Other Parts Discussed in Thread: LMX2572
Tool/software:
Hi there,
We are trying to use the LMX2572LP device as a PLL Local Oscillator in our radio which requires a PLL output from 60MHz to 1024MHz with a 24.576MHz input. For generating a single frequency the PLL works fine. A problem arises when we try to use the LMX2572 for a frequency hopping scheme.
We are utilizing the RAMP registers and RampDir pins to jump from one frequency to another. When testing and the PLL is ramped too far in one direction, the output just stays still and no longer hops. This is with a large VCO calibration threshold. When the VCO calibration threshold is lowered, the full frequency range can be achieved but there is distortion in the audio received when there are calibrations on the PLL.
The data below shows these frequency limits when the PLL is ramped in one direction until it stopped hopping any further. The VCO calibration threshold was set to a high value so that it wouldn't re calibrate.
|
Frequency (MHz) |
Lower Bound (MHz) |
Upper Bound (MHz) |
Bandwidth (MHz) |
VCO Core (div) |
Original Numerator |
|
50 |
49.29 |
50.69 |
1.4 |
VCO 6 ( 64) |
1252698794 |
|
75 |
74.08 |
75.87 |
1.79 |
VCO 4 (32) |
1073741823 |
|
101 |
99.95 |
102.0 |
2.05 |
VCO 1 (16) |
2058005162 |
|
121.368 |
119.9 |
122.77 |
2.87 |
VCO 2 (16) |
0 |
|
125 |
123.47 |
126.5 |
3.03 |
VCO 2 (16) |
3131746985 |
|
170 |
167.67 |
172.17 |
4.5 |
VCO 5 (16) |
1386916522 |
Would you know what is causing these boundaries? they appear to be not related to the VCO core or original numerator when starting to hop.
Is there a better way to hop using the LMX2572LP? The hop bandwidth needs to be 8MHz at 300 hops per second. If the VCO is calibrated every hop (even in partial assist mode) the audio received at the other end is not good quality as the PLL output during calibration is not good. That is why hopping with the RAMP registers was explored.
Thanks for your help
