Hi,
We’re starting a new design with the DW1000.
In our circuit, we have several ICs running at 1.8V and others at 2.8V, so we intend to use those power supplies:
VDDIOA, VDDBATT, VDDAON, VDDLNA, VDDPA: 2.8V
VDDLDOA, VDDLDOD : 1.8V.
Also, I’ll be using a TCXO connected to XTAL1.
So, my question is: To which rail XTAL1 is referenced? Should the TCXO be powered by 1.8V or 2.8V rail?
Thanks,
XTAL1 is powered by VDDBATT. Any disturbance to the VDDBATT rail while operating does damage to the ability of the DW1000 to transmit or receive packets when using a TCXO. A crystal is not as affected due to the crystal not being ground referenced.
Note that VDDPA rail generates current spikes from transmitting very short pulses out the antenna.
If VDDPA are connected with VDDBATT, then the current spikes upset the XTAL1 buffers and that disturbs the DW1000 internal clocks leading to a bad packet being transmitted.
So VDDBATT and VDDPA must be separated. In the datasheet, this is done with a 3.0 volt LDO from the 3.3 volt rail that feeds VDDPA. In your scheme listed above, no isolation is evident from VDDBATT to VDDPA, so you will have difficulty with a TCXO. You need to make VDDBATT from another regulator, say an LDO from your primary power source, say a battery. It can be 2.8 volts as well, but it has to be decoupled from VDDPA.
As to XTAL1 input levels, the TCXO output should be capacitor coupled as XTAL1 is biased by the DW1000. The TCXO needs to generate an 800 mV peak to peak signal. The TCXO can be powered from any voltage which allows it to produce at least an 800 mV output signal. The VDDBATT or TCXO voltage doesn’t matter, just that the amplitude of the output is sufficient.
The most common TCXOs produce a clipped sine output of about 800 mV and work with the DW1000. Finding logic output TCXOs is rare and not needed.
The TCXO + LDO concept is detailed in the DW1000 datasheet, specifically figure 37, the application circuit.
Mike Ciholas, President, Ciholas, Inc
3700 Bell Road, Newburgh, IN 47630 USA
mikec@ciholas.com
+1 812 962 9408
Thank you very much for that detailed answer! That helps!