Relationship between power supply switching transient and the transmitted RF signal
we have an important issue about the power supply switching transient.
We would like to understand if the switching transient of the power supply delivered to a RF trasmitter can cause the appearance of spurs in the RF spectrum at the frequencies multiple of the switching frequency.
In our application, we have a 4 GHz carrier. On spectrum analyzer, we see spurs at
4 GHz +/- 200 kHz;
4 GHz +/- 400 kHz;
4 GHz +/- 600 kHz;
and so on.
Can those spurs be caused by a DC/DC converter, switching type, with 200 kHz switching frequency?
If the above question is positive, which is the theoretical explanation about the issue? How can it happen?
What does it depend on from?
Many thanks in advance.
Sincerely,
Antonio
Absolutely.
200, 400, 600 are the first, second and third harmonics of your switcher. When those signals beat against your 4GHz you get the sum and differences, as you're seeing. How can this happen? LOTS of ways. Through the power supply, especially the ground. Through radiated pickup. Through capacitive coupling of PCB traces.
200 kHz switching frequency for a DCDC converter that supply an RF system, will give a lot of headache in terms of spurs, interferers, birdies, etc.
Modern wireless systems use higher switching frequencies. For example ET (Envelope Tracking) DCDC converters, used in 3G or 4G wireless systems, use switching frequencies in range of few MHz (about 6 MHz) or even higher.
These high frequencies introduces a switching loss in the DCDC converter but their harmonics and spurs are much easier to be filtered.
Also a guaranteed way to avoid DCDC converter emissions is to use frequency mitigation (changing the converter switching frequency dynamically when change the RF frequency/channels/bands etc).
A good PCB layout design is the most important thing avoiding converter emissions.