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intermodulation products at some
level which can interact with
neighboring elements in the RF
chain. Since these neighboring
elements in real-world systems are
often filters, out-of-band signals are
fully reflected back into the mixer
where they produce a secondary
set of spurious signals, which
additionally limit the dynamic range
of the system.
This has been a longstanding
problem in RF systemdesign, against
which designers were more or less
helpless until recently. Various
techniques have been employed to
compensate forthe adverse effects
of stopband reflections (inserting
attenuators or isolation amplifiers
around the mixer, for example), but
these solutions still degrade system
SNR and dynamic range, which
becomes self-defeating.
The results presented in this article
demonstrate how reflectionless
filters may be used with mixers in
up-converter and down-converter
configuration to absorb those
out-of-band signals within the
filter circuit itself. In comparison
to conventional filters in the
same circuit, reflectionless filters
achievesignificant reduction of third
order intermodulation products,
which corresponds to proportional
improvement in system dynamic
range.
Mini-Circuits offers a wide range of
reflectionless filters from stock in tiny
QFN packages and bare die form,
including low pass, band pass and
high pass models with passbands
from DC to 30 GHz. These patented
products give designers a new tool to
achieveperformance improvements
similar to those presented here in
their own systems.
Figure 2. Top-level schematic diagram of the receiving circuit
Table 3: Summary of Test Conditions for Two-Tone Intermodulation Test
in Downconverter Configuration
FILTER
POSITIONS
• Case 1: NO FILTERS
• Case 2: REFLECTIVE FILTERS (VARIOUS) on RF and
IF PORTS
• Case 3: REFLECTIONLESS FILTERS on RF and IF
PORTS
RF INPUT 800 MHz
2200 MHz
TONES TWO TONES SPACING: 1
MHz
(800 MHz AND 801 MHz);
(2200 MHz AND 2201 MHz)
MIXERS
TESTED
•ADE-10H+
•SYM-30DHW+
FILTERS
TESTED
LUMPED ELEMENT (Sharp Filters)
RF
SLP-1000
IF
SLP-150+
SLP-1200+
SLP-2400+
SLP-2950+
LTCC FILTERS (Shallow Slope)
RF
VLF-800+
IF
VLF-160+
VLF-1000+
VLF-2250+
VLF-2500+
REFLECTIONLESS FILTERS
RF
XLF-861+
IF
XLF-151+
XLF-122+
XLF-252+
Test & Measurement
Special Edition
New-Tech Magazine Europe l 55