Hum pickup in PLL

I've completed layout of a PCB which includes a PLL which phase locks a VCXO to an external standard. I was planning on setting the PLL loop bandwidth at a few Hz; but I'm worried that it won't be able to remove 50 Hz mains hum pickup. I would prefer not to electrostatically screen the circuit.

It's all SMT on a 2-layer FR4 board with an almost continuous ground plane. The PLL circuitry occupies a square inch of board area. Between an op-amp active loop filter and the VCXO control input I have an RC filter of 10k and

100nF using 0603 components mounted close to the VCXO.

The specified phase noise of the VCXO at 50 Hz offsets is equivalent to 4uV peak at the control input. Am I likely to see much more than a few uV of 50 Hz pickup around that circuit node?

TIA Andrew.

Reply to
Andrew Holme
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CXO

dth

hum

e.

-amp

and

uV

of 50

You have a capacitive voltage divider. Propose a 50Hz voltage on a plate directly above the PCB and work out the voltage on the surface area of the copper.

Reply to
MooseFET

You aren't likely to have a problem with typical ambient 50 Hz fields. Don't park it right under a fluorescent lamp.

The 10K and 100nF has a corner frequency of 160 Hz, which doesn't help the hum situation much. And it may degrade loop dynamics.

Need details to comment further.

John

Reply to
John Larkin

CXO

dth

hum

What kind of phase detector are you using? A multiplier type detector will reject out-of-band signals, but an XOR or edge-detect counter type won't. The PLL itself can either sense or reject that 50 Hz, independent of other measures taken.

Reply to
whit3rd

Environment is a concern, particularly in a typical plastic box, or situated near 60hz magnetics.

RL

Reply to
legg

Near magnetics, transformers and fans, the issue is usually magnetic loop area more than electric fields. And that falls off very rapidly with distance, cubic pretty much.

Very roughly expect a few microvolts in a 1 cm loop close to a fan or small transformer, "close" being 5 cm maybe. AC and DC fans are similar. PCB layouts can be planned to minimize loop areas.

John

Reply to
John Larkin

I've had that much show up in a linear circuit, with minimal are, at twice that distance from a 60hZ transformer with 'no' gap or rotating parts. Reduced to unmeasurable when re-oriented by 90 degrees.

I'd consider packaging effects, if not wart-powered. Easy enough to wave expected sources around likely victims, or vice versa, while monitoring the output. No profit in guess-work.

RL

Reply to
legg

VCXO

hum

and

50

As I said, the numbers are rough. I had one NMR product that had a couple PPM of 60 Hz hum, sneaking into a current shunt circuit. In NMR, seeing 60 or 120 Hz sidebands is like having a restaurant with roaches in the salad. There were two power transformers in the chassis, and we fixed most of the problem by switching the primary leads on one of them. Got lucky!

Remember old video monitors that had the power transformers mounted at goofy angles?

John

Reply to
John Larkin

VCXO

hum

plane.

op-amp

and

4uV
50

It's still fairly common in audio equipment, to meet test spec numbers. Arbitrary layout in pre-established products has you doing the same thing to parts like common mode chokes, which throw a pretty strong 60Hz field due to intentionally leaky stucture.

RL

Reply to
legg

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