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  1. #1
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    multiple vcos pll

    I need to build a freq synthesizer with wide range. 450 MHz to 850MHz. I'm already found some pll's, but the main problem is vco's. Cause, vco with this range, need very large voltage tunning. (aprox 0 to 25v), and the current from pll ic charge pump is very weak, and loop filter can't reach this voltage.
    Is there a possibility to use vco' in parallel???
    Pleae some suggestion or rf tricks or questions...

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  2. #2
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    pll vcos

    You can have multiple VCO's, and switch between them. It might be easier to use an active loop filter, where you can have DC gain in the loop filter to change the Charge pump output from 3.3 Vdc to the 25 Volts that you need.

    Dave



  3. #3
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    pll vco

    ok, if i use an active loop filter i can reach 25v. But my pcb have a maximum power of 5v.
    Can i use an external charge pump to source the opamp of active loop filter??



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    pll active loop filter dc gain

    You'll have to use a boost regulator to go 5V-27V, followed by a linear regulator to regulate down to 25V to power the op-amp. Be very careful of the noise and ripple, because that will modulate the VCO, and make it noisy.

    Dave
    www.keystoneradio.com



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    Re: PLL VCO - freq synthesizer with wide range. 450 MHz to 850MH

    A few companies have VCOs with this wide tuning Range. Crystek makes the CVCO55BE-0400-0800 which is specified as 400 - 800 Mhz, however, even though the tuning voltage range is specified as 0 to 12 V, you can drive the tuning range up to 15V and then you can virtually guarantee you'll hit 850 MHz. The varactors won't break down at this voltage (I've checked with the company). I've found with this VCO that 800 MHz normally is reached at about 8V so even a 10 V tuning range should be fine as the Kvco is about 50 MHz meaning you'll hit 850 MHz at about 9V. DigiKey stocks this VCO. It is very low phase noise. To drive it, consider the Analog Devices ADF4113HV which accepts a charge pump voltage as high as 15V and eliminates the need for an op amp which just adds noise and makes things messy. Also an op amp means you won't have a true charge pump which will greatly increase VCO noise. The ADF4113HV charge pump works directly at up to 15V so you'll have a very low phase noise source. Since you can probably hit 850 MHz with 10V, a voltage doubling charge pump should work just fine as the charge pump supply draws virtually no current. Make sure you filter this supply well.
    Last edited by quickdraw88; 14th November 2010 at 04:37.



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    Re: PLL VCO - freq synthesizer with wide range. 450 MHz to 850MH

    Pretty close to what is in a varactor controlled UHF television tuner.


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