Hooking up VCO and prescaler

[gogomi]

Hi,

I am working on a fractional-N PLL design and I am having trouble
with the interface from VCO to prescaler. I used a complementary
LC-VCO in my design, which has a 1/2 Vdd DC bias voltage at
the output, and I used CML latches in the prescaler. If I hook them
up directly, it looks the DC voltage from VCO puts the prescaler
latches on at all time. I don't know if there is a good way to solve
this kind of problem without degrading the performance too much.
Thank you for your help!

gogomi

 

[nathan]

Have you tried putting few inverters in between ? You may need to sharpen the VCO wave ...

nathan

 

[v_naren]

hi gogomi
Please use a capacitor which will act as a AC coupling capacitor thereby seperating the DC bias level of your VCO from that required by the prescaler.
Hope it helps.....

 

[gogomi]

Since the VCO output frequency is high ~2.4GHz, it's hard to use
mos inverters to shape the wave. As for adding a AC coupling cap,
I don't know how to determine the cap value and if it degrades the performance. Could it be done by adding a differential amp with a
unit gain?

 

[v_naren]

Since the VCO output frequency is high ~2.4GHz, it's hard to use
mos inverters to shape the wave. As for adding a AC coupling cap,
I don't know how to determine the cap value and if it degrades the performance. Could it be done by adding a differential amp with a
unit gain?

ok here goes....why are we adding a capacitor?..just to block DC alone. Since this capacitor will act as a load to VCO...meaning it changes the frequency of oscillation....so add as small a capacitor as possible. example say 0.1pF. I did the same thing for my design.

cheers

 

[gogomi]

Hi, v_naren,

Thanks, looks it is the easilest way to solve this problem. I guess I am
going to need a good voltage reference to set the input DC level of
CML divider.

gogomi