SC5412A Operating & Programming Manual
Rev 1.1.0
11
There are nine low pass filters in the RF filter bank. These filters are automatically selected when the
user enters the operating frequency. These filters can also be selected manually should the user choose
to do so. As with all filters, there is generally an amplitude roll-off as the frequency nears its 3 dB cutoff
point so it is important to understand that frequencies near the cutoff point may experience a slightly
faster roll-off of their IF bandwidth. A typical 1 dB IQ IF bandwidth is about 160 MHz. The user may want
to choose a higher frequency filter if
this becomes a problem. See the section “
Setting the SC5412A:
Writing to Configuration
Registers”
for more details. The filters in both the RF and LO filter banks are
identical and are listed below.
Filter Number
1 dB Cutoff Frequency
0
400 MHz
1
500 MHz
2
650 MHz
3
1000 MHz
4
1400 MHz
5
2000 MHz
6
2825 MHz
7
3800 MHz
8
6000 MHz
LO Input Section
The SC5412
A requires an external RF signal as its “Local Oscillator”
(LO) for the frequency conversion
process.
The external RF signal must be connected to the “LO in” port.
The typical required input level is
-3 dBm to 3 dBm. These levels are required to sufficiently drive the IQ modulator for good linearity
performance and conversion loss. The LO signal is conditioned through a bank of low-pass filters to
reduce the signal harmonics. Reducing the harmonics produces a “purer” signal tone, improving the
duty cycle of the LO as it drives the mixers of the modulator. Additionally, the LO signal can be passed
out of the device via the
“LO o
ut
” port. This output can be used as the input LO source for another
modulator, for example. Driving multiple modulators (or demodulators when
working with SignalCore’s
SC5312A) with the same derived LO signal optimizes phase coherency between them. When this port is
not in use, it is highly recommended to terminate it into a 50
load.