LTC55933
4
MEASUREMENT EQUIPMENT
AND SETUP
The LTC5593 is a dual high dynamic range down-
converting mixer IC with very high input 3rd order
intercept. Accuracy of its performance measure-
ment is highly dependent on equipment setup and
measurement
technique. The
recommended
measurement setups are presented in Figure 5,
Figure 6, and Figure 7. The following precautions
should be observed:
1. Use high performance signal generators with
low harmonic output and low phase noise, such
as the Rohde & Schwarz SME06. Filters at the
signal generators’ outputs may also be used to
suppress higher-order harmonics.
2. A high quality RF power combiner that provide
broadband 50
Ω
-termination on all ports and
have good port-to-port isolation should be
used, such as the MCLI PS2-17.
3. Use high performance amplifiers with high IP3
and high reverse isolation, such as the Mini-
Circuits ZHL-1042J, on the outputs of the RF
signal generators to improve source isolation to
prevent the sources from modulating each
other and generating intermodulation products.
4. Use attenuator pads with good VSWR on the
demonstration circuit’s input and output ports to
improve source and load match to reduce re-
flections, which may degrade measurement
accuracy.
5. A high dynamic range spectrum analyzer, such
as the Rohde & Schwarz FSEM30, should be
used for linearity measurement.
6. Use narrow resolution bandwidth (RBW) and
engage video averaging on the spectrum ana-
lyzer to lower the displayed average noise level
(DANL) in order to improve sensitivity and to
increase dynamic range. However, the trade off
is increased sweep time.
7. Spectrum analyzers can produce significant
internal distortion products if they are over-
driven. Generally, spectrum analyzers are de-
signed to operate at their best with about
-30dBm at their input filter or preselector. Suffi-
cient spectrum analyzer input attenuation
should be used to avoid saturating the instru-
ment, but too much attenuation reduces sensi-
tivity and dynamic range.
8. Before taking measurements, the system per-
formance should be evaluated to ensure that:
a.
Clean input signals can be produced. The
two-tone signals’ OIP3 should be at least
15dB better than the DUT’s IIP3.
b.
The spectrum analyzer’s internal distortion
is minimized.
c.
The spectrum analyzer has enough dy-
namic range and sensitivity. The measure-
ment system’s IIP3 should be at least 15dB
better than the DUT’s OIP3.
d.
The system is accurately calibrated for
power and frequency.
A SPECIAL NOTE ABOUT RF TERMINATION
The LTC5593 consists of high linearity passive
double-balanced mixer cores and IF buffer amplifi-
ers. Due to the bi-directional nature of all passive
mixers, LO±IF mixing product is always present at
the RF input, typically at a level of 12dB below the
RF input signal. If the LO±IF “Pseudo-Image Spur”
is not properly terminated, it may interfere with the
source signals, and can degrade the measured
linearity and noise figure significantly. To avoid in-
terference from the LO±IF “Pseudo-Image Spur”,
terminate the RF input port with an isolator, di-
plexer, or attenuator. In the recommended meas-
urement setups presented in Figure 6 and Figure
7, the 6dB attenuator pad at the demonstration cir-
cuit’s RF input serves this purpose.
