The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> LNA and Mixer Gain
https://designers-guide.org/forum/YaBB.pl?num=1171998629

Message started by sandman on Feb 20^th, 2007, 11:10am

Title: LNA and Mixer Gain
Post by sandman on Feb 20^th, 2007, 11:10am

Hi,

Would anyone be aware of the simplest method to estimate the gain of a front end comprising of an LNA and a Mixer in Cadence-SpectreRF ? As of now I'm using PSS and PXF (to allow for the frequency translation from the mixer) to estimate combined gain. The results are quite different from what I'd expected. So, any inputs would be much appreciated.

Cheers.

Title: Re: LNA and Mixer Gain
Post by aaron_do on Feb 20^th, 2007, 10:05pm

Are you asking for a simple method to estimate the gain in Simulation or make an estimate based on knowledge of the LNA and mixer.

If you're talking about by simulation then you're using the right method.

Otherwise its just the gain of the LNA in dB plus the gain of the mixer in dB. You have to take the loading into account too. If your mixer uses large device size it may be lowering the voltage gain of your LNA. Maybe you can use the XF (transfer function) simulation of the LNA+Mixer taking the output at the LNA output node to troubleshoot.

Aaron

Title: Re: LNA and Mixer Gain
Post by sandman on Feb 21^st, 2007, 1:17am

This is the simulation I'm concerned about. Thanks for your inputs.

I tried the XF for the LNA and find that it gives me a different result for voltage gain than the PSS simulation. I don't understand how these simulations work, so is there a reason why they are different ?

I wanted to confirm if the PSS+PXF for LNA+Mixer gives the Voltage gain and not the power gain.

Cheers.

Title: Re: LNA and Mixer Gain
Post by Ken Kundert on Feb 21^st, 2007, 9:15am

It is hard to help you because you have given no specifics. For example, how did you set up the PXF and how do the results differ.

PXF and PSS naturally produce different results. PXF produces the gain and PSS calculates a voltage level. So you have to divide the PSS output voltage and input voltage to compare them. Furthermore, if your stimulus is being provided by a port, PXF is computing the gain assuming a perfect match at the input. In other words, it is computing the gain from the voltage source that is internal to the port, and dividing it by two.

If you are using a port as stimulus, you might want to use PAC rather than PXF. This way Spectre will compute both the input voltage and the output voltage, just like in PSS, and you can find the gain by dividing them.

-Ken