here is th issue decription with all figures:
Issue with external VCO.pdf (1,3 MB)
At first here my development environment is the
- DVK2c-AX8052-MAIN V1.9 together with the
- DUK2c-MOD5243-2 Rev 1.1
- To set the IDE I use the AX-RadioLab (v2.11s):
See figure 1 in the appended description!
I want to use the AX5243 in a 27MHz Application and I made several investigations and tests. I had read several application notes and the corresponding datasheets. And I got success with external inductors and internal VCO2 with center frequencies down to 40MHz. But as closer the center frequency was at 27MHz the stability of the PLL was more sensitive.
So, I decided to use an external VCO. The VCO have a differential output within 0V to 3V (VCC). As shown in the following oscillogram.
See figure 2 in the appended description!
I added two voltage dividers (4.7k-4.7k) to get the half voltage level. This should deliver the input voltage for AX5246 within 0V to 1.8V with a typical differential voltage of 0.7V as defined in the datasheet. Furthermore, I changed the PLLVCODIV setting from 0x36 (working properly with the external Inductor) to 0x16 to activating the external VCO.
See figure 3 in the appended description!
But the signals at L1 and L2 didn’t show any frequency. As shown in the following oscillogram the signals at L1 and L2 are at a constant 0.8V level. Therefore, I assume the L1 and L2 pins have rather an output than an input function.
See figure 4 in the appended description!
I made more tests to find out what the AX-RadioLab (v2.11s) recommended as register setting when I change the pin configuration to “external VCO”. But I was surprised, that the only changes form “VCO2 (external inductor)” to “external VCO” was in the PLLCPI register setting from 0x01 to 0x10. PLLVCODIV register setting was unchanged in both cases at 0x24 (or 0x25). But this is the setting for the “VCO1 (internal inductor)”.
See figure 5 in the appended description!
Can you please help me, by answering the following questions?
- Is it realy possible to use the AX5243 with an external VCO?
- Can you tell me what changes I must do to switch the L1 and L2 as frequency inputs?
- Can you give me an approved register setting with external VCO?
If you need mor information, pleas let me know.