Data processing: measuring – calibrating – or testing – Testing system – For transfer function determination
Reexamination Certificate
1999-05-17
2002-05-21
Grimley, Arthur T. (Department: 2852)
Data processing: measuring, calibrating, or testing
Testing system
For transfer function determination
C702S111000, C702S069000, C330S149000
Reexamination Certificate
active
06393372
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to devices which measure noise present in radio frequency (RF) signals. More specifically, the present invention relates to noise measurement test systems for making phase noise and frequency noise measurements of devices that amplify or modify an RF signal.
Amplifiers are devices that increase the gain of a carrier signal. Amplifiers play a key role in many electronic systems today. Parameters such as gain, gain flatness, compression point, intermodulation and others have always been important to optimizing the performance of an amplifier. In addition, new parameters of an amplifier have become important to maintain the performance of an overall system. Unfortunately, in addition to amplifying the carrier signal, amplifiers typically introduce RF energy into an original RF signal in the form of intermodulation and sideband noises, including thermal noise, shot noise and flicker noise. This noise is typically random and is referred to as additive and residual phase noise and frequency noise. In addition, spurious noise signals can be generated by an amplifier. These consist of discreet signals appearing as distinct components called “spurs” which can be related to power line and/or vibration modulation. It is important that this noise be minimized to the greatest extent possible. Since frequency noise is a function of phase noise, these noise components will be collectively referred to herein as phase noise.
The presence of phase noise in RF signal sources is a concern in several applications including applications related to analog and digital communications such as code division multiple access (CDMA) and time division multiple access (TDMA) cellular communication systems. The European Global System for Mobile communications (GSM) has promulgated detailed standards which define the operating requirements for both mobile and base station transmitters because the radio communication system as a whole will work properly only if each component operates within precise limits. Mobile transmitters and base stations must transmit outgoing RF signals with sufficient power, sufficient fidelity to maintain call quality and without transmitting excess power into frequency channels and time slots allocated to others.
For communications equipment particularly, it is important to minimize spurs and distortion products even when they appear below the amplitude level of the signal produced by an amplifier. This is important because the distortion or noise from numerous communication modules in a communications system tend to statistically add, thereby raising the level of noise in the overall system. Accordingly, it has now become important to measure the phase noise levels of an amplifier even below the level of the signal produced by the amplifier.
Phase noise is also of great concern in radar equipment, especially Doppler radar equipment which determines the velocity of a moving target by measuring the shifts in frequencies caused by return echos from a transmitted signal. The return echo signal is typically amplified for measurement. Unfortunately, a very large noise floor, up to 40 dB, has been observed near the carrier frequency. Such high background noise caused by the amplifiers of the radar equipment can result in degradation in target detection sensitivity and prevention of proper operation of land based and airborne active array radar. In some instances, it has been found that the introduction of phase noise caused by the amplifiers can partially or even totally mask the echo signal.
The most straight forward and least expensive technique for measuring the phase noise of an amplifier is to input a signal of known frequency into the amplifier and connect the output to a spectrum analyzer. However, it is difficult to measure the phase noise which is close in frequency to the carrier signal. In addition, using this technique, it is impossible to measure the noise caused by the amplifier which is below the amplitude of the amplifier output signal. In order to minimize the amount of noise generated by a particular system, it is highly advantageous to be able to measure that noise. As a result, there has been a continuing need for equipment which can make phase noise measurements.
Two approaches predominate for making phase noise measurements of an RF signal. The first system is a noise measurement test apparatus that uses a waveguide delay line descriminator. For example, U.S. Pat. No. 5,608,331 issued to Newberg et al. discloses a test system for making phase noise and amplitude noise measurements of microwave signals using a waveguide, coax and fiberoptic delay lines. The delay line descriminator uses the RF input from a unit under test (UUT) to generate a reference signal via the delay line for phase noise evaluation. The signal from the unit under test is split into first and second paths and combined again at a mixer which places the respective signals 90° out of phase (in phase quadrature). Where the test system introduces very low noise or substantially no noise, the mixer outputs demodulated phase noise which can be measured by a sweeping spectrum analyzer.
The second conventional approach for making phase noise measurements makes use of the combination of noise from two phase lock RF sources. A low noise source is provided which provides a carrier signal to a unit under test, typically an amplifier. The low noise source also outputs a second low noise signal, at the same frequency as the carrier signal, which is combined with the carrier signal from the amplifier in a mixer. Using a phase shifter, the mixer places the two signals in phase quadrature.
Assuming no significant noise in the test setup, the mixer output signal represents the noise of the unit under test which can be measured by a sweeping spectrum analyzer.
Unfortunately, measurement of modulation and wide band noise measurements using prior art systems is expensive, difficult and time consuming. At wide offsets, such as 600 kHz, these measurements require high dynamic range which has historically been expensive. For these reasons, wide band noise measurements are typically only performed on a sample basis. Even conducting noise measurements on a sample basis is extremely time consuming as this technique requires a series of separate measurements to be performed requiring a lot of retuning of the test equipment. For example, utilizing the delay line technique described above, one must adjust phase shifters, attenuators and additional amplifiers at several frequency assignments across a broad bandwidth. Similarly, the above-described ultra low noise source test system typically requires making manual adjustments of a low noise source, a phase shifter, an additional amplifier and a buffer for each frequency offset across a broad bandwidth. The manual adjustment of each of these units usually takes ten minutes or more for each test measurement. Moreover, to ensure test accuracy, a large number of test samples must be taken with the increase in the number of sample measurements resulting in a decrease in the standard deviation in error of the noise measurements of the unit under test. Simply, sufficient readings must be taken to verify correct operation of the unit under test.
Unfortunately, performing such tests is both expensive and time consuming. Moreover, the prior art test systems are very expensive, typically costing between $100,000 and $200,000. In addition, these systems must be purchased piecemeal, requiring the separate purchase of amplifiers, low noise sources, signal attenuators, phase shifters, mixers and sweeping noise analyzers, which must be assembled to create a desired test system. The interconnection on each of these components creates additional regions for the introduction of phase noise into the system.
There is thus a need for a noise measurement test system which can accurately measure low level phase noise.
It would also be advantageous if a noise measurement test system were provided which had a high degree of reproducibility in the
Charioui Mohamed
Drummond & Duckworth
Grimley Arthur T.
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