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allen

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How does VCO affect the reference supr?
Jan 17^th, 2005, 3:10am

Hi,

I'm designing a frequency synthesizer. When an ideal VCO, which is described in veriloga, is placed in the loop, the simulated reference spur is below -90dBc. However, when a VCO circuit with nearly the same gain is used, the reference spur is only about -52dBc. Can anybody tell me what has happened? How does the VCO affect the reference spur?

Thanks

Allen

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boa Junior Member Offline Posts: 31	Re: How does VCO affect the reference supr? Reply #1 - Jan 18^th, 2005, 6:34pm In which frequencies do you have spurs? Are they multiples of VCO freq? Also it would be good to learn more about your PLL, its spec and how you simulate it.
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allen

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Re: How does VCO affect the reference supr?
Reply #2 - Jan 19^th, 2005, 5:07am

Thanks boa :)
The frequency of the VCO is 2.4GHz, and the reference frequency is 5MHz. The spur is at 5MHz, 10MHz ... offset from the carry frequency. The VCO's gain is about 150MHz/V, and the open loop bandwidth is 100KHz. The spur is to be designed below -70dBc.
To simulate the spur, I first run a transition simulation with spectre, then use DFT function in the calculator to get the specturm of the VCO's output. Is it right to simulate spur in this way?

Allen

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boa

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Re: How does VCO affect the reference supr?
Reply #3 - Jan 20^th, 2005, 8:55pm

The spurs should appear because of reference freq coupling through PFD/CP to VCO control voltage. Another thing is that your filter should be able to suppress them.

Actually, I do not understand the reason why PLL's output has these spurs when you simulate with a real VCO (I suppose it has a proper phase noise characteristics) if it works fine with an ideal Verilog-A VCO..

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maserati New Member Offline Posts: 3	Re: How does VCO affect the reference supr? Reply #4 - Jan 26^th, 2005, 4:26pm Based on my experience, the DFT function from the calculater is not accurate. But it should NOT have > 10dB difference... try another way to sim the spur and compare
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allen

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Re: How does VCO affect the reference supr?
Reply #5 - Jan 27^th, 2005, 4:50am

Thanks boa and maserati.

The reference spur decreases to -77dBc after I shorten the maximum step in the simulation option. Therefor, I think the degeneration of spur may be caused by simulation error. However, the simulation time is extremly long when the simulation step is short, and I still don't know the simulation result is the correct one( it may continue to decrease if the step gets even shorter). I wonder if there are some more effecent ways to simulation reference spur.

Allen.

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rf-design Senior Member Offline Reiner Franke Posts: 165 Germany	Re: How does VCO affect the reference supr? Reply #6 - Feb 1^st, 2005, 11:38pm If there is no coupling via substrate or supply to the VCO impacting the spur performance the model approach is acurate enough. Only the VCO voltage ripple impact the spur then.
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