LMX2541SQ2060E_NOPB Datasheet, PDF (34/64 Pages) Texas Instruments – LMX2541 Ultra-Low Noise PLLatinum Frequency Synthesizer with Integrated VCO

LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
SNOSB31I – JULY 2009 – REVISED FEBRUARY 2013
www.ti.com
f_VCO = f_PD × N = f_OSCin × N / R
(3)
In order to the reduce the VCO tuning gain and therefore improve the VCO phase noise performance, the VCO
frequency range is divided into many different frequency bands. This creates the need for frequency calibration in
order to determine the correct frequency band given a desired output frequency. The frequency calibration
routine is activated any time that the R0 register is programmed. It is important that the OSC_FREQ word is set
correctly to have this work correctly.
The VCO also has an internal amplitude calibration algorithm to optimize the phase noise which is also activated
any time the R0 register is programmed. The optimum internal settings for this are temperature dependent. If the
temperature is allowed to drift too much without being re-calibrated, some minor phase noise degradation could
result. For applications where this is an issue, the AC_TEMP_COMP word can be used to sacrifice phase noise
at room temperature in order to improve the VCO phase noise over all temperatures. The maximum allowable
drift for continuous lock, ΔT_CL, is stated in the electrical specifications. For this part, a number of +125 C means
the part will never lose lock if the part is operated under recommended operating conditions.
Programmable VCO Divider
The VCO divider can be programmed to any value from 2 to 63 as well as bypass mode if device is in full chip
mode. In external VCO mode or divider mode, all values except bypass mode can be used for the VCO divider.
The VCO divider is not in the feedback path between the VCO and the PLL and therefore has no impact on the
PLL loop dynamics. After this programmable divider is changed, it may be beneficial to reprogram the R0 register
to recallibrate the VCO . The frequency at the RFout pin is related to the VCO frequency and divider value,
VCO_DIV, as follows:
f_RFout = f_VCO / VCO_DIV
(4)
When this divider is enabled, there will be some far-out phase noise contribution to the VCO noise. Also, it may
be beneficial for VCO phase noise to reprogram the R0 register to recalibrate the VCO if the VCO_DIV value is
changed from bypass to divided, or vice-versa.
The duty cycle for this divider is always 50%, even for odd divide values. Because of the architecture of this
divider that allows it to work to high frequencies and always have a 50% duty cycle, there are a few extra
considerations:
• In divider only mode, there must be 5 clock cycles on the ExtVCOin pin after the divide value is programmed
in order to cause the divide value to properly changed. It is fine to use more than 5 clock cycles for this
purpose.
• For a divide of 4 or 5 ONLY, the R4 register needs to be programmed one more time after the device is fully
programmed in order synchronize the divider. Failure to do so will cause the VCO divider to divide by the
wrong value. Furthermore, if the VCO signal ever goes away, as is the case when the part is powered down,
it is necessary to reprogram the R4 register again to re-synchronize the divider. Furthermore, if the R0
register is ever programmed in full chip mode, it is also necessary to reprogram the R4 register.
Programmable RF Output Buffer
The output power at the RFout pin can be programmed to various levels as well as on and off states. The output
state of this pin is controlled by the RFoutEN pin as well as the RFOUT word. The RF output buffer can be
disabled while still keeping the PLL in lock. In addition to this, the actual output power level of this pin can be
adjusted using the VCOGAIN, DIVGAIN, and OUTTERM programming words. The reader should note that
VCOGAIN controls the gain of the VCO buffer, not the tuning constant in of the VCO.
Powerdown Modes
The LMX2541 can be powered up and down using the CE pin or the POWERDOWN bit. When the device is
powered down, the programming and VCO calibration information is retained, so it is not necessary to re-
program the device when the device comes out of the powered down state (The one exception is when the
VCO_DIV value is 4 or 5, which has already been discussed.). The following table shows how to use the bit and
pin.
CE Pin
Low
POWERDOWN Bit
Don't Care
Device State
Powered Down
34
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