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Vector Network Analyzer: VNA

Symbol

Summary

VNA performs power sweep and frequency sweeps, similar to a Vector Network Analyzer. VNA generates one or more tones at each swept power level and/or frequency point. Most Visual System SimulatorTM (VSS) measurements can utilize the VNA block as a test point. Most of the measurements in the System > NW Analyzers category can directly use the power level of the generated signal for the x-axis values of the measurement.

VNA can be used with the Swept Variable Control block (SWPVAR) to perform multi-dimensional sweeps. See SWPVAR for more information.

This block replaces the Vector Network Analyzer (Large Signal) block VNA_LS and the Vector Network Analyzer (Small Signal) block VNA_SS.

When used to generate a frequency response, it will most likely operate slower than the VNA_SS block. However, it offers two significant improvements over VNA_SS: It supports nonlinear blocks, and it directly supports impedance mismatch modeling.

Parameters

Name Data Type Description Unit Type Default
ID N Element ID N/A M
PSTART R Power sweep level start Power in dB 0 dBm
PSTOP R Optional power sweep level stop Power in dB  
PSTEP R Optional power sweep level step dB  
PVARNAME S Optional name of power variable to sweep Text  
FSTART R Frequency sweep start Frequency 10 GHz
FSTOP R Optional frequency sweep stop Frequency  
FSTEP R Optional frequency sweep step Frequency  
FOFFSET R Optional frequency offsets for multiple tones Frequency  
PWRTYP E Interpretation of power level N/A Power per tone
FVARNAME S Optional name of frequency variable to sweep Text  
CTRFRQ R Center frequency Frequency 1 GHz
*PHS R Optional phase offsets Angle 0
*ZS R Source impedance Resistance _Z0
*ZL R Load impedance Resistance _Z0
*T R Output noise temperature Temperature _TAMB
*NOISE E Thermal noise modeling N/A Freq analysis only
*TNRSEED I Thermal noise random number generator seeds Scalar  
*SWPDUR R Duration of each sweep Time  
*SWPCNT I Samples per sweep Scalar  
*RBW R Resolution bandwidth Frequency  
*VBW R Video bandwidth Frequency  
*NFFT I Number of FFT bins Scalar  
*NAVG I Number of averages Scalar  
*WNDTYP E FFT windowing type N/A Auto
*WNDPAR R FFT windowing parameter Scalar  
*WNDWHN E When to window   Auto
*SLDFRC R Fraction of window to slide Scalar 0.5
*SMPFRQ R Sampling frequency Frequency  
*SMPSYM I Samples per symbol Scalar _SMPSYM
*BLKSZ I Samples per pass Scalar _BLKSZ * _SMPSYM

* indicates a secondary parameter

Parameter Details

PSTART. The initial power level to be swept or the power level if power is not swept.

PSTOP. The maximum (minimum if PSTEP is negative) power level to be swept. If this is left empty the power level is not swept and always remains PSTART.

PSTEP. The power level increment between power sweeps. If this is left empty the power level is not swept and always remains PSTART.

PVARNAME. The optional name of a variable to be assigned the power value of the current power sweep. The value is in units of dBW. If a variable is to be assigned, it should be in double quotation marks, such as "A". There must be a corresponding equation on the system diagram with a dummy numerical value assigned to it, such as "A=0".

FSTART. The initial frequency to be swept or the base frequency if frequency is not swept.

FSTOP. The maximum (minimum if FSTEP is negative) frequency to be swept. If this is left empty the frequency is not swept and always remains FSTART.

FSTEP. The frequency increment between frequency sweeps. If this is left empty the frequency is not swept and always remains FSTART.

FOFFSET. If not empty, an array of frequency offsets used to define multiple tones to be generated. The array values are offsets from the current swept frequency. A tone is always generated at the swept frequency, so 0 does not need to be included. The PWRTYP parameter determines how the current power level is applied to the tones.

PWRTYP. Determines how the power level is interpreted when more than one tone is generated:

  • Power per tone: Each tone is set to the current power level.

  • Total power: The current power level is divided evenly between each tone.

FVARNAME. The optional name of a variable to be assigned the frequency of the current frequency sweep. The value is in units of Hertz. If a variable is to be assigned, it should be in double quotation marks, such as "A". There must be a corresponding equation on the system diagram with a dummy numerical value assigned to it, such as "A=0".

CTRFRQ. The center frequency of the output signal. If TONES is empty, a single tone at this frequency is generated.

PHS. The phase offsets for each tone. If there are fewer values for the phases than there are tone frequencies in TONES, the last phase value is repeated.

ZS. The source impedance of the tone generator.

ZL. The load impedance seen by the DUT looking into the measurement node. If this is left empty, the measurement node is treated as a voltage probe.

T. The temperature of noise generated by the block if noise is enabled for the simulation type by NOISE. The noise generated is white noise.

NOISE. Determines how thermal noise is modeled:

  • Freq analysis only: Noise is modeled for frequency analysis simulations such as RF Budget Analysis, but not Time Domain simulations.

  • Generate samples: Noise is modeled for both frequency analysis and Time Domain simulations.

  • Noiseless: The block is assumed to be noiseless.

TNRSEED. The seed for the Time Domain simulation's thermal noise pseudo-random number generator. See RND_D for details.

If this is left empty, a seed is generated based on a hash of the block name and the ID parameter (if the block is within a subcircuit, the ID parameters of the parents are also used). In general, this results in different instances of the block generating different sequences, although it is not guaranteed.

If this is set to -1, the seed varies from sweep to sweep in a single simulation run. An initial seed value similar to that generated from the block name and ID parameter is used, with a different offset added to it each new sweep. The seed sequence is deterministic between simulation runs.

SWPDUR. The minimum duration of each sweep. Either SWPDUR or SWPCNT can specify the duration of each sweep. If both SWPDUR and SWPCNT are left empty, the duration is determined automatically. It is set to at minimum, the larger of 10 times the period of the tone closest to the center frequency or to NFFT * NAVG.

SWPCNT. The minimum number of samples to gather per sweep. Either SWPCNT or SWPDUR can specify the duration of each sweep.

RBW. The default resolution bandwidth for spectrum-based measurements. The spectrum-based measurements approximate the number of points for the FFTs from NFFT = SMPFRQ / RBW. Either this or NFFT can control the spectrum frequency resolution.

VBW. The default video bandwidth for spectrum-based measurements. The video bandwidth in a spectrum analyzer is the bandwidth of the video filter, which is a lowpass filter normally used to smooth the spectrum display. Smaller video bandwidths result in smoother displays. The spectrum-based measurements approximate the effect of the lowpass filter by performing FFT averaging. The number of averages is set from NAVG = RBW / VBW. Either this or NAVG can control spectrum smoothing.

NFFT. The default number of FFT bins to use for spectrum-based measurements. Either this or RBW can control the spectrum frequency resolution.

NAVG.The default number of averages to use for spectrum-based measurements. If set to 0, measurements use cumulative averaging. Either this or VBW can control spectrum smoothing.

WNDTYP. The default type of windowing to perform for the FFT computations used by frequency domain based measurements. When windowing is applied is determined by the WNDWHN parameter.

WNDPAR. The default windowing parameter used for the window type selected by WNDTYP.

WNDWHN. Determines when windowing is applied to FFT computations. When set to Auto, data windowing is only performed when averaging is performed.

SLDFRC. The default percentage of the time domain data window by which averaged FFTs are overlapped. A value of 1 indicates no overlapping, a value of 0.75 indicates 25% overlap, and a value of 0.5 indicates 50% overlap.

SMPFRQ. The sampling frequency of the source signal. If this is left empty, the sampling frequency is determined from either a model that receives data generated by this model and can specify a sampling frequency, or is set to the following if no such model is found:

SMPFRQ=SMPSYM·max(|fi-CTRFRQ|)

where fi are the frequencies of the tones. If only a single tone is generated at CTRFRQ, the sampling frequency is set to the sampling frequency specified in the System Simulator Options dialog box.

SMPSYM.The number of samples representing a symbol.

BLKSZ. The number of samples generated each time the block is visited.

Data Input

Node No. Type Purpose
2 Complex Measured Signal

Data Output

Node No. Type Purpose
1 Complex Source Signal

Implementation Details

This block generates a single or multiple tone signal with stepped power and/or frequency sweeps. The signal is generated until a minimum number of samples are received at the input node. The absolute minimum number of samples required is determined by the RBW/NFFT, VBW/NAVG and SMPFRQ parameters. These determine the minimum number of samples required by measurements that perform spectrum-based computations.

You can increase this minimum by specifying SWPDUR or SWPCNT. If you specify either, the value is adjusted by any signal delay present in the measured signal.

Recommendations for Use

VNA can be used to quickly generate AM-AM and AM-PM curves. For Time Domain simulations use the AMtoAM_PS and AMtoPM_PS measurements. You can also use the PWR_MTR measurement instead of AMtoAM_PS. For RF Budget Analysis simulations, use the P_node measurement, setting its Pwr Sweep to "Use for x-axis".

VNA can also be used to generate frequency response curves. For Time Domain simulations use the S21_PS or V_GAIN. For RF Budget Analysis simulations, use the desired cascaded gain measurement (C_GA, C_GP, C_GT, etc.), setting the Freq Sweep sweep setting of the measurement to "Use for x-axis".

NOTE: When using the VNA block to sweep frequency in RF Budget Analysis simulations, it is best to leave the offset frequencies on the RF Frequencies tab of the System Simulator Options dialog box a single offset of 0 and let the VNA block determine the frequencies.

You can also use the block to measure intermodulation levels versus input power. To generate a two-tone signal, set the TONES parameter to a vector containing the frequencies of the two tones. For example, to set one tone at 1 GHz and a second tone at 1.1 GHz, set TONES={1e9,1.1e9}.

Blocks connected to the nodes only see the impedances ZS, and if specified, ZL, if impedance mismatch modeling is enabled through the System Simulator Options dialog box. VNA, however, always uses ZS to compute the tone voltages. Measurements also always use ZS and ZL (if specified) to compute power. See Chapter 2 for details on impedance mismatch modeling.

Parameters such as RBW, VBW, NFFT and NAVG are typically used to establish system diagram-wide settings for these parameters. For example, to set a default RBW of 10 kHz, you can create an equation such as: RBW_Hz=10e3

You can then assign it to the RBW parameter for all TP, VNA and VSA blocks. Any spectrum-based measurement that refers to one of these blocks uses the RBW setting by default.

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