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Power Supply Noise

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robmitch

New Member
My project is an RC (radio control) based project - I am using STMicro mems gyro and accelerometer and Zilog encore XP mcu. am interfacing the mems at the ADC of the mcu, and while I get it up and running, I have a 20cm ribbon cable from my proto board with the mems sensors etc to the Ziolg development board (this will all be on the one pcb eventually...). I am supplying power (12v) from my rc transmitter, as well as PPM signal and gnd via a 5 core shielded cable - 100cm long - to the proto board. This cable will remain eventually.

Now, my problem is two things:
1) The zero rate gyro output at powerup starts at a high value and asymptotically, after about a minute, settles at a stable value. (Thie is independent of the power supply source)
2) If I use my transmitter to power the proto/dev board setup, I get the same transient start-up gyro drift, but additionally, I get drifting values, depending on the transmitter proximity (even when my hand gets within a couple of cm, the readings go a little funny). My transmitter is at 36MHz, not sure of output power, but it will be in the range of 20-100mW

I cant tell what the problem(s) are - temperature transient at startup, improper grounding, or power filtering...

Could it be noise from the transmitter (conducted through the supply cable), or emitted and being picked up on the board etc...?

What is a general rule of thumb for power supply - should I have say, a 470uF cap where the 12v power enters the pcb (after a long run of cable), do I need an inductor as well?
Sorry, too many questions to start with perhaps:eek:
Cheers,
Rob
 

Diver300

Well-Known Member
Most Helpful Member
The 470µF capacitor will be next to useless at 36 MHz. They are used for storing charge to keep electronics running when powered from 50/60Hz mains. Capacitors like that have far too much inductance to do anything much at 36 MHz.

You need much lower value capacitors to have the least impedance at those frequencies.

I would also suggest that you put resistors in series with an data lines, and capacitors to ground, to filter out higher frequencies than the ones that you need to get through.
 

robmitch

New Member
Diver,
What capacitance/resistance for data lines? (Do you know any reference to look at capacitance/frequency etc)

Any ideas about the startup transient? Do I need filtering on the power supply (I have fixed voltage 3.3V, LDO regulator with suggested input and output capacitors)
Cheers,
Rob
 
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