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Posted by Heider Marconi on April 29, 2009, 8:54 pm
Hey people! Here goes another question. I am designing a PLL that will use an LC differential oscillator. It's output has a 2,4V DC component and a 0,6V RF signal. How can I get this signal into a digital prescaler? The technology I am using is AMS 0.35um. It has a 3.3V bias voltage for the digital circuits(vdd= 3.3V).
I have seen designs that use a voltage amplifier to to that but I am looking for a low power solution...Suggestions?
Thankyou

Replies to This Discussion

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Reply by Jesal Mehta on April 29, 2009, 9:43 pm
A simple resistor feedback inverter with ac coupling cap at the input will do the job
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Reply by Heider Marconi on May 3, 2009, 4:15 pm
at 900MHz?! this buffer should work between the VCO and the feedback logic in RF frequency...
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Reply by Jesal Mehta on May 3, 2009, 9:06 pm
Should be no problem. I've used one upto 6GHZ. If you think of the topology it is shunt feedback with ow input impedance.
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Reply by SHumphreys on May 5, 2009, 7:46 am
The first stage might not have much gain, so a few stages may be needed in series. The value of the resistor trades off the self biasing robustness against the gain in the first stage. Only the first stage needs to be biased, but the following stages should have identical W/L ratios to have the same switch point. Be sure to simulate over PVT and with parasitics. Another option: If you can handle a divide-by-2 (and still meet the frequency spacing requirements), then you could implement a CML divide-by-2 and a CML-to-CMOS stage at 450MHz.
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