Sorry,.
i describe my quqetion in detail.
My mixer is a broadband mixer. And a
off-chip bandpass filter will follow it. The filter needs 50 Ohm source and 50 Ohm load.
Considering the filter requirement, I add a source follower buffer.
The buffer has gm=20ms.
You can see in the uploaded picture file “sch
without bandpass”and “sch
with bandpass”.
In the file “sch with bandpass”, a bandpass is inserted between buffer out and 50Ohm resisitor.
And at the mixer out, I label “MIXO_P” and “MIXO_N”. Also, at buffer out, I label ”BUFFER_P’ and “BUFFER_N”.
The bandpass generates from the “Filter Solution Tool”.
And when I stimulate the filter with SP analysis, I find the input impedance is 50Ohm at 200MHz
My stimulation follows the file “Lab2_Mixer_Tutorial”
When I set up the stimulator such as PSS, PAC and so on, I just change the RF frequency, LO frequency, IF frequency, RF power and LO power.
Other setup are the same to pdf file “Lab2_Mixer_Tutorial”
Basic parameters are :
RF=900M
LO=700M
So: IF=200MHz
Prf: -30dBm@ 50 Ohm
Plo:10 dBm @50Ohm
Because RF signal is wideband, so I try several different frequency in order to confirm my mixer can work well.
Mainly, I use PSS+PAC for CG, PSS+Pnoise for NF QPSS+QPAC for IIP3.
One difference can be found in the
two picture file “me_CG_Dircec_Plot” and “pdf_CG_Dircec_Plot”
In the pdf file “Lab2_Mixer_Tutorial”, the author plot in the method: pac---
voltage---differential—sideband—peak---db20---output Sideband -1 100M-300M.
My method: pac---
voltage gain--+ -Output and + - inputNets---Sideband—dB20---Output Sideband -1 100M-300M Input Side band 0800M-1G.
I find
in my method, the voltage gain of PAC is the same to transient. In transient analysis, I plot differential signal @+ -Output and + - inputNets. Then do dft analysis to find RF and IF signal magnitude. Finally calculate 20log(Vout/Vin)
The +-input net ate RF_N and RF_P. The output net are MIXO_N and MIXO_P or BUFFER_N and BUFFER_P.
Now here are questions.
When doing stimulation “sch without bandpass”, I plot NF, CG.
See picture file
“CG without bandpass @ mixer out”,
“NF without bandpass @ mixer out”,
“CG without bandpass @ buffer out”,
“NF withoutt bandpass @ buffer out”.
Also, I stimulate “sch with bandpass”.
And plot NF, CG.
See picture file
“CG without bandpass @ mixer out”,
“NF without bandpass @ mixer out”,
“CG without bandpass @ buffer out”,
“NF withoutt bandpass @ buffer out
You can see: CG changes huge at differential output. Also NF changes huge, too.
1
NF changes because the load resistor @ buffer out or at filter out contributes noise.
But you say
https://www.designers-guide.org/Forum/YaBB.pl?num=1231854969/5#5
The output would not be a problem if it was not terminated with a port, because all that happens is that the noise gets excluded from the load anyway.
I think 50 Ohm at buffer out can be viewed as a port. So 50 Ohm resistor should not contribute noise.
How can I reduce NF?
Adding buffer and 50Ohm load, I need CG and NF are all bellow -10 dB.
Can this performance be reasonable?
2
CG curve at mixer output has gain
above 3.9 dBb.
It is impossible. The maximum CG is 2/pi, or -3.92dB.
What is wrong with my circuit? The method I plot CG curve is wrong?
According to theory, NF shouleg be equal to CG.
But in my circuit, it is not ,Why?[color=darkred][/color]