Hi all,


just thought i'd get some opinions here. The question is do we really need to match the 50 ohm antenna to receiver input? The only reason I can think of is if you have an off-chip band-select filter, you need to match it to 50 ohm to ensure the proper frequency response. Any response is welcome.


thanks,
Aaron

Hi,


thanks for the reply. So basically antennas don't really get 50 ohm impedance at all. So we should consider the antenna we are using first...

Actually after some discussion with colleagues it seems the 50 ohm antenna impedance is more a concern of the transmitter side. I.E we need to have a well defined antenna impedance to get the maximum power transfer from the PA to the antenna.

If we have a TR switch, however, it seems like we should be able to do away with 50 ohm matching on the receiver side as long as there is some kind of out-of-band filtering...


thanks,
Aaron

I believe 50ohm input impedance is very important for RX even there is no RF filter in between, because when you are connecting the chip to the antenna, you should design a transmission line (TL). If the input impedance of the RX and the (TL) are both 50ohm, the length of the TL can be choose freely, i.e., it matches the practical situation that the chip-design company won't know how their customer in selecting the length of the TL in connecting their antenna.

Just my small opinion.

To recapitulate,

you need a well defined LNA input impedance to:

1_ Terminate a preceding SAW,BAW,... off-chip filter with 50 ohms since these filters are optimized to operate with 50ohms terminations.
2_ If no filter precedes the LNA and the antenna is on-chip, then the LNA should synthesize an impedance which is complex-conjugate of that of the antenna at the RF signal of interest for maximum power transfer.
3_ If you have any transmission line effect whether intentional or not, then you need to consider terminating the TL with its characteristic impedance to avoid reflection. That basically defines the input impedance of the LNA.
5_ It feels that I forgot something....

-Ibrahim

Hi,
Aaron if you plot on a smith chart the reflection coefficients you will find that exists a trade-off between matching and NF, and the NF will never be 0 dB, moreover if you make a perfect noise adaptation probably most of the incoming signal will be reflected back to the antenna so I doubt you will gain anything reducing the noise to the minimum expression.
Just my point of view.

Hi,
I think that it doesn't matter if we obtain a high gain if all the signal it's reflected back to the antenna, but I agree that 50 ohms it's not a useful reference impedance for a complete SOC.

Hi,
Yesterday I read a paper that puts this discussion in perspective:
"From Oxymoron to Mainstream: The Evolution and Future of RF CMOS",Thomas H. Lee,RFIT2007-IEEE International Workshop on Radio-Frequency Integration Technology, Dec. 9-11, 2007, Singapore. It's the typical invited paper for a symposium that makes reflexions,history... but it has an interesting quotation that reminded me this thread(the quotation doesn't reference directly the author but google is a powerful tool):
"CMOS devices take voltage, rather than power,as an input, and they have an almost purely  capacitive input, so they can't absorb any power. [G]etting … voltage gain using passive components that consume zero [power is] the most important factor in overcoming the noise of that device [6]."
doing google search you end up in a interview in  Electronic News: http://www.edn.com/article/CA6339267.html.

Prof.Lee in his paper mades a reasoning about why this idea is wrong, although he isn't completely neutral of course(he cited Shaeffer et al-where et al is Lee himself-), but ended up in the well know trade-off between Nfmin and input matching for power transfer(two-port noise theory).

Just to put things in perspective,

Hi aaron,
As I see you have readed in depth the paper from Cook and his thesis(could you post the link, I'm unable to find online-at the end of the day I lost my google powers-). First I have to say that it's indeed an interesting paper(I'm just in the second reading of it, and just now with pen and paper but it's quite innovative in the solutions they propose), I think that most of the "angry answer" of prof.Lee comes from the interview and not from the paper itself(the part of CMOS doesn't consume power only voltage) and as I said he is not entirely neutral(his work with Shaeffer and his own book for example). I had to read and analyze more in depth the paper(and the thesis if I have the change) but at first some things impresse me a little bit:
-as you said lack of s11 measurement(at 50 ohms or whatever impedance they use in the antenna-which antenna?-)
-at first glance I didn't find the data about sensitivity,data rate(250Kbps?),BER and intended distance for the receiver(I suppose these are carried out in thesis),now I'm making some educate assumptions to pick up numbers.
I agree with you that if you have a big gain at the LNA we can consider(we are engineers) that all the noise is from LNA and if as you said you can survive with big NF it won't be the problem, the problem comes from how much power from the antenna is effectively amplified(enters in the receiver) so the overall SNR at the demodulator input allows the detection.
PS:I'm not sure about NQS but I think that are included in BSIM4 models and not in BSIM3-this means when we switch to BSIM4 we cannot blame anymore the modelling guys for the NF excess...
PS2:I will read again the paper and if I could the thesis-I like classical theory-as you can deduce from my previous posts- but I'm not closed to learn new things.