©2018
Rev 2.0
17
Functional Description
The disadvantage of this approach is the requirement of high IF1 and LO1 frequencies which places
demanding phase noise requirements on the LO1 synthesizer.
The Second Mixer and IF2 Path
The second IF (IF2) is typically centered at 305 MHz and has settable range with 5 MHz steps. The
relationship between LO2, IF1, and IF2 is:
𝐿𝑂2 = 𝐼𝐹1 − 𝐼𝐹2
IF1 and IF2 are settable with the condition that LO2 must be tuned in 5 MHz steps and have a range
less than ± 50 MHz.
The converted IF2 signal out of the mixer is passed through a 500 MHz filter to suppress IF1 and
LO2 leakages. The IF2 LPF is not aggressive, so converted LO1 leakages that are close to the IF1
frequency may appear at relatively high values in the IF2 path. After IF2 LPF, the signal passes
through the selectable bandpass filters BPF0 and BPF1, whose frequency bandwidths are
dependent of the customer option for IF BW. These filters may shape the final BW of the device
for low IF frequencies because there are limited filter selections. If there is a need for further signal
shaping, an external LPF or BPF will need to be placed after the IF OUT port. This signal will be
converted to IF3 via the third IF3 mixer.
For low RF signals, BPF1 can optionally be fitted with a narrow 100 kHz BW filter to suppress LO
leakages, especially for those frequencies that are less than the BW of BPF0.
The Third mixer and IF3 Path
The third IF (IF3) is tunable from
±5
MHz to
±
100 MHz, as LO3 is tuned from 220 MHz to 420 MHz
in 5 MHz steps. The negative value of IF3 indicates that the spectrum is inverted with respect to
the RF input spectrum. When spectral inversion is selected, LO3 tunes below IF2 and it tunes above
for no spectral inversion. There is an LPF filter in this stage to provide filtering of IF2 and LO3
leakages as well as harmonics of the IF3 signal.
Attenuators IF3_ATTEN1 and IF3_ATTEN2 are used to set the output amplitude of IF3. Attenuation
should be applied to IF3_ATTEN2 first and any overflow of more than 30 dB is then applied to
IF3_ATTEN1. However, if better linearity is needed, apply the first 6 dB to IF3_ATTEN1 and then
the rest to IF3_ATTEN2.
Signal Chain Configurations
Recommended signal chain configurations are provided below for various application scenarios.
These are strictly recommendations and not indicative of the downconverter function limitations;
users will need to adapt the device to their own applications.
Low Input RF
If the RF value is on the order of the bandwidth of the narrowest filter, significant LO leakage
may appear at the IF output. Placing an external IF filter will provide further suppression of the
leakage that appears outside of the passband. If the external digitizer can handle the RF directly,
then the RF signal should be switched to the IF port directly, bypassing conversion.