a result, the cut-off frequency of the tracking filter is linearly proportional to the carrier and effectively independent of process parameters.
18 Citations
16 Claims
- 1. A carrier frequency generator for a communication device having a frequency synthesizer employing a current-controlled oscillator, which generates an output frequency in accordance with a control current therefor, said frequency synthesizer having a phase locked loop containing said current-controlled oscillator, said carrier generator being driven by said frequency synthesizer and including a tunable tracking filter stage, operational characteristics of which are controllable in accordance with said control current, such that said tunable filter stage has a cut-off frequency that is linearly proportional to said control current, whereby said frequency of the tracking filter effectively tracks said carrier frequency.
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6. A method of controlling the cutoff frequency of a carrier frequency generator having its operational frequency defined in accordance with a resistor-capacitor network thereof comprising the steps of:
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(a) configuring a resistance component of said resistor-capacitor network as a tunable resistance which varies non-linearly with a control current; and (b) controlling the value of said tunable resistance in accordance with said control current. - View Dependent Claims (7, 8)
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- 9. A carrier frequency generator comprising an emitter-coupled logic configured buffer formed of a matched differential bipolar transistor pair having their collectors coupled to controlled MOSFET resistors of a resistor-capacitor tuning network, their base electrodes coupled to receive complementary square wave drive signals from a controlled oscillator of a phase locked loop, and their emitter electrodes coupled to a current control loop, and wherein said MOSFET resistors are controlled by a control current that establishes the output frequency of said controlled oscillator.
Specification