A RECEIVER FOR SPECTRUM ANALYSIS

Abstract: A receiver for spectrum analysis comprises at least one mixer (230), in which a radio frequency (f1) input signal to be measured is mixed with an oscillator frequency (f2) generated by a voltage-controlled oscillator. In a detector (214), the level of an input signal to be measured is detected, after said signal has been transferred onto a lower frequency. The use of a logarithmic amplifier may be avoided by directing the detected signal to a decision means (215), which, in response to a change in the level of the detected input signal, provides a control voltage (Vcntrl) proportional to the magnitude and the direction of the change. Said voltage commands an adjustable attenuator (210) to attenuate the signal to be measured so that the level of the detected input signal substantially remains constant. The attenuation of the attenuator measured in decibels is linearly dependent on the control voltage, so the values of the control signal directly represent the amplitude values.

Description: A receiver for spectrum analysis This invention relates to a receiver for meas¬ uring a spectrum of a radio-frequency input signal, comprising means for generating a local oscillator sig¬ nal, a mixer in which the input signal and the local oscillator signal are mixed for producing an intermedi¬ ate frequency signal, and a detector for detecting an envelope of the input signal that has been transferred onto the intermediate frequency. In the spectrum analysis, energy distribution of an electric signal is studied as a function of fre¬ quency by means 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 for measuring atten¬ uation, FM deviation and frequency, as well as for stu¬ dying 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 oscill¬ ator. 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 ver¬ tical deflection plates of a cathode-ray tube, whereas said saw-tooth voltage is directed to the horizontal de- flection plates. As a result, an amplitude is drawn on a display as a function of frequency. In older spectrum analyzers, the detector and the video amplifier were combined in the same block. Figure 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 2 . The oscillator frequency f 2 is generated in a YIG oscillator block A, which comprises an oscillator 4 and its control circuits 5 and 6. The frequency f 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 success¬ ive mixer stages 9 and 10 using fixed oscillator fre¬ quencies. The resolution band may thus be made narrower. After amplification carried out in a chain 11 of switchable intermediate frequency amplifiers, band-pass filtering performed in a filter 12 with an adjustable pass band, and amplification carried out in an inter¬ mediate 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 logar¬ ithmic amplification are commercially available. Block 14 provides as an output the amplitude variation of the detected signal as a function of frequency and in acc¬ ordance with the logarithmic scale. The vertical de- flection voltage of the cathode-ray tube varies in response to the output of block 14, whereby the ampli¬ tude of the input signal is drawn on the tube as a func¬ tion of frequency in the decibel scale. In prior art spectrum analyzers, a video ampli- fier 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 price 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 price, but the drawback is a low precision. The dynamic range that may be achieved with prior art ana- lyzers is some 60 dB, which limits the uses of the ana¬ lyzer. Furthermore, video amplifier circuits may be very costly depending on the qualities, and the variation between analyzers caused in the production is rather great, and adjustments of different kinds must thus be carried out for reducing the variation. The third draw¬ back is caused by the fact that costly oscillator solu¬ tions, such as a YIG oscillator must be used since the requirements for the precision of an adjustable oscil¬ lator are high. The object of this invention is to achieve a receiver for spectrum analysis, suited for analysing a radio frequency signal, and to avoid the drawbacks of the prior art receivers. The object is specifically to measure the signal level as a function of frequency in a simple and reliable manner, without using a logar¬ ithmic amplification chain, and achieve a wide dynamic range, up to 100 dB. The object is achieved with a spectrum ana¬ lyzer, which is characterized in what is claimed in claim 1. The receiver of the invention comprises an adjustable attenuator and a first means for generating a control signal for adjusting its attenuation in response to the level of a detected signal so that the level of the detected signal remains unchanged, and a second means for recording the values of the control signal. In such a case, the values of the control signal represent amplitude values. In accordance with a preferred embodiment, the receiver comprises a microprocessor in which the values of the control signal are recorded and which processes them to be displayed in a display unit. The micropro¬ cessor may also carry out calibration of the adjustable attenuator. It may also be informed if the signal exceeds the dynamic range, whereby the operation of the receiver continues in a predetermined manner. In accordance with a preferred embodiment, the oscillator frequency is produced by means of a phase- locked loop, the division values of its dividers being provided by the above-mentioned microprocessor, whereby it is possible to generate the frequencies of the freq¬ uency range to be swept in desired frequency steps, and the frequency information, as well as the value of the attenuator control signal proportional to the amplitude value of the detected signal are recorded in the micro¬ processor. In the following, the invention will be dis¬ closed in greater detail by means of a preferred embodi¬ ment of the invention with reference to the attached drawings, in which Figure 1 shows a block diagram of a prior art receiver for spectrum analyses and Figure 2 shows a block diagram of a receiver for spectrum analysis in accordance with the invention. In the receiver of the invention, the method is partly the same as that in the prior art receiver, cf . Figure 2: an incoming RF signal RFIN to be studied is mixed with the oscillator frequency f 2 in a mixer 230 onto the intermediate frequency of a band-pass filter 280, whereafter detection is carried out in a detector block 214. An essential difference is that there is no logarithmic amplifier after the detector, but the level of the detected signal is studied in a detection block 215. An attenuator 210 attenuates or amplifies the input signal in response to the information provided by block 215 so that the level of the detected signal remains constant. The principle is similar to an AGC principle (AGC, Automatic Gain Control) applied in radio receivers. When the oscillator frequency f 2 is swept over a desired band and the adjustment values of the attenuator 210 are recorded in the memory, it is easy to form a spectral curve in the display unit by process¬ ing the adjustment values as they are proportional to amplitude values. In the following, the operation of the receiver is disclosed in greater detail with reference to Figure 2. An RF signal to be measured, having a frequency f, enters a filter 211, which limits the useful bandwidth of the signal to be measured within the right range. Thereafter, the signal is attenuated or amplified in an adjustable attenuator 210 so that the level of the detected signal remains constant after detector 214 at point B. The adjustable attenuator 210 may be implemen¬ ted in form of a chain of PIN diodes or a commercially available attenuator microcircuit. An important require¬ ment for the attenuator is the fact that its attenuation in decibels depends linearly on the control voltage Vc NTRL' i-e. the change V/dB is linear. A suitable change is e.g. lV/lOdB. For achieving the desired change in practice, it is necessary to use a number of attenuation stages, whereby the calibration voltage V Cabr may be applied to one attenuation stage. The calibration volt- age is obtained from a microprocessor 232 via a D/A converter 219 in a manner disclosed hereinafter. The frequency f, of the signal to be measured, attenuated/amplified in the attenuator 210, is trans- ferred onto a lower frequency by mixing it in the mixer 230 with a oscillator frequency f 2 signal, which is obtained from a voltage controlled oscillator 217. In accordance with a preferred embodiment, frequency f 2 is generated by means of a phase-locked loop PLL, the volt- age controlled oscillator 217 being a part of a phase- locked loop, the other components of which, such as a phase comparator, loop filter, reference oscillator and dividers are represented with a PLL block 231 only. Integrated circuits including said other components, that is, so-called PLL circuits, are commercially avail¬ able. As is generally known, a phase-locked loop forms a frequency synthesizer in a case where its divisors may be changed by means of software. The frequency to be synthesized with the loop, the so-called oscillator fre- quency f 2 is determined by the microprocessor 232 by feeding suitable divisors to a programmable loop divider (not shown) of the PLL block 231. When the divisors are changed, the control voltage V vco of VCO 217 changes, and the frequency generated by the VCO may thus be changed gradually over a desired frequency band. By generating the oscillator frequency with a phase-locked loop, an excellent precision of frequency and a remarkably lower price are achieved in the narrow frequency range used as compared with the YIG oscillator, for instance. An intermediate frequency signal is filtered from the mixing results in a band-pass filter 280. The filter thus limits the resolution to be measured. A signal to be measured, after being transferred onto a lower frequency, is directed to the detector 214, the output of which provides the envelope of the signal. Thereafter, the detected signal is directed to a deci¬ sion block 215, in which it is studied whether the level of the input signal is higher or lower than the values given at point B. If the level is higher or lower than the given values, the decision block 215 provides a control signal V CNTRL , which commands the attenuator 210 via a low-pass filter 216 to attenuate or amplify the input signal RFIN so that it remains on the desired level at point B. The value of the control signal V CNTRL represents the position of the attenuator 210, and that is exactly the information desired, because the vertical deflection of the display unit (not shown) is controlled according to it, in other words, the dB value of the signal may easily be calculated on the basis of said value on a specific frequency. If such an attenuator is used the attenuation of which measured in decibels depends linearly on the control voltage V CNTRL , calcula¬ tion is simple. The value of the control signal V CNTRL , which is the desired information, is converted into a digital form in the A/D converter 218 and recorded in the microprocessor 232. The processor also has a corres¬ ponding frequency information owing to the fact that it has set the oscillator frequency by feeding the divisors into the phase-locked loop. Therefore, frequency infor¬ mation (X coordinates) and dB information on attenuation (Y coordinates) are recorded in the processor, and the spectrum may thus be displayed in the display unit either immediately or later. The recorded data may also be processed easily either in real-time or by post-pro¬ cessing. The decision circuit 215 may be a comparator or an operational amplifier that compares the output of the envelope detector 214 with the reference level, or it may be an intermediate frequency circuit supplied with an RSSI detector, whereby the RSSI voltage of the circuit is maintained constant by adjusting the attenuator 210. A prior art filter used in spectrum analyzers may be used as a low-pass filter suitable for filtering the control voltage V CffrRL , which filter narrows the noise bandwidth of the signal entering the display of the spectrum analyzer, in this case the noise bandwidth of the signal entering the A/D converter 218. The decision circuit 215 also provides an enve- lope information, which is applied to the comparator 233. When the level of the envelope exceeds a given reference level, the comparator 233 provides the micro¬ processor 232 with an information on exceeding the ref¬ erence level, indicating that the dynamic range of the receiver has been exceeded, i.e. the signal to be measured goes beyond the measuring range . In response to this information, the microprocessor carries out a predetermined function, that is, it gives an alarm, for instance. It would also be possible to obtain the above- mentioned information from the control voltage informa¬ tion V CNTRL , that is, the information that the attenuator is in its extreme position, and attenuation thus cannot be increased any more. It is remarkably faster, however, to act on the information provided by the envelope. It is also possible to program a calibration table for the microprocessor 232. The table comprises loading information of a PLL circuit 231, that is, in¬ formation on the frequencies generated by it, and in¬ formation on the level error correction value, indicat- ing how much the position of a calibration attenuator, which is a part of the attenuator 210, must be adjusted to achieve the correct value of the input level. Cal¬ ibration control V Cahbr is converted into an analog form in the D/A converter 219 and applied to the attenuator 210. Calibration is carried out in the manufacturing of the receiver on the production line. It is also possible to implement a receiver arrangement in which an input gate is automatically connected to a calibration generator, which may be a comb generator or a noise generator, and the microprocessor 232 carries out the calibration phase by studying the input level as a function of frequency, and produces correction values of the information. If the filter 211 of the input signal of the receiver and/or the filter 280 of the mixing results are voltage-controlled filters, the control voltage V vco of the VCO 217 may be used for adjusting these filters, whereby the passband of the filters may be shifted in accordance with the signal frequency to be studied. This possibility is shown in Figure 2 with control lines leading to the filters. By means of a receiver for spectrum analysis of the invention, a simple and reliable measurement of the level of the signal is accomplished as a function of frequency without a costly logarithmic detector. It should be understood that the above explanation and the illustrations associated therewith are only intended to illustrate the present invention. Different variations and modifications of the invention will be obvious to persons skilled in the art without deviating from the scope and the spirit of the invention set forth in the attached claims. Therefore, the detector and the decis¬ ion circuit may be combined into one commercially avail¬ able circuit, such as an intermediate frequency circuit supplied with an RSSI circuit. This is illustrated in Figure 2 with a broken line surrounding the detection and decision block. The microprocessor may also combine temperature calibration with the calibration signal. In the case of the example, there is only one mixer stage, but, by increasing the number of the mixer stages, it is possible to achieve very narrow resolution bands. It is also possible to employ direct mixing, whereby th band-pass filter 280 is replaced with a low-pass filte and an amplifier, that is, an active filter. The direct mixing is, however, attended by the drawback that since the frequency of the VCO is the same as that of the input signal, the frequency of the VCO may leak into the input gate RFIN. The attenuator may also be placed afte the mixer at a suitable position prior to signal detect- ion.

Claims: Claims : 1. A receiver for spectrum analysis, comprising at least one mixer (230) , in which a radio fre- quency (f,) input signal to be measured is transferred onto a lower frequency by mixing it with an oscillator frequency (f 2 ) generated by a voltage-controlled oscill¬ ator (217) , a detector (214) for detecting the level of the input signal to be measured, after said signal has been transferred onto a lower frequency, c h a r a c t e r ¬ i z e d by further comprising: decision means (215) for providing, in response to a change in the level of the detected input signal, a control voltage (V..,.,) proportional to the magnitude and the direction of the change, an adjustable attenuator (210) , to which the signal to be measured is directed and which, in response to said control voltage (V..,,,) , accomplishes attenuation or amplification of a signal to be measured, coming from the output of the attenuator, so that the level of the detected input signal substantially remains constant, whereby the control voltage information is proportional to the level of the signal to be measured, and a control and recording means (232) for record¬ ing and processing the control voltage information and the oscillator frequency information. 2. A receiver as claimed in claim ^ c h a r a¬ c t e r i z e d in that the adjustable attenuator (210) is connected before the mixer (230) . 3. A receiver as claimed in claim ^ c h a r a¬ c t e r i z e d in that the decision circuit (215) comprises means for comparing the level of the detected input signal with a reference level and that the control voltage (V cmr: ) is proportional to the deviation of the level of the input signal from the reference level. 4. A receiver as claimed in claim ^ c h a r a¬ c t e r i z e d in that the voltage-controlled oscil- lator (217) is a part of a frequency synthesizer and that the control and recording means (232) directs the synthesizer to generate a desired oscillator frequency (f 2 ) . 5. A receiver as claimed in claim 1, c h a r a- c t e r i z e d in that the control voltage (V..,.,) is directed to an A/D converter (218) , whereby the digital value of the control voltage obtained from said A/D converter and directed to the control and recording means (232) is said control voltage information. 6. A receiver as claimed in claim ^ c h a r a¬ c t e r i z e d in that the receiver further comprises means (233) for monitoring the dynamic range, said means providing the control and recording means (232) with an exceeding information in response to the level of the detected input signal rising above a predetermined value . 7. A receiver as claimed in claim 1 or 2, c h - a r a c t e r i z e d in that the adjustable attenuator (210) comprises a number of adjustable attenuation stages, the control and recording means (232) adjusting attenuation of one stage by providing the stage with a calibration voltage . 8. A receiver as claimed in claim 4, c h a r a- c t e r i z e d in that the control voltage (V vco ) of the voltage-controlled oscillator (217) of the frequency synthesizer is also an adjusting voltage for at least one of the adjustable filters (211, 280) of the receiver. 9. A receiver as claimed in claim 1, c h a r a- c t e r i z e d in that the attenuation of the adjust- able attenuator (210) measured in decibels is linearly dependent on the control voltage (V cntr| ) . 10. A receiver as claimed in claim ^ c h a r ¬ a c t e r i z e d in that the control and recording means (232) is a microprocessor that processes the con¬ trol voltage information and the oscillator frequency information so that the amplitude of the signal to be measured as a function of frequency may be displayed with a display means.

Inventors: WECKSTROEM MIKKO, WECKSTROEM, MIKKO
Applicants: NOKIA TELECOMMUNICATIONS OY, WECKSTROEM MIKKO, WECKSTROEM, MIKKO
Application year: 1996
Main classification: G01R23
IPC codes: G01R23