"1. Bottom noise
The so-called bottom noise refers to the bottom limit that makes the actual measurement impossible. Only interference will be detected here. Visually, the Spectran spectrometer will display a collection of small "dots" or strips, which stay at the same place statically or change (noise) every scan. However, depending on the frequency and selected settings, the bottom noise can vary widely and must be determined before starting the measurement. Generally, the bottom noise increases as the frequency decreases - for example, the bottom noise at 50Hz (power grid) is significantly higher than that at 500Hz.
2. 0Hz spike and bottom noise
When using the spectrum analyzer, the closer you are to 0Hz (the so-called 0Hz spike), the stronger the natural bottom noise will make the measurement accuracy worse. The measurement quickly reaches the physical limit in this range. In order to achieve the best possible sensitivity, at least the residual noise of the amplifier, that is, the electronic measuring equipment itself, must pass through an advanced very low noise amplifier, such as in our Spectran equipment, whose accuracy is very close to the actual physical limit.
When the measurement is close to 0Hz, the following suggestions must be paid attention to:
The closer to 0Hz spike, the stronger the received noise and the more inaccurate the measurement:
The simulation diagram shows that a steep curve starts at 0Hz (left) and the peak at 0Hz quickly smoothes to a higher frequency. At the same time, you can also see that a similar 20-40hz scan result (about 2.5) is much higher than another 50-70hz scan result (about 0.2), although they are the same span.
When the measurement is close to 0Hz spike, always use a small span as far as possible to reduce the bottom noise:
Due to the steepness of the filter, span itself also affects the bottom noise intensity, and a larger span is smoother. At the same time, it also starts to capture adjacent frequency signals.
Although this is almost no problem for higher frequencies, it is unusually important near 0Hz spike. Despite the same starting frequency, the bottom noise is stronger when using a large span than when using a small span.
Therefore, if possible, try to avoid measurement at the 0Hz limit.
Try not to use 0Hz as the starting frequency
If possible, try to replace 0khz - 100k with 1kh or higher, such as 10kHz - 100k. The spectrum analyzer tries to suppress the bottom noise when scanning at the starting frequency of 0Hz, which has the side effect of very low sensitivity in the "low frequency range". Therefore, when using 0hz-100khz scanning, do not expect to correctly measure 50Hz signal.
However, if you want to measure as close to 0Hz spike as possible (for example, 50Hz transmission network or 16.7hz traction power), you should use the smallest possible filter (0.3hz, 1Hz or 3Hz). However, it naturally leads to an extremely slow scan. Because this is not practical, we have developed a very fast scanning mode: DFT scanning. In fact, it uses FFT. In order to avoid its typical mixing, DFT mode will start automatically under the following two conditions:
a) Use 0.3hz, 1Hz or 3Hz filter; as well as
b) Span is less than 200Hz, but at least more than 10Hz.
In the current software version, there are two other limitations:
1) Only span is a multiple of 15, such as 15Hz, 30Hz, 45Hz, etc
2) Always use the interval between span less than 0Hz and the start frequency "flow". Otherwise, you will get the wrong result.
For example:
Flow = 30Hz and fhigh = 60Hz (span = 30Hz): correct (ex.: 50Hz main)
Flow = 15Hz and fhigh = 30Hz (span = 15Hz): correct (ex.: 16.7hz traction power)
Flow = 29hz and fhigh = 59hz: error. Compared with span, the distance between span and 0Hz is too small. The distance is 29hz. Span is set to 30Hz, so the distance between start frequency and 0Hz is at least 30Hz.
Flow = 16Hz and fhigh = 30Hz: error. Span is not a multiple of 15 (because the setting is 14Hz), so it is an incorrect measurement result.
For more details, please refer to "DFT mode (1-3hzrbw fast measurement)"
3. Select the correct filter (RBW)
The correct filter setting depends on various factors. Generally, the filter should be less than the selected span. Otherwise, signals (visible) outside the span will also be received and cause false display, which is tolerated only in rare cases. Most of the time, a filter should be at least 10 times smaller than the selected span.
For example:
SPAN=100Hz； Set filter 10Hz (or less)
SPAN=500kHz； Set filter 100kHz or 30kHz (or less)
However, please note that the selected filter will directly affect the scanning speed and display accuracy: if a larger filter is selected, the scanning will be faster, but the display results will also be less accurate.
The following illustration explains the problem of the filter, assuming that the scanning range is from 50-60 kHz and the visible range displayed by the grid line:
It can be seen that the 46khz signal outside the scanning range is not considered, because the 1kHz filter only "captures" the signal in the small frequency range outside the 50 - 60KHZ scanning range, and the result will display 1 μ The correct value of T.
On the contrary, when a 30kHz filter is used, the 46khz signal outside the 50-60khz sweep range will be captured, because the 30kHz filter will also contain the broadband range outside the 50-60khz sweep, and the result will be displayed as 10 μ Error value for t.
If you like to make a very accurate level measurement, you should not set the bandwidth of the filter too narrow. Due to the internal phase noise, too narrow filter may cause the displayed value to be lower than the actual value.
For example, if a 100kHz signal is measured, a 10kHz filter is used. Changing the filter to 3kHz or 1kHz may not cause any significant change in the display level. However, at some frequency points, such as 300Hz or 100Hz filters, the displayed signal level will suddenly drop slightly due to phase noise, thus providing an incorrect reading.
4. Select the correct sampling time
(SpTime)
If you want to measure the level, it is very important to select the appropriate sampling time. If you use too little sampling time, you may get low level results. Especially if you use low filters (rbws), you must set a fairly high sampling time (depending on the filter, even up to seconds).
The correct sampling time depends on the filter (RBW) used. You should set the synchronous adjustment rbws and sampling time:
10Hz=70s, 30Hz=30s, 100Hz=7s, 300Hz=3s,
1kHz = 700ms, etc
If the scan span is 20 times larger than RBW, you must use a higher sampling time. Please note that the sampling time limit in DFT mode (1Hz and 3Hz filters) is 5S, and the sampling time in DFT mode is too small: the level will change frequently after each scan. If you use our PC software, you will see a snake on the display. Simply change the sampling time to a higher value, and you will get a stable reading after each scan.
5. Measure the electric field above 500KHz and all fields
The correct measurement of potential free electric field or magnetic field above 500KHz requires a little technical knowledge. The measurement results may soon become inaccurate due to the proximity to nearby objects, such as walls, trees, or the operator itself. Therefore, please pay attention to:
——You should maximize the distance between your body and the measuring instrument. Therefore, you should use insulated wooden tripod or similar auxiliary equipment, and increase the distance between your body and the equipment by about 1-2m. In addition, please note that the electromagnetic field will weaken rapidly when it is close to the ground. That's why you'd better always hold the device for measurement. You should stretch out your arm to measure. In addition, since the detector is installed near the front end of the device, please hold the tail of the device as much as possible. However, please note that the measured value when measured with "manual" will be amplified by 3-4 times.
——Confirm that no other person is near the measurement location because they may affect the measurement results.
——Auxiliary equipment such as equipment and tripod shall not be covered by other things, otherwise extremely inaccurate results may be caused—— The operation of connecting USB or audio cable or attached power supply may affect the electromagnetic field and lead to extremely inaccurate results.
When measuring, always use battery power without any cable connection.
——When using "panning approach" to determine the maximum electric field, the detector can only conduct one-dimensional measurement, otherwise,
The wrong (too low) level will be measured when several wires, signal sources or other various interference sources are mixed
Especially when. On the other hand, if you measure a single object, such as high voltage for a free electromagnetic field
The line is measured without panning approach, and the measurement inaccuracy is usually within 5% of the tolerable lower value.
Warning: please pay attention to all kinds of so-called "construction"
"Biologists" and their "measuring instruments"
Wrong measurement method requiring grounding. This method violates any applicable EMC measurement standard, and the results will be wrong and legally worthless.
Therefore, the results measured by such measuring equipment are indistinguishable from the Spectran equipment measured according to the standard
Comparability.
6. Sensitivity
Please understand that the bottom noise and sensitivity of the spectrum analyzer are subject to significant fluctuations according to different measurement frequencies. Generally, the lower the frequency, the higher the bottom noise and the worse the sensitivity. In fact: using the same filter settings, weaker signals can be measured at 100kHz than at 50Hz. Refer to the chapter "0Hz spike and bottom noise".
7. Measurement error
Annoni specifies typical accuracy for each model of Spectran spectrometer. However, this means that a higher deviation is also entirely possible. Especially when you approach the so-called bottom noise or the maximum sensitivity of the spectrum analyzer, the accuracy will be reduced in principle.
Even though Spectran spectrometer provides impressive accuracy, considering the price, it should be assumed that there is a higher measurement accuracy than the basic instrument itself in practice. The reason behind this is that there are many factors that can affect the measurement accuracy: they are temperature correlation, repeatability, antenna error, modulation correlation, etc. These factors must be explained in each test equipment.
8. Cursor and zoom functions
Using the wheel knob, a cursor can be activated in spectrum analysis mode. This cursor allows you to determine the exact frequency and signal level of a specific point when it is displayed. To activate the cursor, simply turn the dial. The cursor is displayed as a line in the spectrum. You can also press the menu key twice to close the cursor. The spectrum display part below the cursor is displayed in reverse color, so its information is retained.
By turning the wheel button, the cursor can be freely positioned on the display screen. The reading displayed on the main display only corresponds to the exposure limit display at the current cursor position and the demodulation at the current cursor position only represents this frequency! The current frequency and level display at the cursor position is updated only after a complete scan and displayed in the first marking area (leftmost). Naturally, in this mode, the automatic marking function is disabled.
When you press the scroll knob and lock the cursor, you can zoom this signal. Here, set the center frequency at the current cursor position and / or frequency, reduce the span by half, and start a new scan. That is, you reduce the signal by 50%. You can use this feature to achieve the resolution you need. This is an incredible and very convenient function. Press the menu button twice to deactivate the function. Note: the cursor cannot be used in hold mode.
9. DFT mode (fast measurement of narrow rbws)
All 50xx series Spectran models have "ultra high speed" DFT mode (digital Fourier transform). When the following C settings are set, the DFT mode will start automatically.
a) RBW is 0.3hz, 1Hz or 3Hz;
b) Span less than 200Hz (10Hz is the minimum)
DFT mode allows very fast scanning when using very narrow filters, which is especially suitable for measuring main power network (50Hz) or traction power (16.7hz). Compared with conventional FFT, DFT (digital Fourier transform) does not produce the well-known poor mixing phenomenon of FFT (fast Fourier transform).
Note: in DFT mode, you can even use a very low scanning time setting to realize fast signal search. It allows very fast scanning, but inherently may produce wrong results. In order to prevent users from misunderstanding, when a too short scanning time is selected, the UNC display will start automatically, which indicates that the measurement result is unreliable. However, it is still possible to quickly locate the signal source and verify its existence.
For more information about DFT mode, refer to the chapter "0Hz spike and bottom noise".
10. Select the appropriate attenuator
The Spectran device contains a highly accurate built-in attenuator that attenuates the input signal to prevent overload of the internal amplifier.
Without attenuator, serious measurement error may occur and even damage the internal amplifier. To select the appropriate attenuator for the measurement, the steps are as follows:
First, always select the highest attenuator (40dB), and if there is a reading available, remember its value. Then change to a smaller attenuator (30dB). If the new reading is significantly lower than the previous one, it indicates that the amplifier has been seriously overloaded, so you should return to a higher attenuator. But if the readings are almost the same, this attenuator is the right choice.
If the wrong attenuator is selected, more signs are unstable readings and cluttered graphical displays.
When providing external signals through SMA connectors, we recommend the following attenuator settings（
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