Noise Reduction Techniques

Random electrical noise which shows up in the analyzer receiver chain can reduce measurement accuracy. The following features help reduce trace noise and the noise floor which can lead to better dynamic range and more accurate measurements.

See Also

Group Delay

Increase Dynamic Range

Other topics about Optimizing Measurements

Averaging

Averaging is a feature that reduces the effects of random noise on a measurement. There are two types of averaging: Point or Sweep.

The Point averaging type computes averaging on each data point before stepping to the next data point. You determine the number of measurements by setting the averaging factor (enabled by clicking the Averaging button). The higher the averaging factor, the greater the amount of noise reduction.

The Sweep averaging type computes averaging on subsequent sweeps until the required number of averaging sweeps are performed.

See also this Keysight support article: HOW TO ACQUIRE DATA USING AVERAGES ON A VNA WITH SCPI EXAMPLE IN PYTHON

Effects of Sweep Average

Both Averaging and IF Bandwidth can be used for the same benefit of general noise reduction. For minimizing very low noise, Averaging is more effective than reducing IF bandwidth. Generally, Averaging takes slightly longer than IF bandwidth reduction to lower noise, especially if many averages are required. Also, changing the IF bandwidth after calibration results in uncertain accuracy.

How to Set Averaging

Using Hardkey/SoftTab/Softkey

Using a mouse

  1. Press Avg BW > Main > Averaging.

  2. Enter the Averaging number.

  1. Click Response.

  2. Select Avg BW.

  3. Select Averaging....

Average dialog box help

Average ON   Check to enable Averaging.

Average Factor   Specifies the number of measurements that are averaged. Range of 1 to 65536 (2^16).

Average Type

Sweep  Each data point is based on the average of the same data point measured over consecutive sweeps. When the number of sweeps = Average Factor, the averaging continues following the Sweep Averaging formula.

(Sweep) Restart   Begins a new set of measurements that are used for the average. Applies only to Sweep averaging - NOT Point.

Point   Each data point is measured the number of times specified by the Average Factor, and then averaged, before going to the next data point.  

  • On subsequent sweeps, averaging is automatically restarted by measuring each data point again the number of times specified by the Average Factor.

  • Because measurements occur quickly in the background, the Average Counter is NOT updated.

  • Point averaging is NOT available in Gain Compression, or Noise Figure Apps.

Notes

  • An Average Counter appears on the screen when Sweep averaging is selected, displaying the number of sweeps that has been averaged. The effect on the signal trace can be viewed as the Average Factor increases. This can assist in the selection of the optimum number of sweep averages. The Average Counter is NOT updated for Point averaging.

  • Channel-wide scope- Averaging is enabled and the factor is set for all measurements in a channel. The Average counter is displayed for each channel.

  • Calibration - Because averaging is a mathematical process that occurs after the raw measurement is made, averaging can be turned ON before or after calibration without invalidating the error correction terms. If averaging is ON before calibration, the measurement of calibration standards are averaged measurements. More time is needed to perform the calibration, but there will be less noise in the resulting error correction terms. Subsequent corrected measurements will also have less noise error. In addition, noise is further reduced by turning Averaging ON after calibration.

  • Triggering is implemented separately from Averaging. For example, setting averaging factor to 4 has NO effect on the number of triggers that are required to achieve 4 sweeps or 4 data points.

  • Unratioed measurements - Although averaging unratioed (single receiver) measurements is allowed, you may see unexpected results.

  • The noise floor does not drop when averaging unratioed measurements as on ratioed measurements.

  • Phase results may tend toward 0.  This is because phase measurements are relative by nature. Measuring absolute phase with a single receiver appears random. Averaging random positive and negative numbers will tend toward 0.

Sweep Averaging Formula

NewAvg = (NewData/n) + [OldAvg*(n-1/n)] 'where n = average factor

From the formula, you can see that data from the first n sweeps continues to be included in the results of subsequent sweeps. Its effect is increasingly smaller but never diminishes to zero.  For example, with n = 5, the average of the 5 sweeps is displayed. On the 6th sweep, you see 4/5 the average of the first 5 sweeps plus 1/5 the new sweep.

The effects of older data can be eliminated by clicking Restart.

Learn more about Averaging (scroll up)

IF Bandwidth

The received signal is converted from its source frequency to a lower intermediate frequency (IF). The bandwidth of the IF bandpass filter is adjustable down to a minimum of 1 Hz. The maximum IF varies depending on the VNA model.

Reducing the IF receiver bandwidth reduces the effect of random noise on a measurement. Each tenfold reduction in IF bandwidth lowers the noise floor by 10 dB. However, narrower IF bandwidths cause longer sweep times.

Effect of Reducing IF Bandwidth

 

How to set IF Bandwidth

Using Hardkey/SoftTab/Softkey

Using a mouse

  1. Press Avg BW > Main > IF Bandwidth.

  2. Enter the IF Bandwidth value.

  1. Right click on the BW icons on the status bar.

  2. Select an IF Bandwidth....

IF Bandwidth dialog box help

For M938x,

IF Bandwidth  Specifies the IF (receiver) bandwidth. The value of IF bandwidth is selected by scrolling through the values available in the IF bandwidth text box. The IF BW is set independently for each channel.

Right click on the BW icons on the status bar then select IF Bandwidth... to display the IF Bandwidth dialog:

    Reduce IF BW at Low Frequencies : This is the same as LF Auto BW.

    OK  Selects the IF BW value shown in the text box.

     

    Learn about IF Bandwidth (scroll up)

     

    Modulation Distortion Bandwidth Settings (Option S9x070xB, S9x070A/B Modulation Distortion only)

    How to access Bandwidth settings

    Using Hardkey/SoftTab/Softkey

    Using a mouse

    1. Press Avg BW > Main

    1. Click Response.

    2. Select Avg BW.

    3. Select Bandwidth....

    Bandwidth dialog box help

     

    Noise BW is equal to the Resolution BW divided by the Vector Average factor. You cannot directly set the Resolution BW or Vector Average in a Modulation Distortion channel. Resolution BW is always set to its maximum value for the given signal chosen. Vector Average is automatically set depending on the Noise BW setting.

    Auto - Check to set the Noise BW to its maximum possible value. This results in the fastest possible sweep but increases the noise floor. If Auto is checked, then the Vector Average is set to 1, resulting in the widest Noise BW.

    The Noise BW is displayed in the Softkey and in the middle of the display x-axis annotation.

     

    LF Auto BW

    How to enable/disable LF Auto BW

    Using Hardkey/SoftTab/Softkey

    1. Press Avg BW > Main > LF Auto BW.

    2. ON enables and OFF disables LF Auto BW.

    On VNA models with a maximum frequency of 20 GHz and higher, the trace noise becomes worse below about 400 MHz. This is especially obvious between 10 MHz and 45 MHz and also when Time Domain is ON.

    This setting:

    When LF Auto BW is ON, the VNA uses a smaller IF Bandwidth than the selected value at low frequencies.

    1. User set IF Bandwidth.
    2. Closest IF Bandwidth less than or equal to (Receiver Frequency / 4).

    For example:

    180 kHz / 4 = 45 kHz is lower than User set IF bandwitdth 100 kHz. Hence 45 kHz is chosen.

    Then, next lower IF Bandwidth is 30 kHz. 30 kHz IF bandwidth is applied for this point.

    Use the following calculations to determine the actual IF Bandwidth that is used. If the result is NOT a selectable IF BW, the next higher selectable value is used.

    For M9834A/37A models, if LF Auto BW is ON, use smallest value of below:

    1. (At least one path is Coupler Path)

    M983xA

    Frequency Upper Limit

    Ratio

    46,750,000

    0.005

    50,000,000

    0.0075

    62,500,000

    0.01

    78,125,000

    0.015

    142,000,000

    0.025

    175,000,000

    0.075

    296,500,000

    0.1

    470,000,000

    0.25

    810,000,000

    0.5

    1,140,000,000 and above

    1

     

    2. Paths of all receivers are other than Coupler Path like as DRA, NF Path

    Example: The coupler path is used (The test port is used for measurement), IF BW 30 kHz, frequency point: 1.1 MHz

    Since 3 Hz is the lowest number on three of above, IF BW 3 Hz is used for this points.

    Trace Smoothing

    Trace smoothing averages a number of adjacent data points to smooth the displayed trace. The number of adjacent data points that get averaged together is also known as the smoothing aperture. You can specify aperture as either the number of data points or the percentage of the x-axis span.

    Trace Smoothing reduces the peak-to-peak noise values on broadband measured data. It smooths trace noise and does not increase measurement time significantly.

    Because Trace Smoothing follows Format in the data processing map, the formatted data is smoothed. Smoothing is automatically turned off if the format is Polar or Smith Chart.

    Learn more about Data Format Types.

    See the data processing map.

    Tips:

    Effects of Smoothing on a Trace

     

    How to set Trace Smoothing

    Using Hardkey/SoftTab/Softkey

    1. Press Avg BW > Smoothing > Smoothing ON|OFF.

    Smoothing ON  When checked, applies smoothing to the displayed trace.

    Percent of Span  Specify percent of the swept stimulus span to smooth. For example, for a trace that contains 100 data points, and specify a percent of span = 11%, then the number of data points that are averaged is 11.

    Points  Specify the number of adjacent data points to average.

    Learn about Trace Smoothing (scroll up)