Datasheet

LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
www.ti.com SNOSB31I –JULY 2009–REVISED FEBRUARY 2013
MHz typically has much better phase noise than a direct 20 MHz signal, if it comes from a signal generator.
Another technique 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 multiplied 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 reference 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
RFout
/ f
OSCin
) . A
characteristic of this noise is that 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 reference source, not the LMX2541 device. Another characteristic 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
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 resistors 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 subtract away this noise. The divider noise floor tends to be flat, whereas the VCO phase noise decreases
with offset frequency, 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 desired 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 divider could be set to 10 and then 20 dB is subtracted from
the VCO phase noise to figure its contribution at 400 MHz. Provided 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.
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E