Using AX5243 with external VCO

Hello OnSemi-Team,
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.

Kind regards


Hello OnSemi-Team,
Here some further test with the external VCO:

  1. DC-Decoupling
    One idea was DC decoupling the voltage divider to the L1/L2 inputs. Therefore, I added two 100nF (X7R) capacitors in series to the L1/L2 inputs as shown in the following schematic:

AX5243 with external VCO SCM2

Figure 6: DC decoupling of the differential input signals

But this didn’t also work. I can’t either measure any Frequency at the L1/L2 inputs nor get a frequency range where the PLL locked can be locked.

  1. Tests without Voltage divider
    Assuming impedance of the 4.7kΩ resistors are too high, I made one test with shorted resistors R1 and R2. Then I get clock signals at L1 and L2 with an amplitude of ca. 2.5Vpp. But the PLL will also not lock under these conditions.

  2. Investigating the L1/L2 input impedance
    I tried to find out if the input impedances of L1 and L2 are changing at different settings in the with the different PLLVCODIV settings. Therefore, I replaced the R1 and R2 with a 1.0kΩ resistor and I get a small frequency signal (but with a bad signal form) at L1 and L2. Under this condition, I observed the signal with all possible PLLVCODIV register settings for VCOI MAN, VCO2INT and VCOSE: 0x06, 0x16, 0x26, 0x46, 0x56, 0x66, 0x76. But the signal forms at L1 and L2 was always the same. This means, that input behavior can’t be changed with this register.
    But at previous tests I saw that the PLL loop can changed with PLLVCODIV from internal inductor with VCO1 to external inductor with VCO2. This means, that the software accesses to the right register during those tests.

  3. Checking the behavior of FILT output
    At least I checked the behavior the FILT output I saw a ramping up voltage signal at this output, and this causes a frequency change at the VCO. And I measured, that this behavior can be controlled by the PLLLOOP register setting. Its semes, that the setting for the PLLLOOP is ok.

  4. Investigation results
    My result of those tests is that with PLLVCODIV the internal structure of the PLL can be adapted. And for internal or external inductor the behavior of L1 and L2 can be the same. Even when the internal inductor didn’t need the external pins. But for an external VCO the L1 and L2 inputs must have another behavior. And this must be set up in another undescribed register.

So all the previous questions are still activ:

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?

Kind regards,