RBW bandwidth and VBW bandwidth of the spectrum analyzer

RBW (Resolution Bandwidth) represents the difference in bandwidth between two signals of different frequencies that can be clearly distinguished. If the bandwidth of two signals of different frequencies is lower than the RBW of a spectrum analyzer, the two signals will overlap and it is difficult to distinguish. .
2. The resolution bandwidth is also called the reference bandwidth, which indicates how much bandwidth power is being tested. For example, when testing a GSM 2W dry-amplified single-carrier output with full power, the RBW is set to 100KHz to measure 30dBm, and it is set to 200KHz to measure 33dBm. RBW is actually the bandwidth of the internal filter of the spectrum analyzer. Setting its size can determine whether two closely adjacent signals can be separated. Its setting has an influence on the test result. The reading is accurate only when the RBW is set to be greater than or equal to the working bandwidth, but if the signal is too weak, the spectrum analyzer cannot distinguish the signal. At this time, even if the RBW is larger than the working bandwidth, the reading will be inaccurate.
VBW: display bandwidth-a wider frequency range can be seen during the test. If the signal to be observed is finer, it needs to be reduced; RBW: analysis bandwidth; for example, the power of the test CDMA, neither too large nor too small , Should correspond to the bandwidth of the signal; as well as test link noise, etc., there are also certain requirements for RBW.
RBW, resolution bandwidth, also called reference bandwidth by some, indicates how much bandwidth power is being tested. For example, when testing a GSM 2W dry-amplified single-carrier output, when the RBW is set to 100KHz, it measures 30dBm, and when it is set to 200KHz, it measures 33dBm. . RBW is actually the bandwidth of the internal filter of the spectrum analyzer (it is the 3dB bandwidth of the IF filter). Setting its size can determine whether to separate two close signals. Its setting has an influence on the test result. The reading is accurate only when the RBW is set to be greater than or equal to the working bandwidth, but if the signal is too weak, the spectrum analyzer cannot distinguish the signal. At this time, even if the RBW is larger than the working bandwidth, the reading will be inaccurate. VBW, video bandwidth, indicates the accuracy of the test. The smaller the value, the higher the accuracy. For example, if VBW is set to 100KHz, it means that a sample is taken every 100KHz to test its level. Therefore, it can be seen that the smaller the VBW setting, the smoother the test curve.
VBW is the filter bandwidth after peak detection, mainly to make the test signal more smooth. It is also a 3dB bandwidth. Other manufacturers have 6dB bandwidth. RBW is much more important than VBW. Generally HP recommends VBW<0.01RBW. But many experiments show that VBW=0.1RBW is reasonable.
RBW is the resolution bandwidth, and VBW is the video bandwidth. RBW should not exceed one tenth of SPAN. VBW can be set to automatic. If the signal is too weak, VBW can be reduced.
Adjust the RBW and the signal amplitude does not change significantly, and the RBW bandwidth at this time can be used.
A wider RBW can more fully reflect the waveform and amplitude of the input signal spectrum, but a lower RBW will be able to distinguish signals of different frequencies. That is to say, RBW represents the difference in bandwidth between two signals of different frequencies that can be clearly distinguished. If the bandwidth of two signals of different frequencies is lower than the RBW of a spectrum analyzer, the two signals will overlap and it is difficult to distinguish. A low RBW is certainly helpful for the resolution and measurement of signals at different frequencies. A low RBW will filter out higher frequency signal components, resulting in distortion when the signal is displayed. The distortion value is closely related to the set RBW. A higher RBW Although it is helpful to the detection of broadband signals, it will increase the noise floor value (Noise Floor), reduce the measurement sensitivity, and easily hinder the detection of low-intensity signals.
The following figure explains the effect of different RBW on sensitivity:


If you still don’t understand enough, you can look at the picture below to make it more vivid:


  Tips:
Ingeniously use the RBW of the spectrum analyzer to set the DUT transmit power (the high-end spectrum analyzer has ACLR test items, you can call the default mask test):
RBW is the bandwidth value of the internal IF filter of the spectrum analyzer, which means that the power value measured by the spectrum analyzer is measured based on this bandwidth. For example, if you set the RBW to 20MHz, the spectrum analyzer displays all signals in the 20MHz bandwidth. + Noise power value: Take a GSM mobile phone as an example. The GSM channel bandwidth is 200kHz (including the guard interval). When you use a spectrum analyzer to test the 1 channel power of a GSM mobile phone, if you set the RBW to 200kHz and 1MHz, the test results The peak power of 1 channel is basically the same.
The unit of the power read in the spectrum analyzer is dBm/XXHz, XXHz=RBW Bandwidth. When you set the RBW larger and larger, this value will naturally increase until the RBW is equal to the bandwidth of the signal. Is rising. The effective bandwidth of WiFi (802.11abg) is 20M. When you set RBW=20M, or test channelpower (Bandwidth=20M), you can get the accurate power, which should be within 1 of the power meter.
The following is a detailed description of the test precautions:
: The settings of Ref and ATTEN are very important to prevent the test results from being problematic due to the fact that the actual power is higher than the value displayed by the spectrum analyzer.
Note, but in fact, the reference value of many spectrum analyzers can only be set to 0dBm, so you can only change the attenuation value. Usually the attenuation value is set to the IF filter just without overflow (when it overflows, the spectrum analyzer will display "IF OVLD" ), be sure to pay attention to the prompts when testing, otherwise the test results will be incorrect. In addition, when the IF filter does not overflow, the attenuation value setting has little effect on the test results of higher power, because the IF amplifier automatically compensates ATTEN, such as 20dB, 25dB, 30dB, or even 40dB, the result is almost the same of. But it should be noted that the greater the ATTEN setting, the greater the noise, which will have a greater impact when the signal is small.
The linkage between the input attenuator (ATTEN) and the intermediate frequency amplifier (IF) is shown in the figure below:


Second: RBW is set to 1M, VBW is set to 3MHz and above. It is mentioned in the standard that the RBW is greater than or equal to 1% of the occupied bandwidth. If the value of RBW is too small, the test result will be too small.
Third: The so-called channel power refers to testing the power on the occupied bandwidth of the channel, so the channel bandwidth must fully comply with the meaning of this test item. IEEE802.11b, g, n20 are 22MHz bandwidth, and n40 is 40MHz bandwidth.
Fourth, the detection method will affect your measurement results. The peak detection is about 3dB higher than the Avg detection.
Remarks: Generally, the bandwidth of RBW is related to the bandwidth of the spectrum analyzer, and many low-end spectrum analyzers cannot be set to 20M.
The following briefly introduces the concept of signal and spectrum analysis, which is convenient for everyone to deepen their understanding:
You often use oscilloscopes, spectrum analyzers, and vector analyzers (ESA) in your work, but you don’t understand them. The following picture illustrates the differences and their respective functions:

Remarks: Usually high-end spectrum analyzers will have vector analysis functions.
The following picture characterizes the different display of the same signal in the time domain (oscilloscope) and frequency domain (spectrum analyzer):


Below are the frequency domain indicators of the signal (power, phase noise, spurious, harmonic), all of which can be tested with a spectrum analyzer.


After watching so much, do you understand? Well, the following debut questions, review the homework:
The low noise of a certain model of spectrum analyzer is -165dBm/Hz. When the RBW is set to 100KHz, can it distinguish a single tone signal with an amplitude of about -120dBm? If you want to accurately measure the amplitude of this single tone signal, and also want to save the test time as much as possible, how should RBW be set?
The answer is: No, why? What is the noise of the 100KHz RBW?
Pn=-165dBm/Hz + 10log (100KHz/Hz) = -115dBm
-115dBm> -120dBm, so it cannot be tested accurately.
When more than 10KHz RBW is set, it can be distinguished barely.