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Design >> RF Design >> Measuring INPUT impedance of the Passive Switching Mixer
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Message started by Ricky Chen on Jul 20^th, 2011, 11:23am

Title: Measuring INPUT impedance of the Passive Switching Mixer
Post by Ricky Chen on Jul 20^th, 2011, 11:23am

Hi All,

I want to plot the input impedance ( in RF Port side) of a passive switching Mixer [2-Phase i.e. 2- 180 degree out of phase non-over lapping LOs with rise and fall time of 1 fs & Two switches Loaded with Resistors of 50 Ohm at the IF ports] with respect to Frequency.

LO frequency was kept 1 GHz and the RF input frequency is swept from 0 to 5 GHz. The RF port impedance is kept at 50 ohm. Also I used analogLib delay element to generate 180 degree out-of pahse 2nd LO.

From theory I know this input impeadnce at the RF port will be a Frequency translated version of the IF side Impedance. But since I am using simple resistor at the IF side the RF side impedance will be the same resistor i.e 50 Ohm

I did PSS and PSP analysis and then I plotted the S11 but I got 0dB across the frequency instead of getting a very large -ve value !!

Can anybody please tell me Where I am going wrong in doing the simulation ?

I have attached the circuit schematic.

My Netlist is as Below:--

// Library name: PASSIVE_MIXER
// Cell name: IN_IMPEDANCE
// View name: schematic
V0 (net7 0) vsource type=pulse pacmag=1 pacphase=0 val0=1 val1=0 period=1n \
delay=0 rise=1f fall=1f width=500.0p
PORT0 (net1 0) port r=50 num=1 type=sine freq=frf dbm=prf fundname="rf"
DELAY0 (net033 0 net7 0) delay td=500.0p gain=1.0
R1 (IF_N 0) resistor r=50
R0 (IF_P 0) resistor r=50
SW1 (IF_N net1 0 net033) relay vt1=0 vt2=1 ropen=1T rclosed=1.0
SW2 (IF_P net1 0 net7) relay vt1=0 vt2=1 ropen=1T rclosed=1.0

simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
digits=5 cols=80 pivrel=1e-3 ckptclock=1800 \
sensfile="../psf/sens.output" checklimitdest=psf
pss pss fund=500M harms=51 errpreset=moderate annotate=status
psp psp sweeptype=absolute start=0 stop=5G step=100M
+ portharmsvec=[0] ports=[PORT0] donoise=no annotate=status
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
saveOptions options save=allpub

Title: Re: Measuring INPUT impedance of the Passive Switching Mixer
Post by Larry_80 on Jul 20^th, 2011, 8:46pm

From your schematic, your circuit is terminated in 50 ohms load, so assuming you are using ideal switches, your s11 should be a large negative value in dB i.e perfectly or near perfect match. The only thing i can think of is maybe you are doing the measurement when the switch is off (OPEN CIRCUIT), thus u are observing a 0dB S11 value! I am not too sure, but this is what i think might be wrong!

Title: Re: Measuring INPUT impedance of the Passive Switching Mixer
Post by Ricky Chen on Jul 21^st, 2011, 2:19am

Thanks,
Your reason may be valid but why the switch will be always open ??

Title: Re: Measuring INPUT impedance of the Passive Switching Mixer
Post by RFICDUDE on Jul 21^st, 2011, 3:30am

Before debugging the PSS PSP result, you should run a transient simulation where the input signal is a sinusoidal signal at some offset to the LO pulse frequency. Do this just to make sure the switches are actually switching properly with the LO pulses.