SMIQ
Arbitrary Waveform Generator ARB
1125.5555.03
2.343
E-9
The I/Q data are loaded by the host computer via the DATA IN interface to the DSP which passes them
into a non-volatile FLASH RAM. The latter is organized in 22 blocks of 64ksamples, each. At least one
block is occupied by each waveform. The I/Q data are subject to a convolution filter which limits the
signal bandwidth. This is required for further processing.
If a waveform is selected, the I/Q data are loaded into the output memory. They are convoluted by a
correction filter, which compensates all errors in the path to the I/Q modulator. In particular, the Si
frequency response of the D/A converter as well as the frequency responses of the analog filter on the
SMIQB60 and of the I/Q filter of the option SMIQB47 are corrected. Fig. 2-216 illustrates the further
signal flow in the frequency and time domains.
Fig. 2-216
Signal flow SMIQB60
A 1-MHz sinewave signal with fa = 3 MHz is sampled by way of example. The sampling procedure
causes aliasing to occur in the frequency range (n*3 ± 1) MHz. The following interpolation filter
increases the sampling rate by the factor 4. This corresponds to a correct reconstruction of the
additional sampling values in the time domain. Due to the sample-and-hold process at the output of the
subsequent D/A converter, a stair-step signal is generated in the time domain which corresponds to a
SI-weighting in the frequency domain. The analog filter compensates for the aliasing of the interpolated
sampling rate, the output signal is a sinewave signal in the time domain. SMIQB60 automatically sets
the interpolation rate such that aliasing is suppressed by the analog filter.
The sampling rate is increased internally by the factor 4, in order to avoid errors with CCDF
determination of the waveform with low sampling rate.
The absolute value of the I/Q output signal is 0.5 Vs at 50
Ω
(= 0 dB) in Normal mode. This is the
nominal output of the I/Q modulator. The output level may be varied in Manual mode between -6 dB and
3 dB in order to optimize the ACP in various channel offsets, e.g. The range above 0 dB is not specified,
signal frequencies above 10 MHz may lead to a limitation.
The internal calibration of the SMIQB60, which is performed automatically with calibration of the vector
modulation, corrects offset and gain errors to a minimum.
