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Design >> RF Design >> connecting Current starved VCO and TSPC divider
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Message started by geeksy on Jun 24^th, 2016, 3:17am

Title: connecting Current starved VCO and TSPC divider
Post by geeksy on Jun 24^th, 2016, 3:17am

Hi..! I am designing a PLL, i have a problem when i connect VCO and the divider(TSPC). i have some fluctuations in VCO output when i connect it with divider. i tried connecting inverter buffer in between but it is not working too.. i cant trace out what exactly the problem is..? i think its because capacitance of TSPC (true ?).. how should i solve it.?..

I need your help :'(

Thanks..!!

Title: Re: connecting Current starved VCO and TSPC divider
Post by vroy_92 on Jul 20^th, 2016, 3:09am

What is the fluctuation you are talking about? Frequency is off? Then it is probably due to the capacitance. Voltage waveform is showing sudden spikes? Then it is probably due to kick-back or something else.

Title: Re: connecting Current starved VCO and TSPC divider
Post by geeksy on Jul 20^th, 2016, 3:13am

Thanks for the reply,
i have sudden voltage spikes, can you give some details about kick-back..?

Title: Re: connecting Current starved VCO and TSPC divider
Post by vroy_92 on Jul 23^rd, 2016, 1:28am

Lets say you your VCO is driving an inverter. When the output of the inverter is switching, through the Cgd cap, it will couple back to the input. The similar thing is happening to your case as well.
You cannot reproduce this by driving an inverter with an ideal pulse voltage source. You need to connect a source resistance in series.
You can think of connecting an isolating inverter between the inverters in your ring and the TSPC divider.