VCO Tuning at 27MHz

So I have been working on trying to reach 27MHz using the following link as a starting point.

Link to 27MHz post on forum

First, I have made sure all the parts I used for the external PLL and VCO are at least a 2% tolerance or less. I created two radio board using the same parts, one board for transmitting and the other for receiving using the CMSIS examples. The configuration file was adjusted to accommodate for the different TCXO and to change the frequency.

When the two devices are set to carrier wave, the output measured on an oscilloscope shows there mean frequency’s are the same out to three significant figures ( one lock at 27.14 and the other at 27.18). However when set to Tx and Rx the two devices are unable to communicate.

Another experiment was with the VCO value. When I attempted to use the 820nH inductor by itself, the locking frequency was close to 22MHz. When I use a lower inductor value, such as 580nH, the frequency goes up closer to 30MHz. I mention this cause when I put a capacitor in parallel with my inductor, the frequency tends to drop.


  1. How close do the frequency’s need to get to communicate?
  2. Is there a more accurate way to determine the lock besides carrier wave.
  3. What is the internal VCO with the external inductor?
  4. Anything you can recommend to get more consistent frequency’s between devices?
  5. Can you explain how tuning with the capacitor works? How did you achieve 27MHz using the 820nH inductor? Did the increased capacitance raise the frequency, or did the the capacitor act like a really high inductor and decrease inductance total?
  6. Is there a specific capacitor type to use?
  7. Is there an equation for picking the capacitor, or is it just trial and error?


We wanted to let you know that we are looking into this one on our end and we will share the relevant information here.


  1. What is the internal VCO with the external inductor?

Fully integrated VCO allows to operate the device in the frequency ranges 800 – 1050 MHz and 400 – 520 MHZ. The carrier frequency range can be extended to 54 – 525 MHZ and 27 – 262 MHz by external inductor between device pins L1 and L2. The bits VCO2INT and VCOSEL in the PLLVCODIV register must be set to high to enter this mode.

Two VCO selection:

  1. Internal (VCOSEL=0, inside the register PLLVCODIV)
    Frequency range for RFDIV=1 is 400 to 525 MHZ, for RFDIV=0 is 800 to 1050 MHz (table 8, AX5043-D.pdf).
  2. Internal with external inductors at L1 and L2. (VCOSEL=1, VCO2INT=1 inside register PLLVCODIV)
    Frequency range for RFDIV=1 is 27 to 262 MHz, for RFDIV=0 is 54 to 525 MHZ (table 8, AX5043-D.pdf).

(For choice of Lext values as well as VCO gains see Figure 3 and Figure 4 page 10 in AX5043-D.pdf)

For Fmind=27MHz it should be nearly 720 nH (nano Henry) inductor between L1 and L2 pins and RFDIV=1. But it is just start value for trimming.

Checked by table 9, page 11, for L=22nH Fmind is 308 MHz. Devided by 2 for RFDIV=1 is 154 MHZ. Calculated Fmind from figure 3 is 310,633 MHz, for RFDIV=1 is 155,316 MHz.

So for part number selection AX5043 433MHz it should be (VCOSEL=1, VCO2INT=1 inside register AX5043_PLLVCODIV_(TX), AX5043_PLLVCODIV_(RX) under address 032H), table 28 control register map AX5043-D.pdf.

At the address 032H should be 0011-0100 it is 34H but last two bits REFDIV (input devider) depends on used fXtals see table 73, page 46 in AND9347-D.pdf.

  1. External (it is not considered now)