Lambda SR1, Руководство по эксплуатации, Страница: 103 / 317

SR1 Operation 103
© 2014 Stanford Research Systems
control. For the variable cases the "Sample Control" will be initialized with the current OSR or ISR, the
user should enter the actual sample rate that will be used during testing.
The multitone generator can output either a "mono" signal, in which the A and B channel output signals
are the same, or a "stereo" signal, in which the tone placements for the two channels can differ. The
latter mode is useful for measuring crosstalk by the examining the amplitude in bins containing a tone in
one channel, but not in the other. Note that the default tones created by the Multitone Configuration
panel are always the same on both channels, even if stereo mode is selected. The user must explicitly
change the tone placement on one of the channels using the "Edit Tones" panel. This is especially
important when configuring crosstalk measurements where the two channels need to have different tone
placements.
The signal length determines the frequency resolution of the generated signal. For a signal length of N
and a sampling rate of Fs, there are N/2 bin frequencies each separated by (Fs/N) ranging from DC to
just under Fs/2. Increasing the signal length increases the number of possible tone positions and allows
for more differentiation of tones, harmonics, and IMD products, but increases the amount of time
necessary to acquire the FFT record. The "Repeat Count" is only used when the generator is operated in
burst mode. In this mode, each burst will contain the indicated number of repetitions of the complete
multitone signal. Increasing the repeat count beyond "one" is useful in burst situations as it gives the
analyzer more time to recognize the multitone stimulus and increases the dynamic range of the final
measurements.
Tone Generation
Tones are placed at bin frequencies between the values entered in the "Start Frequency" and "End
Frequency" fields according to the algorithm selected in the "frequency distribution" control:
Multitone Frequency Distribution
Linear The "desired number of tones" will be distributed between the frequency limits with an
approximately uniform frequency separation.
Log The "desired number of tones" will be distributed between the frequency limits with an
approximately constant ratio between the frequencies of adjacent tones.
Octave Starting with the first bin with a frequency greater than or equal to the "Start Frequency",
tones will be placed with each tone having a frequency two times greater than the previous
tone. The final tone has a frequency less than or equal to the "End Frequency."
Decade Starting with the first bin with a frequency greater than or equal to the "Start Frequency",
tones will be placed with each tone having a frequency ten times greater than the previous
tone. The final tone has a frequency less than or equal to the "End Frequency."
Prime Tones are placed at each prime numbered bin whose frequency falls within the selected
range. This placement algorithm guarantees that no tones will fall on harmonics of other
tones which is useful in separating the harmonics distortion due to each individual tone.
Linear
Prime The "desired number of tones" are placed on prime-numbered bin frequencies within the
selected range with an approximately uniform frequency distribution.
Log
Prime The "desired number of tones" are placed on prime-numbered bin frequencies within the
selected range with an approximately uniform ratio between adjacent tone frequencies.
The phase distribution of tones in a multitone signal directly affects the crest factor of the signal. Crest
factor is important because it limits the amount of power that can be placed in each tone for a given
peak amplitude of the resultant signal. SR1 offers several phase distribution choices for multitone
signals. "Zero" simply sets all tone phases to 0 which usually results in a signal with very high crest
factor but which may be useful for comparison purposes. "Random" assigns random phases to each
tone. The remaining choices, "Newman", "Schroeder", "Zygmund", and "Rudin", are various