Designer’s Guide Community

T3N

New Member

Offline

Posts: 5

passive mixer design
Sep 18^th, 2010, 1:53am

I am designing a simple double-balanced passive CMOS mixer. When I ran the transient simulation and measured the IF voltage, I noticed that the IF voltage is very small. It is in the range of nV. The RF and LO voltages are in mV range. It means that my circuit has a hug insertion loss.
Do I need input and out-matching to simulate a mixer? Currently I have no input and output matching.

Thank all

Back to top

IP Logged

vp1953

Senior Member

Offline

Posts: 172

Re: passive mixer design
Reply #1 - Sep 18^th, 2010, 8:47am

Hi T3N,

passive mixers do not have as much gain as active mixers, so output in general will be smaller, though it looks really small in your case. Why LO is so small, it generally is at least several 100mV. What is the input impedance looking into the mixer and the output impedance of the LNA. If possible, please post a schematic of your mixer.

Your mixer has any filters at the output, if yes the downconverted signal is within the bandwidth of the filter?

Back to top

IP Logged

T3N New Member Offline Posts: 5	Re: passive mixer design Reply #2 - Sep 19^th, 2010, 6:22pm I don't have any filter at the output. I also don't have any input or output matching. Right now, I am simulating a stand alone mixer.
Back to top	mixer_001.png IP Logged

vp1953 Senior Member Offline Posts: 172	Re: passive mixer design Reply #3 - Sep 20^th, 2010, 6:47pm Hi T3n, Can you try increasing the LO to maybe 1V in amplitude? I think in your current setup, none of the fets are turning on really.
Back to top	IP Logged

T3N

New Member

Offline

Posts: 5

Re: passive mixer design
Reply #4 - Sep 22^nd, 2010, 2:19am

Hi vp1953,

Thank a lot for your help. Now I bias the gate of all the transistors at 1V and the source at 0.5V. The result is much more reasonable. I notice that if I connect the IF to a 50ohms port, and measure the IF at the output of the 50ohms port, the IF voltage is very low. But if I measure the the differential IF nets ( as seen in the schematic), the output is exactly what I expect.
Is it because of the 50ohms load of the port? Do I need to add a buffer?

Back to top

MIXER.png

IP Logged

vp1953 Senior Member Offline Posts: 172	Re: passive mixer design Reply #5 - Sep 22^nd, 2010, 5:03pm Hi T3N, 50 ohms is like a short load for your mixer. Try using a larger port resistance like 1k or 10k or higher.
Back to top	IP Logged

T3N New Member Offline Posts: 5	Re: passive mixer design Reply #6 - Sep 22^nd, 2010, 6:38pm But for measurement, aren't we supposed to design a mixer for 50ohms load?
Back to top	IP Logged

vp1953 Senior Member Offline Posts: 172	Re: passive mixer design Reply #7 - Sep 22^nd, 2010, 10:17pm Hi T3n, Well not necessarily, it does not always have to be 50ohms. But if you have 50 ohm load, then obviously you need to increase the drive on those fets - use larger sizes, larger biases and larger amplitude for LO. I think there is not enough drive current that is why the voltage goes low for 50ohms.
Back to top	IP Logged