matching of mixer
How to design the LO port matching of mixer?
While calculating the Input impedance of the LO port, what's statuses should be set for the switching pair? One is on, the other is off, or both of them are on? (take the single-balanced mixer as an example)
Thank you.
Ruri
Do you intend to apply an external LO (50 ohms)?
:)
Yes, almost.
I heard some guys said LO port impedance matching is very important, but many others don't think so. And in most of the related papers, LO port impedance matching is even not an issue...
I'm just a little confused. What if one wanna do the LO port matching? What's the impedance looking into the two LO ports?
Rgs.
Ruri
It depends on if you want to apply external LO or not in the final prototype or product. If the LO is external, we can assume that the external LO source is very powerful, so a 0dBm LO power is available. Since LO power is not a problem in this situation, we can directly attach a 50ohm shunting between the LO port and ground. Note that most of the LO power is dissipated on the 50ohm, but since the LO source is external and powerful, we don't care about the power loss due to 50ohm loss.
On the contrary, if the LO is provided by another RFIC or designed in the same chip with mixer, the output power of LO is limmited to its driving ability. So if you want to save the DC power consumed by LO driver, you'd better match the impedance between mixer's LO port and output port of LO driver.
Hi
My experience is that for an integrated solution matching is not really an issue (that's why not many mention it). As long as you have good buffering of your LO source to the mixer, the problem is usually having high enough voltage swing to switching the mixer, not the impedance.
G.L. :)
Hi,
As LO and RF signals are in almost all application connected to the same (input)side of a device (diode, transistor), it is always a compromise which one to match. Mismatching RF is often too expensive as it automaticaly means worse conversion gain (loss). So, as a trade-off, RF port is allways match almost perfectly and then LO is left as good as it can be. In these cases, VSWR of 2 at LO port is more than acceptable.
flyhigh