Understanding Time and Frequency Parameters
Sometimes it can be hard to understand exactly how all the Time, Resolution Bandwidth, Windowing, Gating, and frequency Span parameters are related and just how they determine what data is captured from the measurement hardware and what is shown on the time traces and spectrum traces.
The first section describes these parameter relationships for the and measurement types. The second section below describes how Digital modulation analysis differs from simple Vector measurements. Digital modulation analysis measurement types determine some parameters through the dialog instead of the MeasSetup dialog.
Vector and Analog Demod measurement types
When you choose a frequency span and a ResBW or Time Length setting, the VSA uses the following calculations to determine what is captured and shown on the traces.
Action |
Description | Example |
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User Actions |
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1. Set frequency span |
Determines sample rate: f = 1.28 * span (for zoom data, i.e. complex time samples) f = 2.56 * span (for baseband data, i.e. real time samples) And of course, the sample period is Dt = 1 / f |
Span = 10 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second.
f = 12.8 MHz (zoom) f = 25.6 MHz (baseband)
Dt = 78.1 ns (zoom) Dt = 39.1 ns (baseband) |
2. Set number of frequency points (Nf) for spectrum traces |
Determines maximum Main Time Record length: max TimeLen = Nfft * Dt = (Nf - 1) / span where Nfft is the size of the FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. and is a power of 2 (512, 1024, ...) Nfft = 1.28 * (Nf-1) (for zoom data, i.e. complex time samples) Nfft = 2.56 * (Nf-1) (for baseband data, i.e. real time samples) Instead of setting the number of frequency points manually, you can enable Auto Frequency Points to determine the number of points based on your selection of Time Length (or ResBW, which determines Time Length). |
Set Nf = 801.
Nfft = 1024 (zoom) Nfft = 2048 (baseband)
max TimeLen = 80 us (zoom & baseband) |
3. Choose window shape |
Determines relationship of TimeLen and ResBw (see next table row). Each time window has an Equivalent Noise Bandwidth (ENBW), which is the bandwidth of a rectangular filter that passes the same noise power as the filter defined by the time window. |
Hanning window selected. Normalized ENBW = 1.50 Hz-sec |
4. Set either desired Resolution Bandwidth or desired TimeLen |
TimeLen and ResBw are directly related by: TimeLen = Normalized ENBW / ResBW Resolution Bandwidth is a qualitative measure of the minimum separation required between two frequency components to be able to visually separate them and, for the VSA, is defined as the Equivalent Noise Bandwidth of the filter, which is determined by the window type that you select and the length of the window. |
Specify ResBW = 100 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). TimeLen = 1.50 Hz-sec / 100 kHz = 15 us |
5. Set Time Gate parameters (optional) |
Gate Length and Gate Delay determine what section of the Main Time Record is used. |
Disable gating. |
VSA actions |
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6. VSA determines number of time points to acquire for analysis (Na) |
Number of Main Time Record points (Na) is determined by Na = TimeLen / Dt The actual number of time points acquired from the hardware will be greater than Na to allow resampling filter settling. You can see the actual points captured from the hardware in the Raw Main Time trace. |
Na = 192 (zoom) Na = 384 (baseband) |
7. VSA performs time gating |
Gate Time Record length (Ng) is determined by Ng = Gate Length / Dt and gate position within the Main Time Record is determined by Gate Delay. |
No gating. |
8. VSA calculates Spectrum trace data |
The VSA performs these steps to compute the frequency Spectrum trace data:
You can see all the spectrum points (Nspec) by selecting Show All Frequency Points. |
Nspec = 1024 (zoom) Nspec = 1025 (baseband)
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Digital Modulation Measurement Types
The process outlined above applies to Vector mode and Analog Demod measurement types, but for digital demodulation measurement types, most of the parameters cannot be set directly. The table below describes how the time and frequency parameters are interact as you configure the VSA:
Action |
Description |
Example |
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User actions |
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1. Set frequency span |
Determines sample rate: f = 1.28 * span (for zoom data, i.e. complex time samples) f = 2.56 * span (for baseband data, i.e. real time samples) And of course, sample period is Dt = 1 / f |
Span = 25 Mhz f = 32 MHz (zoom) f = 64 MHz (baseband)
Dt = 31.25 ns (zoom) Dt = 15.625 ns (baseband) |
2. Choose window shape |
Each window has an Equivalent Noise Bandwidth (ENBW), which is the bandwidth of a rectangular filter that passes the same noise power as the filter defined by the time window, multiplied by the length of the time window. |
Hanning Window selected. Normalized ENBW = 1.50 Hz-sec |
3. Set length of trace |
For most demods, the length of the TimeLen) is determined by the parameters on the tab in the Demod Properties dialog. Usually the parameter is named . trace (Some demods have the ability to specify some way of searching for a particular section of the modulated signal. In this case, the length of the Time trace can vary each measurement sweep. The trace shows the pre-demod time data that is analyzed by the demodulator and that is FFT'd and shown in the trace. The actual time record captured from the measurement hardware is shown in the trace and the resampled data that is used to search for the analysis interval is shown in the trace (some demods do not have this trace).Time Gating is not supported with digital modulation measurement types. |
TimeLen = 50 ms |
VSA actions |
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4. VSA determines ResBW |
Resolution Bandwidth is a qualitative measure of the minimum separation required between two frequency components to be able to visually separate them and, for the VSA, is defined as the Equivalent Noise Bandwidth of the filter, which is determined by the window type that you select and the length of the window. To calculate the ResBW of a window, the normalized ENBW of the window is divided by the length of the TimeLen). trace (TimeLen and ResBw are inversely related by: ResBW = Normalized ENBW / TimeLen |
ResBW = 1.50 Hz-sec / 50 ms = 30 Hz |
5. VSA determines number of time points to acquire for analysis (Na) |
The Na points) is determined by a parameter (usually) named or : length (Na = Search Time Length / Dt The actual number of time points acquired from the hardware will be greater than Na to allow resampling filter settling. You can see the actual points captured from the hardware in the Raw Main Time trace. The trace is only available in some demods. |
Search Length = 35 ms
Na (zoom) = 35 ms / 31.25 ns = 1,120,000 points Na (baseband) = 35 ms / 15.625 ns = 2,240,000 points |
6. VSA searches for analysis start boundary within Search Time record |
The VSA searches within the Search Time record for some boundary to set as 0 ms. This can be the beginning of the burst or a frame or subframe boundary, depending on the demodulator. The 0 ms time location does not necessarily have to be within the begin and end times of the trace. |
Analysis boundary found 5.5 ms into Search Time record. Beginning of Search Time trace set as -5.5 ms. |
7. VSA creates the Time record |
The VSA creates the pre-demod Time record (shown in the Nt) by selecting part of the Search Time record as specified by certain parameters on the tab (generally called and ). trace with number of pointsThe trace data is also resampled to align the time record with the beginning of the analysis boundary (in case the boundary falls between two sample points).See the documentation for the particular demod you are using for more information. In particular, see the and trace topics and the topics. |
Time trace length = 10 ms
Nt (zoom) = 10 ms / 31.25 ns = 320,000 points Nt (baseband) = 10 ms / 15.625 ns = 640,000 points |
8. VSA calculates Spectrum trace data |
The VSA performs these steps to compute the frequency Spectrum trace data:
You can see all the spectrum points (Nspec) by selecting Show All Frequency Points. |
Nfft (zoom) = 2^19 Nfft (baseband) = 2^20
Nspec (zoom) = 2^19 Nspec (baseband) = 2^19 + 1
Nf (zoom) = 409,601 Nf (baseband) = 409,601 |
See Also