SC5412A Operating & Programming Manual
Rev 1.1.0
9
condition changes significantly, nulling may need to be performed again. For example, if the LO is nulled
at 25
O
C, then it will most likely need to be nulled again at 45
O
C although the frequency and input
amplitude is held relatively constant. Digital data must be corrected to compensate for the modulator IQ
amplitude and phase imbalances in addition to nulling out the LO leakage for best application results.
The IQ core, comprising two mixers driven at LO quadrature signals, should not be driven too hard with
IF signals because the resulting IMD3 of the RF signal may be too high. It is recommended to use an IF
level of 0.5 V peak-to-peak differential or less to lower the IMD3. However if the IF is too low, the RF
signal-to-noise ratio may suffer. Subsequently, the user must look at the RF result to determine the
optimal IF input level.
Figure 2. IF input section.
RF Output Section
The output section comprises adjustable digital step attenuators (DSA), RF amplifiers (one is selectable),
and a bank of low pass filters to suppress RF harmonics. These are used to condition the RF signal
generated in the IQ core component. The RF signal has its best performance exiting the IQ core;
therefore it is important to select a suitable RF gain to minimize further degradation of the signal. A
general rule is to apply more attenuation earlier in the RF path (close to the IQ core) to improve
linearity, and more gain to improve signal-to-noise performance. Generally, for RF frequencies less than
3 GHz, to vary signal levels greater than -20dBm the attenuators RF ATTEN#1, and RF ATTEN#2 should
be used as controls. To lower signal levels below -20 dBm, ATTEN#3 and ATTEN#4 should then be used.
Although all attenuators in the path have 30 dB of adjustment with 1 dB steps, it is not always
recommended to maximize the full range of an attenuator before moving to the next one in the path. As
an example, attenuation of ATTEN#1 should rarely be more than 10 dB because any further attenuation
will degrade the signal-to-noise ratio at a faster rate than the improvement of IMD3. If any further IMD3
improvement is needed, ATTEN#2 or ATTEN#3 should be used to create the desired attenuation.
ATTEN#4 should be fully utilized first to generate signals lower than -30 dBm.
The selectable RF amplifier is used to improve the overall gain of the device for frequencies in the 5 GHz
to 6 GHz range, as the RF loss in the RF path become more significant at these higher frequencies. The
gain flatness roll-off over the entire operating band is typically 15-16 dB. The RF amplifier has 14 dB of
gain. If the overall gain is too excessive at the desired operating frequency, ATTEN#2 should be used to
readjust the gain to the required level.
Vocm
100
100
100
100
Vocm
DAC
200
25
25
100
Input Diff
Impedance
IQ MOD
I
25
to 100
LPF
160 MHz LPF
Differential
Driver