Will not go into the merits inherent in the specification or the detailed characteristics of a spectrum analyzer, but I will try to introduce only the elements relevant to the execution of a measure of power (assuming no measurement options normally found in these types of tools Already for some years, such as the measurement of called "Channel Power").
The spectrum analyzers, contrary to power meters (also called bolometers), measure signals in the frequency domain and on a specific portion of band configurable.
Fundamental elements that characterize the result will be reflected in the measure are the resolution bandwidth (Resolution Bandwidth (RBW)), the Band Video (Video Bandwidth (VBW)), the speed sweep (sweep), and of course, the intrinsic specifications of the instrument.
Fundamental elements that characterize the result will be reflected in the measure are the resolution bandwidth (Resolution Bandwidth (RBW)), the Band Video (Video Bandwidth (VBW)), the speed sweep (sweep), and of course, the intrinsic specifications of the instrument.
Therefore you can easily misinterpret the readings, as it is very dependent on the configuration of the instrument (and its characteristics).
Consider the following:
If we measure a signal with very narrow bandwidth (essentially a pure unmodulated carrier, CW)with a power of -30 dBm, the reading with bolometer (measuring instrument broadband) will correspond to the same value; the same signal (CW) measured with a spectrum analyzer set at a ratio RBW / VBW = 1 will give approximately the same value of the bolometer and that is - 30 dBm.
In this case in fact the width of the carrier band CW is so small, therefore the risk of error in the reading on the analyzer is minimized.
If we measure a signal with very narrow bandwidth (essentially a pure unmodulated carrier, CW)with a power of -30 dBm, the reading with bolometer (measuring instrument broadband) will correspond to the same value; the same signal (CW) measured with a spectrum analyzer set at a ratio RBW / VBW = 1 will give approximately the same value of the bolometer and that is - 30 dBm.In this case in fact the width of the carrier band CW is so small, therefore the risk of error in the reading on the analyzer is minimized.
If instead we measure a signal having a bandwidth of more substantial (eg a modulated signal in phase with bandwidth of 30 MHz), the bolometer will always give us the same value, while the spectrum analyzer will give us the reading given by "+ Prx | J0 |. "
Where J0 is the error due to the distribution of power density that depends on the ratio between the bandwidth of the signal to be measured and the resolution bandwidth set on the analyzer used for the measurement.
In our case the error potra be approximately calculated with the following formula:
Assuming the example above, and a 3 MHz RBW (RBW = VBW with), we have:
For which we will read in a video signal with a power of less than 10 dB compared to the actual value of power read with a "Bolometer", therefore:
If the actual signal is - 30 dBm spectrum analyzer can give me a reading of about - 40 dBm.
Naturally more shake bandwidth and most get worse the spectral density ratio between the signal to be measured and the bandwidth used to measure it. By way of example, with the same signal but with a RBW of 300 Khz will be calculated approximately 20 dB of error.
There is, therefore, to consider the use in some cases, a bit 'naive and misleading that is made of spectrum analyzers, especially when they are to be used as a power meter.
By way of example, if one analyzes a signal having the characteristics:
Where J0 is the error due to the distribution of power density that depends on the ratio between the bandwidth of the signal to be measured and the resolution bandwidth set on the analyzer used for the measurement.
In our case the error potra be approximately calculated with the following formula:
Assuming the example above, and a 3 MHz RBW (RBW = VBW with), we have:For which we will read in a video signal with a power of less than 10 dB compared to the actual value of power read with a "Bolometer", therefore:If the actual signal is - 30 dBm spectrum analyzer can give me a reading of about - 40 dBm.Naturally more shake bandwidth and most get worse the spectral density ratio between the signal to be measured and the bandwidth used to measure it. By way of example, with the same signal but with a RBW of 300 Khz will be calculated approximately 20 dB of error.
There is, therefore, to consider the use in some cases, a bit 'naive and misleading that is made of spectrum analyzers, especially when they are to be used as a power meter.
By way of example, if one analyzes a signal having the characteristics:
- Center Frequency = 7471.00 MHz
- Span = 56 MHz
- Ref: -74 dBm
- dB / div = 3 dB
- RBW = 10 kHz
- VBW = 10 Khz
- Marker = Measured Signal Level - 94 dBm
Assuming a measure applied to a reference signal of 14 MHz of bandwidth (typically an 8 PSK modulation on a radio system PDH), we will have that the real value of equivalent power will be:
For which the level that displays the analyzer will be lower than 31.46 dB with respect to the real value. The actual value will be - 62.54 dBm.
The above also becomes essential if the goal of the measure is to seek real signals of the order of -90 dBm. In fact, with the above described conditions will be impossible only with the spectrum analyzer to measure signals with intensity so low.
For which the level that displays the analyzer will be lower than 31.46 dB with respect to the real value. The actual value will be - 62.54 dBm.The above also becomes essential if the goal of the measure is to seek real signals of the order of -90 dBm. In fact, with the above described conditions will be impossible only with the spectrum analyzer to measure signals with intensity so low.
In case of search for signals modulated by a large bandwidth, is not expedient to reduce the resolution bandwidth of the instrument, but should keep it out of relatively large values and act on the sensitivity of the measurement, by using low noise amplifiers (LNA Low Noise Amplifier) with low level of intermodulation and a high gain antenna. The use of antennas with a gain of at least 15 dB and low noise amplifiers with amplification medium of about 40 dB then make the checks possible.
In the calculation of the received power equivalent, in the case of checking interference must be considered as a value of Band B, the following rules.
In the calculation of the received power equivalent, in the case of checking interference must be considered as a value of Band B, the following rules.
- If you do the calculation and the radio link to be used, use the bandwidth of the system to be installed.
- In case you want to make a specific measure of power of a signal, in this case one must consider the overall bandwidth of the signal being analyzed.
In interferential, total estimates of the interfering power () will therefore:
Where:
The above can be considered valid with an accuracy of a few dB to 1 but maintaining the ratio between RBW / VBW (ie assuming RBW = VBW).
Otherwise you can easily introduce errors of assessment even of the order of a few tens of dB.
Useful demos and remote control of a spectrum analyzer, Agilent is available on the website:
Feature Sensitivity test bench and in the analyzer bench Measure for Research interference, the quality of the amplifier LNA and the spectrum analyzer have a major impact on the overall quality of the measures.
In a generic way we can verify that the sensitivity instrinseca of the spectrum analyzer at a given resolution of Banda (RBW) turns out to be:
Where "Nsa (RBWrif)" and the average sensitivity of noise stated by the manufacturer to a given value of RBW reference (Displayed average noise level).
By way of example for spectrum analyzers Agilent HP 4408B ESA the reference values may be:
In a generic way we can verify that the sensitivity instrinseca of the spectrum analyzer at a given resolution of Banda (RBW) turns out to be:
Where "Nsa (RBWrif)" and the average sensitivity of noise stated by the manufacturer to a given value of RBW reference (Displayed average noise level).By way of example for spectrum analyzers Agilent HP 4408B ESA the reference values may be:
- 10 MHz to 1.0 GHz ≤ -116 dBm
- 1 GHz to 2.0 GHz ≤ -115 dBm
- 2 GHz to 6.0 GHz ≤ -112 dBm
- 6.0 GHz to 12.0 GHz ≤ -110 dBm
- 12.0 GHz to 22.0 GHz ≤ -107 dBm
- 22.0 GHz to 26.5 GHz ≤ -101 dBm
The above measured with a RBW reference at 1 KHZ and VBW of 30 Hz and a level of -70 dBm.
If we consider, therefore, that the measuring bench for interfering research will consist of an antenna and an amplifier Horn reference LNA, more of the same spectrum analyzer, interference minimum level will be measured:
Conclusion
Let us remember that in case of measurements of modulated signals the power value displayed by the instrument will be lower than the real power; there will be to perform a correction that will depend on the bandwidth used by the analyzer to measure the signal and the real bandwidth of the signal (or band equivalent to consider).
In order to correct this error, the value of Pm (Power measured) will be:
Where:
Let us remember that in case of measurements of modulated signals the power value displayed by the instrument will be lower than the real power; there will be to perform a correction that will depend on the bandwidth used by the analyzer to measure the signal and the real bandwidth of the signal (or band equivalent to consider).
In order to correct this error, the value of Pm (Power measured) will be:
Where:- Pm = equivalent power measured (basically the input power).
- Prx = equivalent power reading on the monitor of the instrument (average value measured over 100 points).
- B = equivalent bandwidth RF signal on which the study is carried interferential (specific band of radio product) or bandwidth of the RF signal measured
- RBW = resolution bandwidth set on the spectrum analyzer
The formula above, allow with a good approximation to calculate the effective power in ingesso to the instrument. The above is true and only the values of RBW and VBW are equal (in the case other than the error could also be of some tens of dB).
One must also consider the lines of amplification and attenuation of the bench.
Whereas the test bench for a search interfering is composed of elements Amplifiers (antenna and amplifier) and elements Attenuators (cables and adapters), considering negligible errors due to other factors, the equivalent value of Pi interfering power (dBm) measured in the different sections of measurement, is determined using the following basic formula:
where:
Pm = equivalent power measured.
Ga = antenna gain reference.
Glna = Gain Low Noise Amplifier.
Ac = total attenuation of the measurement leads and transitions.
Whereas the test bench for a search interfering is composed of elements Amplifiers (antenna and amplifier) and elements Attenuators (cables and adapters), considering negligible errors due to other factors, the equivalent value of Pi interfering power (dBm) measured in the different sections of measurement, is determined using the following basic formula:
where:Pm = equivalent power measured.
Ga = antenna gain reference.
Glna = Gain Low Noise Amplifier.
Ac = total attenuation of the measurement leads and transitions.
Thank you to my friend and big Telecom Technician Gaetano Elifani for the documents.
Nessun commento:
Posta un commento