For a Hi-Z output driving another Hi-Z input, you typically want to buffer it.
ie, buffer Amp, or maybe a simple Source Follower, depending on your situation.

Hi VP1953.

But how do you decide the LC values if the filter output is connected to a high impedance?   Say I calculate the LC values for RS=50 and RL=100.  When I use that set of values and desconnect the RL (which is the same as connecting the filter output to a high impedance node), the filter peaks by around 10dB.   I assume it's because the LC values are not chosen for RL=infinite.  But if you look at the equations, the L will go infinite for RL=infinite.  That's where I am stucked.

Thanks.

Hi VP1953,

It's basically noise.  There is a noise requirement for the stage (it's a VGA) after the filter.  The VGA input is biased to a voltage through pull up resistors, and they are in the Kohms range for minimum noise.  So as far as the filter is concern, the Rl is pretty high.  Not to mention the corner of the LC filter needs to be around 500MHz, so the inductor is huge.  Since I am not too familiar with the LC filter, I am not sure whether you can build the LC for a infinite RL condition.  

I guess you are saying the LC filter needs to be designed with a specific RL then?  If that's the case, I can look at what's the maximum R VGA can use (trade that off with the noise) and design the filter based on that R then.

Thanks.

To filter out te LO+RF from the mixer before it goes to the on chip VGA.  Originally, the mixer out just goes straight to VGA (high input impedance), now if I need a LC low pass filter in between, it kind of force a specific termination on both ends. Doesn't that also introduces voltage loss too?  Since the the input of VGA is no longer high impedance , but say 100 or 200 ohms (to keep the L reasonable).  Is there a better way to filter out the LO+RF?

Hi rfidea,

Nothing that's what I need!  I just didn't know how to do it.  All the papers I saw are using remination on both ends.

Can you please send me the link or papers on this, thanks.

Hi RFidea,

Thanks.  I simulated the one you posted, the output looks really peaky (>5dB).  Maybe I am not looking at this right?
Thanks.

Your original post was regarding onchip LC.
If the inductor is on chip then the Q will be very low at 500MHz (2 or less).

The series resistance will be (assuming a Q=2 at 600MHz) about 65 ohms and the self resonant frequency will likely be around 1GHz.

So under these conditions is the filter still useful?

You could consider an offchip inductor where the Q will be much better, but then you have to offchip and back onchip.