Signal Generator
SpectraRTA
QuickGuide
Digital Recorder – Acquisition
The recorder utility allows you to record and playback from standard WAV files.
In addition, the RTA will process and display the spectral data during
recording/playback so that you can analyze the contents of the file.
Because the recorder and the analyzer must use the same format (sampling
rate, sampling precision, channels), some of the analyzer settings may change
when you open a file. For instance, the analyzer will switch to single channel
operation if you open a mono WAV file.
Make Backup: When this option is checked, a temporary backup file will be
created so that original file contents can be restored if necessary. Use the
<File><Set Paths> menu command to specify the directory to be used for these
temporary files. All backup files are removed when the application closes. You
can save time and disk space by turning off this option (you will be warned if
you try to overwrite the file).
Continuous Recording Capability
Wave files can store up to 2 GB of data in a single file (the corresponding time
depends upon the sampling format). When this limit is reached, the program
will momentarily stop recording, save the file and then start recording again in a
new file. If the file is does not yet have a name, the program will give it a name
using the computer date and time. The file will be stored in the \wave folder as
specified in the <File><Set Paths> dialog.
Example: c:\specpro\wave\recording_2002_11_26_102011.wav
(in this example the date and time were 10:20:11 Nov, 26 2002 when the file
was saved)
Playback Filtering
This feature allows you to apply user defined filtering to the audio being played
back. The filtering is applied in real time to both the audio stream and the
computed spectral data.
Filter Gain: This allows you to apply an overall gain to the filtered data. This is
useful to prevent clipping.
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SpectraRTA
QuickGuide
Signal Generator
The signal generator utility operates in conjunction with the playback (D/A)
channel of any Windows compatible sound card to produce a variety of test
signals. Selecting the <Utilities><Signal Generator> menu command or
pressing the F11 key will launch the signal generator.
Basic Operation
•
Select the type of waveform you wish to generate by clicking on the list of
waveforms.
•
Click the "Details" button to modify the specified waveform (not all waveform
types have user options).
•
Click the "Level" button to specify the generator output level.
•
Click the "Run" button to start the generator.
•
When the generator is running, the window will change to show only a single
"Stop" button.
Generator
Generator Output Level
Clicking on the "Level" button on the signal generator will allow you setup and
control the generator output level.
In order to accurately control the generator output level we need to know the
"full scale" signal level that your sound card can produce. This is the signal level
produced when a 100% full scale 8, 16 or 24 bit sine wave is generated.
Because most sound cards have the ability to control the output volume, "full
scale" is not necessarily the same thing as the "maximum possible" signal level
unless the volume control is set to its maximum value.
If you don't know this full scale level, you can still control the relative output
level. This value can be entered in either dB or Percent Full Scale.
If you have a volt meter or scope handy, you can easily measure the full scale
output level. Simply set the output level to 0 dB Full Scale and generate a 1kHz
tone. Now measure this value and enter the level in Volts RMS.
Some sound cards such as Digital Audio Labs CardD allow only fixed gains. We
have included some predefined values for this and other selected cards.
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SpectraRTA
QuickGuide
Keep in mind that if your sound card has a volume control, you will need to reset
the output level if the volume control is adjusted. Most sound cards save the
volume settings between sessions.
Refer to the CardD user's guide for information on setting the jumpers.
Because of variations between individual sound cards, you may wish to measure
the output level of your specific card for more accurate results.
When a single tone is generated, the output level is as specified above. When
two tones are generated, the level will drop by 6dB so that clipping will not occur
when the tones are in phase.
White Noise
White noise is a commonly used random noise waveform which has uniform
amplitude for all frequencies. Its averaged spectrum is flat when viewed on a
narrowband spectrum analyzer.
Pink Noise
Pink noise is a commonly used random noise waveform in acoustics. It has
more energy at the lower frequencies. It's amplitude decreases at a rate of 3dB
per octave. Its averaged spectrum is flat when viewed on an octave spectrum
analyzer such as SpectraRTA.
Noise Burst
This option allows you to generate a burst of noise. The "Burst Time" defines
the length of the noise period. The "Cycle Time" specifies the repetition period.
For instance, if the Burst Time is set to 1000 msec and the Cycle Time is set to
2000 msec, the generator will output 1 second of silence followed by 1 second of
noise. This pattern will repeat until the generator is stopped.
1 kHz Tone
This 1,000 Hz sinewave test tone is widely used for calibration and distortion
measurements.
Multiple Tones
This signal generator options allows you to configure your own set of tones. The
tone waveform consists of up to 10 user defined tonals. You can select the
number of active tones, their frequencies and relative amplitudes. The check
box to the left of the frequency setting determines whether or not the tone is
enabled. If the box is not checked, the associated tone will not be generated.
The frequency setting determines the tone frequency in cycles per second (Hz).
The relative power setting determines the amplitude of the tone compared to the
other tones being generated. The power is specified in decibels (dB) relative to
the overall output level.
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SpectraRTA
QuickGuide
Tone Burst
This option allows you to generate a tone burst. The "Burst Time" defines the
length of the tone period. The "Cycle Time" specifies the repetition period. For
instance, if the Burst Time is set to 1000 msec and the Cycle Time is set to 2000
msec, the generator will output 1 second of silence followed by 1 second of tone
signal. This pattern will repeat until the generator is stopped.
Frequency Sweep (Swept(Swept-Sine)
The swept sine waveform produces a sine wave which changes frequency over
time. Another term for this is a chirp. The start and stop frequencies determine
the limits of the sweep signal. The start frequency can be less than the stop
frequency in which case the sweep frequency will decrease over time.
The sweep type can be either Linear or Logarithmic. The sweep time specifies
how long the frequency takes to change from the start to the stop limits. This
value is specified in milliseconds. The sweep will repeat continuously until the
generator is stopped.
Swept sine waveforms are useful for frequency response testing. Set the
averaging to "Off" and set the Peak Hold option to "Forever" to capture the peak
amplitude as the signal is swept across the frequency span.
Level Sweep
The level sweep waveform produces a sine wave which changes amplitude over
time. You select the start and stop levels in decibels, the tone frequency, and
sweep time. The start level can be less than the stop level in which case the
signal level will decrease over time.
The sweep time specifies how long the level takes to change from the start to
the stop limits. This value is specified in milliseconds. The sweep will repeat
continuously until the generator is stopped.
Note: The accuracy of the signal is partly determined by the sound card itself.
The accuracy of the sampling rate clock in the card will govern the accuracy of
the frequency characteristics. In addition, many of the consumer grade sound
cards will not generate DC components. These cards are "AC coupled" and as
such, intentionally filter out or reject any DC component.
The result of this filtering is that signal components below approximately 20Hz
will be distorted. The flatness of the pulse and square waves will be affected
and the ramp signal will become non-linear.
Professional grade sound cards are often capable of generating DC components
(DC coupled) and will have no problems in this regard. The key specification
governing this is the frequency response; DC coupled cards will specify a range
from DC to 20kHz.
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SpectraRTA
QuickGuide
IMD Test Tones
InterModulation Distortion (IMD) testing requires two independent tones at
different amplitudes. The amplitude of the higher of the two tones is typically
12dB (4:1 ratio) lower than the low frequency tone. Several standard tone pairs
are provided as well as the ability to select your own custom tones.
User Defined Waveform
The user defined waveform option allows you to use a WAV file as the signal
source. The signal will be looped continuously until you press the Stop button.
Simply click on the "Pick" button to choose the .WAV file. Since the signal
generator loads this file into memory, you must have sufficient RAM on your
computer.
Digital Zero
Generates a signal containing all zero's or silence.
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SpectraRTA
QuickGuide
THD – Total Harmonic Distortion
Selecting the <Utilities><Total Harmonic Distortion> menu item will activate a
window containing the THD for the current spectrum.
Real world audio devices introduce unwanted side effects called distortion in
addition to the desired response. Total Harmonic Distortion (THD) is a widely
used measurements of this quantity and can be expressed as a percentage or in
decibels. The lower the THD value, the "cleaner" the response.
THD is the ratio of the harmonic power to the fundamental power. It is
computed by searching the entire spectrum to find the peak frequency
(fundamental) and then calculating the total power in the harmonic frequencies.
The THD level is then computed as the ratio of the total harmonic power to the
fundamental power. Residual noise is not included in this calculation.
The THD % and THD dB values are related as follows:
THD dB = 20 log ( THD % / 100 ).
A typical THD measurement will use a 1kHz reference test tone. This reference
signal is passed through the audio device under test and its spectrum computed.
It is often useful to employ long term averaging to cancel the random noise
components and enhance the measurement accuracy.
It should be noted that the purity of both the sound card and the reference
signal must exceed the device being measured if meaningful results are to be
expected. The purity of the signal generator is directly dependent on the THD of
the sound card used. Sound cards such as the "Turtle Beach Systems Tahiti" are
capable of THD values of less than 0.0025% (-92 dB).
This utility window can be resized as required by the user and its contents will
update anytime a new spectrum is computed. If the analyzer is configured to
display the spectrum of both the left and right channels, this utility window will
also show the value for both channels. The left channel value will appear above
the right channel value
Note: The contents of any open utility window will be printed on the right margin
of the spectrum plot.
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SpectraRTA
QuickGuide
IMD – InterModulation Distortion
InterModulation Distortion (IMD) is a measure of the distortion caused by the
interaction (mixing) of two tones.
When multiple signals are injected into a device, undesired modulation or mixing
of these two signals can occur. The IMD level is calculated by first computing
the frequencies and amplitudes of the two strongest tones in the spectrum. The
total power in each of the intermodulation product frequencies is then computed.
IMD is the ratio of the intermodulation power to the rms sum of the tone power.
A typical IMD measurement will use tones at 250 Hz and 8020 Hz with the
higher frequency tone reduced by 12dB (4:1 ratio) below the low frequency
tone. It is often useful to employ long term averaging to cancel the random
noise components and enhance the measurement accuracy.
This utility window can be resized as required by the user and its contents will
update anytime a new spectrum is computed. If the analyzer is configured to
display the spectrum of both the left and right channels, this utility window will
also show the value for both channels. The left channel value will appear above
the right channel value.
Note: The contents of any open utility window will be printed on the right margin
of the spectrum view.
SNR – Signal Noise Ratio
Selecting the <Utilities><Signal-to-Noise Ratio> menu item will activate a
window containing the SNR for the current spectrum.
The Signal to Noise Ratio (SNR) is the ratio of the signal peak power level to the
total noise level and is expressed in decibels (dB).
The SNR is computed by searching the entire spectrum to find the peak
frequency and then calculating the total noise power in the remaining spectrum.
The SNR is then computed as the ratio of the noise power to the peak power and
expressed in decibels.
This utility window can be resized as required by the user and its contents will
update anytime a new spectrum is computed. If the analyzer is configured to
display the spectrum of both the left and right channels, this utility window will
also show the value for both channels. The left channel value will appear above
the right channel value.
Note: The contents of any open utility window will be printed on the right margin
of the spectrum plot.
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SpectraRTA
QuickGuide
SINAD
SINAD is a common sensitivity measurement normally associated with FM
receivers. It is an acronym for Signal plus noise and distortion and is equal to
(S+N)/N in our system. It is expressed in dB. If the signal is much stronger
than the noise then SINAD value approaches the SNR value. Otherwise SINAD
will be greater than the SNR.
This utility window can be resized as required by the user and its contents will
update anytime a new spectrum is computed. If the analyzer is configured to
display the spectrum of both the left and right channels, this utility window will
also show the value for both channels. The left channel value will appear above
the right channel value.
Note: The contents of any open utility window will be printed on the right margin
of the spectrum view.
NF – Noise Figure
The Noise Figure (NF) is a measure of the signal-to-noise degradation by a
component under test. It is expressed in dB.
The Noise Figure is a two channel measurement similar to a Transfer Function
where the input is compared with the output. In the case of the Noise Figure, it
is the difference between the SNR at the output and the SNR at the input of the
device. An ideal device has a Noise Figure of 0 dB; practical devices always
have a larger Noise Figure than this.
A sinusoidal input test signal such as a 1kHz tone should be used as the source.
The Noise Figure can be setup as either "Left vs Right" or "Right vs Left". If the
input signal is on the right channel and the output on the left, you should use
"Left vs Right" otherwise the Noise Figure will be negative.
This utility window can be resized as required by the user and its contents will
update anytime a new spectrum is computed.
Note: Te contents of any open utility window will be printed on the right margin
of the spectrum view.
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SpectraRTA
QuickGuide
Peak Frequency
The peak frequency utility will display the frequency of the strongest spectral
component in the entire span. This utility window can be resized as required by
the user and its contents will update anytime a new spectrum is computed. If
the analyzer is configured to display the spectrum of both the left and right
channels, this utility window will also show the value for both channels. The left
channel value will appear above the right channel value.
Notes:
•
•
The frequency value reported is the ISO center frequency of the specified Octave
scaling format.
The contents of any open utility window will be printed on the right margin of the
spectrum view.
Peak Amplitude
The peak amplitude utility will display the amplitude of the strongest spectral
component in the entire span. This utility window can be resized as required by
the user and its contents will update anytime a new spectrum is computed. If
the analyzer is configured to display the spectrum of both the left and right
channels, this utility window will also show the value for both channels. The left
channel value will appear above the right channel value.
Wideband Power Level
Selecting the <Utilities><Wideband Power Level> menu item will activate a
window containing the wideband power level/SPL for the entire spectrum. This
wideband level is identical to the value displayed on the "Wide" bar on the right
side of the spectrum plot.
This utility window can be resized as required by the user and its contents will
update anytime a new spectrum is computed. If the analyzer is configured to
display the spectrum of both the left and right channels, this utility window will
also show the value for both channels. The left channel value will appear above
the right channel value.
Note: The contents of any open utility window will be printed on the right margin
of the spectrum plot.
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