Modulators – Frequency modulator – Including stabilization or alternatively distortion – noise...
Reexamination Certificate
1999-02-02
2001-01-09
Kinkead, Arnold (Department: 2817)
Modulators
Frequency modulator
Including stabilization or alternatively distortion, noise...
C331S017000, C455S260000, C455S076000
Reexamination Certificate
active
06172579
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, generally, to the field of radio frequency signal generation and wireless communications, and particularly to low power portable wireless communications, examples of which include cellular radio and Personal Communications Systems (PCS), point to point data communications, wireless data acquisition, and wireless automatic identification, remote control, and status reporting.
2. Description of the Related Art
The radio link performance for “MicroRadio™” equipment such as wireless data acquisition, local area data links, garage door openers, vehicle toll tags, RF auto ID, and remote keyless entry systems is often limited by the high emphasis placed upon small size and low cost. In recent years, designers of portable communications equipment such as cellular and PCS handsets have also been under severe pressure to reduce size, cost, and power consumption. Progress in equipment design in these fields requires advances in performance where the necessary low cost and size are still maintained. MicroRadio is a trademark of Cleveland Medical Devices Inc.
Early radio equipment used frequency control based on free running (not contained in a control loop) inductor/capacitor (LC) resonator oscillators. The equipment was plagued with frequency stability problems, and had no facility for anything other than manual tuning. With the advent of software controlled phase locked loop (PLL) frequency synthesizers, a stable, frequency agile, software controlled RF source became available. Advances in electronic technology, and particularly in integrated circuit design, has resulted in size reduction to the point of high portability. These compact synthesizers allow transmitters and channel defining local oscillators in receivers to be set via software control to any of a large number of frequencies uniformly distributed across the band of interest. The most recent advances make feasible the application of frequency synthesis to MicroRadio systems provided that modulation can be provided for without unduly increasing the size and power consumption.
There is a long history of attempts to directly modulate PLL synthesizers, with varying degrees of efficiency and success. The structure and operation of a PLL makes high performance efficient modulation appear quite feasible. The PLL has within it an Electronically Controlled Oscillator (ECO), which typically uses voltage as the frequency control variable and is thus usually referred to as a Voltage Controlled Oscillator (VCO). Although for the sake of convenience “voltage” is often referred to as the signal or control variable, it is understood that current or another electronic variable could be substituted. The generation of the precise voltage (or other electronic variable as the case may be) needed to steer the VCO to the desired frequency is the main function of the system. Simply summing a modulating voltage into the resulting steering voltage will frequency modulate the PLL around the carrier frequency, but results in serious difficulties. The primary problem revolves around the fact that the PLL views the modulation as error that it must correct, resulting in distortion of the desired modulation. A long patent history testifies to the work that has been invested in attempting to effectively solve this elusive problem, the net result of which has been only a partial success.
U.S. Pat. No. 3,393,380 granted to Webb and assigned to the National Aeronautics and Space Administration, PHASE LOCKED PHASE MODULATOR INCLUDING A VOLTAGE CONTROLLED OSCILLATOR, 1968, presents the earliest example of frequency modulation of a phase locked loop. This simple early form injects the modulation into the phase detector output, which has the disadvantages of limited modulation rate due to the filtering action of the subsequent low pass loop filter, as well as allowing distortion to be introduced by the response of the phase locked loop to the modulation. It did, however, represent an advance on the frequency modulation techniques used at the time. The intended application was space communications. No method of preventing the phase locked loop from distorting the desired FM is presented.
A fundamental method of improving the modulation capability of a PLL is to modulate its reference frequency in sympathy with its electronically controlled oscillator. The goal of this is to prevent the loop phase detector from detecting the modulation as frequency error, and leading to the PLL distorting the modulation through its closed loop action of driving all frequency error to zero. There does not appear to be any existing patent on this method, but it has been known in the art for many years. For example, it is implied in U.S. Pat. No. 4,052,672 granted to Enderby et al. in 1977, and described in detail without reference to any patents as well known prior art by Drucker in U.S. Pat. No. 4,313,209, filed in 1980 and issued in 1982. The method is limited by the low modulation bandwidth of practical voltage controlled crystal oscillators (VCXOs) normally used as reference frequency sources. Again, it is noted that a frequency steerable reference frequency source that is crystal based is in more general terms described as an Electronically Controlled Crystal Oscillator (ECXO), and that in the most general terms where the reference source is not necessarily crystal based it may be referred to as an Electronically Controlled Reference Source (ECRS). Unmentioned in previous patents, but shown in detail herein, is that for low distortion modulation the normally crystal based reference oscillator modulation bandwidth must greatly exceed the PLL loop bandwidth. Since the modulation bandwidth of crystal oscillators is very limited, the PLL loop bandwidth becomes extremely limited. This results in long lock times and elimination of the benefits of wider loop bandwidths, such as suppression of vibration induced noise. The nonlinearity of the common VCXO is a source of large distortion in this method, particularly for wider frequency deviations. However, practical linearizing techniques to deal with the high nonlinearity of typical VCXO's are not commonly known in the art.
U.S. Pat. No. 4,052,672, EXTENDED PHASE-RANGE, HIGH FIDELITY MODULATION ARRANGEMENT issued to Enderby in 1977 and assigned to Motorola, Inc., presents another fundamental improvement to the modulated PLL that, like the modulated reference method, has the purpose of eliminating phase detector output that would normally result from modulation. A “correcting integrator” is added to the system that generates a voltage of equal magnitude and opposite sign to the phase detector output component due to modulation applied to the VCO. This correcting integrator output is summed with the phase detector output to cancel loop response to modulation. This breakthrough concept is quite elegant in the way it attacks the root of the problem, and subsequently evolved into the major technique applied for frequency modulating the phase locked loop synthesizer. Numerous subsequent patents have incorporated this excellent basic technique. The method shall be referred to herein as the “two point correcting integrator form”, as modulation is applied to the correcting integrator and the voltage controlled oscillator, or as the “Enderby form”. While the Enderby form is effective and highly circuit efficient, it does not allow DC modulation due to the fact that build up of phase error associated with DC modulation will eventually cause either the correcting integrator or the phase detector to exceed their dynamic ranges, at which point the PLL will either lose lock or lose the effect of the correcting integrator. Also not mentioned is the fact that with digital phase detectors the encoding of phase error as pulse width introduces noise and distortion that may set severe limits on the low frequency content of the modulation or on the allowed phase locked loop bandwidth.
The Enderby two point correcting integrator form is incorporated and extended in numerou
Dacus Farron L.
Hendrix Steven P.
Cleveland Medical Devices Inc.
Kinkead Arnold
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