Datasheet
OSCin
DC
Blocking
Capacitor
Signal Generator
Spectrum Analyzer
SMA Cable
100 MHz
Device
Under
Test
Evaluation Board
SMA Cable
Power Supply
3.3 V
RFout Pin
Matching
Network
LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
SNOSB31I –JULY 2009–REVISED FEBRUARY 2013
www.ti.com
Figure 29. Charge Pump Sink vs. Source Current Mismatch
RFout Output Power Test Setup
(1)
The output power is tested by programming the VCO output to a desired frequency and measuring with a
spectrum analyzer. A 3 dB pad is used and this gain as well as any losses from the cable are added to the actual
measurement. As for the DC blocking capacitor, typically 100 pF is used for frequencies above 2 GHz and 0.1 uF
are used for frequencies below 2 GHz. It turns out that the measurement is not as sensitive as one would expect
to this blocking capacitor value. The output power is mainly a function of the frequency of the output buffer and
the settings of the VCO_DIV (1 or >1), OUTTERM, VCOGAIN, and DIVGAIN bits. It is not very sensitive to the
actual frequency option of the part. For instance, the LMX2541SQ2060E and the LMX2541SQ2380E both should
have similar output power at 2.2 GHz. Note that this same test setup can also be used to measure harmonics.
Phase Noise Measurement Test Setup
The basic setup technique for all noise tests is to measure the noise at the output of the RFout pin in Internal
VCO Mode (MODE=0) with a phase noise analyzer. For all measurements, the internal loop filter components
(LF_R3, LF_R4, LF_C3, and LF_C4) should be set to their minimum values. There are some special
considerations depending on what kind of noise is being measured.
PLL Phase Noise Measurement
To get an accurate measurement of the PLL phase noise, one needs to ensure four things.
• The PLL loop bandwidth is sufficiently wide so that the VCO noise does not degrade the measurement
• The measurement is not corrupted by peaking in the loop filter response.
• The reference source is sufficiently clean so that this does not degrade the measurement.
• A distinction is made between the PLL flat noise and the PLL 1/f noise
If the PLL loop bandwidth is made as wide as possible, then this helps keep the peaking of the loop filter
response and the VCO noise from degrading the measurement. For the ultimate accuracy, this loop filter
response can be factored into the measurement. As for the cleanliness of the reference source, the best sources
tend to be those that are fixed, such as a 100 MHz Wenzel oscillator. Signal generators tend to be noisy, but if
that is all that is available, then there are a few things that can help compensate for this. One technique is to use
a higher frequency and divide this down to a lower frequency. For instance, a 500 MHz signal divided down to 20
(1) The measurement of the output power is sensitive to the test circuit. All the numbers in the electrical specifications and typical
performance curves were obtained from a characterization setup that accommodate temperature testing and changing of parts. In a
more optimized setup the measured RF output power is typically on the order of 1.5 to 2.4 dB higher.
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E