LMX2541SQ2060E/NOPB National Semiconductor, LMX2541SQ2060E/NOPB Datasheet - Page 30

• Manufacturer Part Number: LMX2541SQ2060E/NOPB
• Description: IC PLL FREQ SYNTH W/VCO 36LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Clock/Frequency Synthesizer (RF)r

Specifications of LMX2541SQ2060E/NOPB

• Pll: Yes
• Input: Clock
• Output: Clock
• Number Of Circuits: 1
• Ratio - Input:output: 2:2
• Differential - Input:output: No/No
• Frequency - Max: 2.24GHz
• Divider/multiplier: Yes/No
• Voltage - Supply: 3.15 V ~ 3.45 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-LLP
• Frequency-max: 2.24GHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LMX2541SQ2060E

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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 measure-
ments, 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.
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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 fre-
quency. For instance, a 500 MHz signal divided down to 20
MHz typically has much better phase noise than a direct 20
MHz signal, if it comes from a signal generator. Another tech-
nique is to measure the noise of the reference source and
then multiply it up and then subtract it from the measurement.
For instance, if the signal source was 500 MHz and the output
frequency was 4 GHz, this signal source noise would be mul-
tiplied up by a factor of 20·log(4 GHz / 500 MHz) = 18 dB.
Once that is done, the 1/f noise and the flat noise can be
measured.
PLL Phase Noise Measurement - 1/f Noise
The 1/f noise dominates closer to the carrier. Special care
should be taken to ensure that this is not the noise of the ref-
erence source. The noise contribution of the reference source
at the RFout pin can be calculating by measuring what is
coming into the OSCin pin and then adding a correction factor
of 20·log( f
it follows a 10 dB/decade slope. If the slope of the measured
noise looks more than 10 dB/decade, it is likely to be the ref-
erence source, not the LMX2541 device. Another character-
istic of the 1/f noise is that it is independent of phase detector
frequency. So to fully expose the 1/f noise, raise the phase
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
RFout
/ f
OSCin
) . A characteristic of this noise is that
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detector frequency as high as possible, since this lowers the
flat noise, but not the 1/f noise.
PLL Phase Noise Measurement - Flat Noise
The PLL flat noise is measured at an offset that is not too close
to the PLL 1/f noise, but also well inside the loop bandwidth.
Many phase noise profiles have a point where the PLL noise
flattens to a minimum value between the carrier and the loop
bandwidth. This is where the flat noise should be measured.
To measure the 1 Hz normalized phase noise, it is often easier
to measure this with a lower phase detector frequency so that
this flat noise is higher and easier to measure.
VCO Phase Noise Measurement
In order to measure the VCO phase noise, the loop filter re-
sistors should be set to their minimum value to reduce their
noise contribution. The loop bandwidth should also be made
as narrow as possible. Because the loop bandwidth is very
narrow, the cleanliness of the OSCin signal is therefore not
as important. The phase noise is measured outside the loop
bandwidth of the system.
An alternative way that might not be as accurate, but is much
easier to do is to lock the device to a frequency and then set
the CPT bit to 1 to disable the charge pump. The VCO will
drift a little, the averaging on the equipment should be reset
after this bit is changed and one can not take to long to take
this measurement. Test equipment that tracks the signal
source is better if using this open loop technique.
Divider Phase Noise Measurement
The basic method for measuring the divider noise is to drive
the divider with a noise source of known value and then sub-
tract away this noise. The divider noise floor tends to be flat,
whereas the VCO phase noise decreases with offset frequen-
cy, so this measurement is made at as far of an offset that is
possible. When using Internal VCO Mode (MODE=0), the raw
VCO phase noise with VCO_DIV=1 can be measured. Then
the VCO divider can be programmed to get close to the de-
sired frequency. For example, the VCO frequency can be set
to 4 GHz and the phase noise measured. This phase noise
data can be saved or downloaded. Suppose then that one
was interested in the divider noise at 400 MHz. The VCO di-
vider could be set to 10 and then 20 dB is subtracted from the
VCO phase noise to figure its contribution at 400 MHz. Pro-
vided that the actual phase noise measured at 400 MHz with
VCO_DIV is above this, then one can assume that this is the
noise of the divider.
An alternative way to measure this is to drive the OSCin pin
and use Divider Only (MODE=2) to measure the phase noise.
This gives direct control of the frequency, but one should be
sure that the noise being measured is the device and not the
frequency source.