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filtering crystal noise in a sensitive application

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Feb 12, 2006
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I have a severe problem problem here, I don’t know where to put it exactly, I am having severe problems in isolating analogue GND from the digital ground in pcb design for a critical RF application. I have tried numerous filters using high quality inductors and capacitors, strictly speaking as long as I don’t have a good GND free of noises from crystals of the digital circuitry, filtering would be of no use. When we monitor the circuit on spectrum analyzer, we see first and second harmonics of these crystal frequencies with -50dbm of power which are deadly in our application. How could we get rid of these unwanted harmonics? How should I design digital and analogue grounds? Do the commands from the microcontroller carry these harmonics to the RF board? If so, what’s the solution? After all I can’t filter digital commands before applying them to the target board, because if I do so, there would be no commands. Help me brother.

first of all, you have to identify the frequencies which your noise acts at.
Second, design both analog and digital grounds as large as possible and connect one to another thru a single point only with a trace that could be 6mm large (rule of thumb). Keep both ground on the same layer. For example, in 4-layer applications, put on 2nd layer. On 1st layer, put signals (analog upon analog ground and digital upon digital ground planes); on the 3th, put power planes (analog below analog and digital below digital); on the 4th, signals again.
Third, separate digital and analog power thru a ferrite bead (check the resistance is as higher as possible at the frequency of your noise) which has capacitors at each pins. Capacitors could be 0.1u, 0.01u and 10uF large. Put also capacitors to bypass anyother noise you could have (GSM for example).
Forth, at each power pins, put suitable bypass capacitor for noise, in such a way to bypass noise to ground.
Fifth, put crystal really close to the device whom it is connected but distant from any other signal traces.

Hope to be helpful.
Let me know.


That’s a good tip for start, I mean separating AGND and DVDD and their corresponding supplies from each other using ferrite beads and trying to minimize the signal path from the crystal to the microcontroller and keeping it a safe distance away from other signals, I am planning to use two sets of supplies and grounds for analogue and digital sections which are completely isolated from each other, I am planning to send all the digital commands through opt couplers. I have thought of putting the microcontroller into power down mode soon after issuing commands to the RF board so that crystal noise is eliminated by software. But About connecting AGND and DGND by means of a ferrite bead, I am very worried. Could you give me a reference, a manual or something like that as to where i should start from? That would be very kind of you.

Separate the ground and use opto-isolator to interface the signal.

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