2. Noise Signal

The noise signal parameters are used to design the noise signal. The application provides complete control over the number of tones and tone density in the noise signal as well as control of the noise amplitude profile.

Waveform Name

The name assigned to the waveform segment stored in the arbitrary waveform generator. The Waveform Name can contain up to 22 alpha numeric characters. The name can also contain the following special characters: _ $ & # + - [ ]. The Waveform Name is not case sensitive.

Multiple waveforms can be stored in the arbitrary waveform generator. Just change the waveform name before each download. The different waveforms can be played back by changing the waveform name and executing Play.

Tone Count

Range: 101 - 10,000,001
Default: 801

Enter the number of tones in the NPR signal. The number of tones is always an odd number.

Enter a minimum of 101 tones and a maximum of 10,000,001 tones. The default value is 801 and the resolution is 2. Increasing the number of tones will increase the time needed for the NPR software to generate the correction file. You can estimate the time needed by calculating approximately .25 seconds per tone per iteration.

A waveform setup with many tones will have less dynamic range as the power is distributed to more tones.

Tone Spacing

Enter the frequency separation between the tones specified in Tone Count. Enter a minimum of 10 Hz and a maximum of 10 MHz tone spacing.

Coupled Parameters

Sample Rate Calculation
Sample Rate
Waveform Size

Noise Bandwidth

Range: 1 kHz to 1 GHz
Default: 80 MHz

Displays the bandwidth of the noise power ratio signal. (Read-only.) This value is computed using the following equation:

Noise Bandwidth = (Tone Count  - 1) * (Tone Spacing)

Noise Offset Calculation

Choices: Manual | Auto
Default: Manual

• Manual − the Noise Offset value is set back to 0 Hz.

• Auto − the NPR application calculates the noise offset automatically during a Download.

This parameter allows the notch to be centered in the noise bandwidth without the carrier being present. If the notch is offset from the center, the noise offset may be 0 Hz.

The maximum noise bandwidth is set by the Arb sample rate and the noise offset parameter. In general the maximum noise bandwidth is:

(Arb Sample Rate) / 1.25 – (2*Noise Offset)

 The maximum noise bandwidth establishes the system limits for Tone Count and Tone Spacing.

Coupled Parameters

Frequency

Noise Offset

Range: -312.5 MHz to +312.5 MHz
Default: 0 Hz

Enter a value to move the center of the noise bandwidth relative to the carrier. This allows the notch to be in the center of the noise bandwidth without the carrier leakage. Image tones may be visible in the notch of an uncorrected signal but these are much easier to suppress.

The Noise Bandwidth must be less than the maximum available bandwidth in order to offset it. The noise offset frequency resolution is determined by the tone spacing.

Coupled Parameters

Frequency
IQ Skew
IQ Gain Balance

See also NPR Noise Offset.

Phase Distribution

Choices: Random | Constant | Parabolic
Default: Random

Double-click or use the drop-down menu to select the type of phase distribution of the tones.

• Random − the phase distribution of the tones is determined by a random seed value entered in Phase Seed. Different seed values will give different phase offsets. The random phase offset is pseudo-random, so a particular phase seed value will always produce the same phase offset.

• Constant − aligns the tones so that they all have the same phase and produces a signal with the highest possible peak-to-average ratio. This results in the worst possible crest factor.  An NPR signal with constant phase between the tones is essentially an impulse. It will be difficult or impossible to level the signal power with the ALC.

• Parabolic − produces the signal with the lowest peak-to-average ratio (crest factor) and is, therefore, the least stressful.

Phase Seed

Range: 0 to 2147483647
Default: 1

Set the seed used by the random number generator when Random phase distribution is selected. When Phase Distribution is set to Random, Phase Seed allows you to specify the seed for the randomizer. The seed value is used to start the sequence of random numbers.

Amplitude Profile

The amplitude profile is a way to modify both the amplitude and phase of the NPR signal. Typically it is used to account for system amplitude roll-off and phase deviations. The amplitude profile is contained in an ASCII file that contains two or three columns of numeric data separated by tabs, white space, semi-colons or colons. A two-column file is assumed to contain frequency-amplitude pairs. The frequency values are absolute values and assumed to be in Hz. The amplitude values are assumed to be in dB and represent the relative amplitude response of the system.

The three column file is assumed to contain frequency, amplitude and phase triplets. Phase is in degrees and represents the relative phase response of the system. All rows in the file must contain 3 items to be imported as frequency, amplitude and phase. If any of the rows contains less than three items, all the phase information is ignored and the file is imported as an amplitude profile only. This is done to preserve backwards compatibility. Any additional columns are ignored.

The frequency values must be sorted in order, from lowest to highest frequency without any duplicates. Linear interpolation is used for frequencies not found in the file. The frequency values do not need to be equally spaced and may only contain frequency ranges of interest. Only values that are required by the NPR signal are kept after the file import. This means that a single amplitude profile file can contain several frequency bands each with its own relative minimum and maximum amplitude value.

The relative amplitude values have a range of -30 to +30 dB. However, the NPR correction measurement has been optimized for a 10 dB or less range. The correction measurement can fail when the amplitude variation is greater than 20 dB (Max - Min > 20 dB).

When not using the NPR signal corrections, the amplitude profile can be used as a way to put multiple notches in the NPR signal. The amplitude profile is interpreted as the system amplitude response and used to flatten the signal amplitude response. Basically, power is added to the signal at any frequencies where the amplitude profile is less than zero. Conversely, at any point where the amplitude profile is greater than zero, power is removed from the signal. If values of +200 dB are placed in the amplitude profile, the actual signal power will be set to -200 dB, effectively creating a notch. The actual amplitude range in the amplitude profile file is -30 to 200 dB.

See also Modify the NPR Signal Amplitude Flatness.

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