Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Patent
1997-09-18
1999-06-29
Do, Diep N.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
324 7626, G01R 2316
Patent
active
059173157
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a receiver which measures a spectrum of a radio-frequency input signal. The receiver includes a local oscillator signal generator, a mixer in which the input signal and the local oscillator signal are mixed to produce an intermediate frequency signal, and a detector which detects an envelope of the input signal that has been transferred onto the intermediate frequency.
BACKGROUND OF THE INVENTION
In spectrum analysis, energy distribution of an electric signal is studied as a function of frequency of a spectrum analyzer, which allows graphic representation of the amplitude as a function of frequency in a portion of the spectrum. The analyzer may be used as a sensitive receiver to measure attenuation, FM deviation, and frequency, as well as to study RF pulses. A simple spectrum analyzer is based on a superheterodyne receiver and an oscilloscope. In the receiver, the input signal is mixed in the mixer with a frequency obtained from a voltage-controlled oscillator. The control voltage of the oscillator is a saw-tooth voltage, whereby the frequency of the oscillator sweeps over a certain frequency range. An intermediate frequency signal, obtained as the mixing result, is amplified and detected. The detected signal, which is amplified in a video amplifier, is directed to the vertical deflection plates of a cathode-ray tube and the saw-tooth voltage is directed to the horizontal deflection plates. As a result, an amplitude is shown on a display as a function of frequency. In older spectrum analyzers, the detector and the video amplifier were combined in the same block.
FIG. 1 shows a simplified block diagram of a prior art spectrum analyzer primarily intended for monitoring. An incoming RF signal is attenuated in an attenuator 1, whereafter it is low-pass filtered in a filter 2 before mixing it in a mixer stage 3 with a oscillator frequency f.sub.2. The oscillator frequency f.sub.2 is generated in a YIG oscillator block A, which comprises an oscillator 4 and its control circuits 5, 6. The frequency f.sub.2 of the YIG oscillator varies as a function of a ramp voltage obtained from a ramp generator 7, whereby the oscillator frequency sweeps over a desired frequency band. The horizontal deflection voltage of a cathode-ray tube CRT changes in response to a change in the voltage of the ramp generator 7. From the mixing results obtained in the mixer 3, a desired frequency is filtered in an adjustable band-pass filter 8, whereafter the signal frequency is further lowered in two successive mixer stages 9, 10 using fixed oscillator frequencies. The resolution band may thus be made narrower. After amplification carried out in a chain of switchable intermediate frequency amplifiers 11, band-pass filtering performed in a filter 12 with an adjustable pass band, and amplification carried out in an intermediate frequency amplifier 13, the signal is applied to a block 14 for performing a logarithmic conversion. In this block, detection is also performed. Combined component packages that carry out detection and logarithmic amplification are commercially available. Block 14 provides as an output the amplitude variation of the detected signal as a function of frequency and in accordance with the logarithmic scale. The vertical deflection voltage of the cathode-ray tube varies in response to the output of block 14, whereby the amplitude of the input signal is drawn on the tube as a function of frequency in the decibel scale.
In prior art spectrum analyzers, a video amplifier is employed. The name originates from a detector in which a logarithmic amplifier, that is, a chain of saturated amplifiers with a limiter output is used after or before envelope detection. The precision of a video amplifier of this kind is high, but its cost is high. An IF amplifier circuit with an RSSI (Received Signal Strength Indicator) circuit at the output may also be used. The advantage of amplifiers applying this solution is a low cost, but the drawback is low precision. The dynamic range that may be a
REFERENCES:
patent: 3876946 (1975-04-01), Le Clair et al.
patent: 4430611 (1984-02-01), Boland
patent: 4607215 (1986-08-01), Takano et al.
patent: 5262957 (1993-11-01), Heam
patent: 5731841 (1998-03-01), Rosenbaum
Do Diep N.
Nokia Telecommunications Oy
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