Customer reports that FSS will not always be enabled at startup using the recommended 267kOhm resistor. Is this indicative of hidden features?
In addition to the above observation, the customer would like comment on the rest of the story:
We having been looking at the compensation networks and cleaning up the various rails. We tried the values that you gave us but there appeared to be strong subharmonics that were produced. We used some different values that lowered the noise floor and didn’t produce the subharmonics. The issue we are having at this point is that there is still a strong 600 kHz component at the TX RF output amplifiers on the board. There is gain in the TX amplifiers so it doesn’t take much noise at the digital side of the board to create a spur in the TX output. All the supply phase margins are around 70 degrees with the compensation networks that I am using.
We have enabled the FSS feature but the 267 kohm resistors don’t appear to always guarantee that the supplies will come up with FSS enabled. We had to drop this resistor down to a 200k resistor. We will keep watching to see that the supplies always have the FSS feature enabled.
We are thinking about shifting the switching energy even more and alternating the supply rails by using a mixture of the TPS40303 (300 kHz) , TPS40305 (1200 kHz) and the TPS40304 (600 kHz) part that we are using now.
