SNAS736C June 2017 – April 2019 LMX2595
To reduce the VCO tuning gain and therefore improve the VCO phase-noise performance, the VCO frequency range is divided into several different frequency bands. The entire range, 7.5 to 15 GHz, covers an octave that allows the divider to take care of frequencies below the lower bound. This creates the need for frequency calibration 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 with the FCAL_EN = 1. It is important that a valid OSCin signal must present before VCO calibration begins.
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 recalibrated, some minor phase noise degradation could result. The maximum allowable drift for continuous lock, ΔTCL, is stated in the electrical specifications. For this device, a number of 125°C means the device never loses lock if the device is operated under the Recommended Operating Conditions.
The LMX2595 allows the user to assist the VCO calibration. In general, there are three kinds of assistance, as shown in Table 5:
|ASSISTANCE LEVEL||DESCRIPTION||PROGRAMMABLE SETTINGS|
|No assist||User does nothing to improve VCO calibration speed, but the user-specified VCO_SEL, VCO_DACISET_STRT and VCO_CAPCTRL_STRT values do affect the starting point of VCO calibration. For oscillation to start up properly and for VCO to calibrate correctly, TI recommends setting VCO_SEL = 7, VCO_DACISET_STRT = 300 and VCO_CAPCTRL_STRT = 183 for all frequencies except 11.9 GHz ~ 12.1 GHz. For frequencies within 11.9 ~ 12.1 GHz, user must use VCO_SEL = 4 for proper VCO calibration.|| QUICK_RECAL_EN=0
|Partial assist||Upon every frequency change, before the FCAL_EN bit is checked, the user provides the initial starting point for the VCO core (VCO_SEL), band (VCO_CAPCTRL_STRT), and amplitude (VCO_DACISET_STRT) based on Table 6.||QUICK_RECAL_EN=0
|Close Frequency Assist||Upon initialization of the device, user enables QUICK_RECAL_EN bit.
The VCO uses the current VCO_CAPCTRL and VCO_DACISET_STRT settings as the initial starting point.
|Full assist||The user forces the VCO core (VCO_SEL), amplitude settings (VCO_DACISET), and frequency band (VCO_CAPCTRL) and manually sets the value. If the two frequency points are no more than 5MHz apart and on the same VCO core, the user can set the VCO amplitude and capcode for any frequency between those two points using linear interpolation.||QUICK_RECAL_EN=0
To do the partial assist for the VCO calibration, follow this procedure:
Find a VCO Core that includes the desired VCO frequency. If at the boundary of two cores, choose one based on phase noise or performance.
VCO_CAPCTRL_STRT = round (CCoreMin – (CCoreMin – CCoreMax) × (fVCO – fCoreMin) / (fCoreMax – fCoreMin))
VCO_DACISET_STRT = round (ACoreMin + (ACoreMax – ACoreMin) × (fVCO – fCoreMin)/(fCoreMax – fCoreMin))
In the range of 11900 MHz to 12100 MHz, VCO assistance cannot be used, and the settings must be: VCO_SEL = 4, VCO_DACISET_STRT = 300, and VCO_CAPCTRL_STRT = 1. Outside this range, in the partial assist for the VCO calibration, the VCO calibration runs. This means that if the settings are incorrect, the VCO still locks with the correct settings. The only consequence is that the calibration time might be a little longer. The closer the calibration settings are to the true final settings, the faster the VCO calibration will be.