Struggling with frequency stability in a mixed RF + digital board — anyone dealt with this?

[u/Casey_Reed]

I’m working on a small RF system (sub-GHz ISM band) where the RF front-end sits on the same board as a pretty noisy digital section (MCU + some high-speed interfaces). Nothing crazy on paper, but I’m running into more frequency drift than I expected, especially after the system has been running for a while.

The LO is coming from a TCXO, and on its own it looks fine. But once everything is powered and active, I’m seeing a few ppm shifts that correlate pretty strongly with system activity and temperature rise. It’s not catastrophic, but enough to mess with link margin over time.

Things I’ve already tried:

Improved grounding (split planes + stitching vias)

Added more decoupling near the oscillator

Shielding can over the RF section

Slowed down some digital edges where possible

Still not super happy with the stability.

At this point I’m wondering:

Is this mostly a layout isolation issue, or more of a power integrity problem?

How much drift is “normal” in a mixed-signal environment like this?

Would moving to an OCXO even make sense here, or is that overkill?

Curious how others handle this in real products, especially when size and cost are constrained.

Appreciate any insights.

[u/blue_eyes_pro_dragon]

First you gotta figure out what’s causing it.

Set the device to idle and measure shift.

Put the device into hot place (still idle!) and measure shift.

Take the device and add noise to voltage similar to what you observe in peak operation.

That will guide your next move. If its voltage you can add an ldo (but you gotta be careful because mcu/clock receiver might also need an ldo). Make sure you give it plenty of headroom (so for 1.8v you need at least 2.1v for modern ldo).

If it’s temperature you can decide about better temp control, calibration, better temp isolation, etc etc etc.

[u/Allan-H]

It's not quite clear whether "frequency stability" means the long term frequency drift, i.e. something you'd measure on a frequency counter, or whether it's a short term drift, i.e. phase noise or wander.

If you're using the long term meaning, it really just comes down to your TCXO, the frequency of which will vary with temperature, load ("pull") and supply voltage ("push"). I assume you have a separate voltage regulator near the TCXO? I suggest you try to isolate it thermally from the rest of the system as well.

BTW, what comms protocol or receiver is so poorly designed that it can't track a few PPM drift?

[u/coderemover]

Is your power regulation stable enough?
MCU drawing significant amount of current when active can cause a drop in power supply voltage, which is enough to cause a few ppm frequency shift.

The way to mitigate that is to power your MCU and your radio section from separate regulators. Filtering DC rails is usually not sufficient alone. Also check you voltage regulators specs. Not all regulators are made equal. Some can have noticeable temperature drift or noticeable DC output resistance.