JAJSIB5D November 2019 – March 2022 LMX2694-EP
PRODUCTION DATA
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 (7550 to 15100 MHz) 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 recommended operating conditions.
The LMX2694-EP allows the user to assist the VCO calibration. In general, there are three kinds of assistance, as shown in Table 7-4.
ASSISTANCE LEVEL | DESCRIPTION | VCO_SEL | VCO_SEL_FORCE VCO_CAPCTRL_FORCE VCO_DACISET_FORCE | VCO_CAPCTRL VCO_DACISET |
---|---|---|---|---|
No assist | User does nothing to improve VCO calibration speed. | 7 | 0 | Don't care |
Partial assist | Upon every frequency change, before the FCAL_EN bit is checked, the user provides the initial starting VCO_SEL. | Choose by table | 0 | Don't care |
Full assist | The user forces the VCO core (VCO_SEL), amplitude settings (VCO_DACISET), and frequency band (VCO_CAPCTRL) and manually sets the value. | Choose by readback | 1 | Choose by readback |
For the no-assist method, just set VCO_SEL = 7 and this is done. For partial-assist, the VCO calibration speed can be improved by changing the VCO_SEL bit according to frequency. Note that the frequency is not the actual VCO core range, but actually favors choosing the VCO. This is not only optimal for VCO calibration speed, but required for reliable locking.
fVCO | VCO CORE (MIN) |
---|---|
7550 - 8740 MHz | VCO1 |
8740 - 10000 MHz | VCO2 |
10000 - 10980 MHz | VCO3 |
10980 - 12100 MHz | VCO4 |
12100 - 13080 MHz | VCO5 |
13080 - 14180 MHz | VCO6 |
14180 - 15100 MHz | VCO7 |
For fastest calibration time, it is ideal to use the minimum VCO core as recommended in Table 7-5. The Table 7-6 shows typical VCO calibration times (in µs) for this choice in bold as well as showing how long the calibration time is increased if a higher than necessary VCO core is chosen. Realize that these calibration times are specific to these fOSC and fPD conditions specified and at the boundary of two cores, sometimes the calibration time can be increased.
fVCO (GHz) | VCO_SEL | ||||||
---|---|---|---|---|---|---|---|
VCO7 | VCO6 | VCO5 | VCO4 | VCO3 | VCO2 | VCO1 | |
8.1 | 650 | 540 | 550 | 440 | 360 | 230 | 110 |
9.3 | 610 | 530 | 540 | 430 | 320 | 220 | Invalid |
10.4 | 590 | 520 | 530 | 430 | 240 | Invalid | |
11.4 | 340 | 290 | 280 | 180 | Invalid | ||
12.5 | 270 | 170 | 120 | Invalid | |||
13.6 | 240 | 130 | Invalid | ||||
14.7 | 160 | Invalid |