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.

Note: The trace noise in microwave PNAs becomes worse below 748 MHz and is especially obvious between 10 MHz and 45 MHz. See Reduce IFBW.

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 front-panel hardkey [softkey] buttons	Using Menus
Press Avg then [Averaging]	Click Response then Avg then Average

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. See the data processing map.
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 from 40 kHz (for most PNA models) down to a minimum of 1 Hz.

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.

Channel - IF bandwidth can be set independently for each channel
Segment sweep - IF bandwidth can be set independently for each segment of segment sweep.
Calibration - Changing the IF bandwidth after calibration will cause a 'C-delta' correction level, which means that calibration accuracy is uncertain.

Effect of Reducing IF Bandwidth

How to set IF Bandwidth
Using front-panel hardkey [softkey] buttons	Using Menus
Press Avg then [IF Bandwidth]	Click Response then Avg then IF Bandwidth

IF Bandwidth dialog box help

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.

The following IFBW values are common to all models:

1 | 2 | 3 | 5 | 7 | 10 | 15 | 20 | 30 | 50 | 70 | 100 | 150 | 200 | 300 | 500 | 700 | 1k | 1.5k | 2k | 3k | 5k | 7k | 10k | 15k | 20k | 30k | 50k | 70k | 100k | 150k| 200k | 280k | 360k | 600k | 1M | 1.5M | 2M | 3M | 5M | 7M | 10M | 15M

For the highlighted IFBW settings (1 MHz and above):

- 7 MHz to 15 MHz settings are available ONLY with DSP version 5.0 and above.
- The primary use for IFBW > 600 KHz is for wideband pulsed and pulse profile measurements. They do NOT provide faster sweep speeds for non-pulsed measurements.
- A slight shift (1dB or more) in Log Mag traces may be seen when switching in and out of these bandwidths.
- Available in Step sweep mode only - NOT available in Analog sweep.

Reduce IF BW at Low Frequencies

On PNA 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. See PNA models / maximum frequencies.

When this box is checked, the PNA uses a smaller IF Bandwidth than the selected value at frequencies indicated below.

This setting:

can be made for each channel.
is ON (checked) by default.
also applies to segment sweep.

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.

In the following table and example, the next band starts at .01 Hz above the Stop Frequency. This is indicated with '+'.

	PNA Model
Stop Frequency	N5241A N5242A	N5221A N5222A	N5244A N5245A	N5224A N5225A	N5247A	N5227A	N5234A N5235A	N5249A
14 MHz	0.05	0.05	0.005	0.005	0.005	0.005	0.025	0.05
19 MHz	0.05	0.05	0.005	0.005	0.005	0.005	0.025	0.05
27 MHz	0.05	0.05	0.01	0.005	0.01	0.005	0.025	0.05
38 MHz	0.05	0.05	0.01	0.005	0.01	0.005	0.025	0.05
53 MHz	0.05	0.05	0.015	0.005	0.015	0.005	0.025	0.05
75 MHz	0.1	0.1	0.025	0.025	0.025	0.025	0.025	0.1
105 MHz	0.1	0.1	0.05	0.025	0.05	0.025	0.025	0.1
146 MHz	0.14	0.1	0.1	0.025	0.1	0.025	0.025	0.14
175 MHz	0.14	0.1	0.15	0.025	0.15	0.025	0.025	0.14
205 MHz	0.29	0.29	0.15	0.15	0.15	0.15	0.025	0.29
250 MHz	0.5	0.29	0.25	0.15	0.25	0.15	0.025	0.5
396 MHz	1	1	0.5	0.5	0.5	0.5	1	1
396+ MHz and above	1	1	1	1	1	1	1	1

Example:

On a N5224A, the selected IF BW is 30 KHz.

With Reduce IF BW at Low Frequencies checked, the actual IF Bandwidths used are:

From 53+ MHz to 175 MHz: 30,000Hz * .025 = 750 Hz (next higher selectable value: 1 kHz.)
From 175+ MHz to 250 MHz: 30,000Hz * .15 = 4.5 kHz (next higher selectable value: 5 kHz.)
From 250+ MHz to 396 MHz: 30,000Hz * .5 = 15 kHz
From 396+ MHz to stop sweep = 30 kHz

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

Learn about IF Bandwidth (scroll up)

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:

Start with a high number of display points and reduce until you are confident that the trace is not giving misleading results.
Do not use smoothing for high-resonance devices, or devices with wide trace variations. It may introduce misleading information.
Smoothing is set independently for each trace.

Effects of Smoothing on a Trace

How to set Trace Smoothing
Using front-panel hardkey [softkey] buttons	Using Menus
Press Avg then [Smoothing]	Click Response then Avg then Smoothing

Smoothing dialog box help

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)