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Design >> RF Design >> Negative effects of poor matching
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Message started by Pashtet on May 16^th, 2011, 5:30am

Title: Negative effects of poor matching
Post by Pashtet on May 16^th, 2011, 5:30am

Dear, all.
Let's imagine the situation that we have RF receiver working with passive antenna. LNA of this receiver has acceptable parameters(NF, S12, S21, S22, and so on), except poor S11. Suppose there are no filters between LNA and passive antenna. What would be the drawbacks of such curcuit from the point of view of overall receiver performance?

One thing thing is that part of input power would reflect back, re-radiate by antenna and may corrupt input signal for other receivers working near the one we observe. But i doubt if this phenomenon is of big importance.
Any comments?
Thanks.

Title: Re: Negative effects of poor matching
Post by rfcooltools.com on May 16^th, 2011, 11:26am

Pashtet,

Here is how I would view your question.

The power will not be reflected, rather not absorbed. A wireless transmitter is not matched to the receiver, but rather matched to the volume of space around the antenna. The receiver antenna is also not matched to the transmitters antenna but rather the volume of space around itself.
Think of it like this.. your ears are matched to sounds conducted in the presence of air. Your vocal coords are also matched to deliver sounds through the same air medium. If someone is giving a speech to several others in a given space, how is the speech any different to an observer if one of the listeners is wearing earplugs?

http://rfcooltools.com

Title: Re: Negative effects of poor matching
Post by RFICDUDE on May 17^th, 2011, 4:17am

The way you pose the problem is that you have a LNA designed to have low NF with a somewhat poor S11 referenced to 50 ohms. Now you are going to interface it with an antenna, and you want to know the impact.

An implicit assumption is that the antenna impedance is 50 ohms. If it is not (and it doesn't have to be) then you have to figure out what the NF performance is with the antenna impedance (could be better or worse). If the impedance is 50ohms then the performance of the LNA should be about the same as you simulate with a 50ohm port.

It is also possible (although I'm not an antenna expert) that the radiation efficiency of the antenna is impacted by the impedance presented by the LNA. If this happens then some of the EM energy incident to the antenna will be scattered rather than absorbed. The net effect would be the receiver front-end with the antenna would be "less sensitive" that it would be if the radiation efficiency of the antenna were unchanged. The problem here is that you cannot predict this effect without some sort of working model of the antenna. A thermal 50 ohm port doesn't capture this at all.

So, I think you need to dive into the antenna question a bit to find a good answer to your original question.

Title: Re: Negative effects of poor matching
Post by Pashtet on May 18^th, 2011, 8:44am

Thanks for your feedback)
I'm also not antenna expert but really can't imagine physical effects which may cause reducing of radiation efficiency of passive antenna due to impedance mismatch. I have found nothing similar in literature yet. May be somebody guide me ?

Title: Re: Negative effects of poor matching
Post by RFICDUDE on May 18^th, 2011, 4:49pm

On second thought ...
It was a poor choice on my part to imply the radiation efficiency of the antenna itself is changed by the matching. But certainly the overall efficiency could be degraded.

Title: Re: Negative effects of poor matching
Post by Pashtet on May 19^th, 2011, 12:01am

What do you mean by the term "overall efficiency"? For example, if we math LNA and 50 Ohm antenna - we get 4 dB NF for all receiver. But in case when optimum input impedance for minimum NF doesn't equal 50 Ohm - we can convert 50 Ohm impedance of antenna to optimum NF impedance and get 3.5 dB receiver's NF at expense of poor matching at input. In what case receiver would be better?

Title: Re: Negative effects of poor matching
Post by rfcooltools.com on May 19^th, 2011, 1:56pm

Pashtet,

I think what RFICDUDE is implying by overall efficiency, he is implying that consider the LNA and antenna as a system the optimum as a system is not achieved.
The antenna will have gain vs an isotropic antenna (which uniformly collects energy in all directions). Antennas have this gain, which is specified as dBi. If you lose a 1dBi because of poor matching but your noise figure improves by .5 dB then your "overall efficiency" is degraded.

http://rfcooltools.com

Title: Re: Negative effects of poor matching
Post by Pashtet on May 23^rd, 2011, 4:21am

rfcooltools.com wrote on May 19^th, 2011, 1:56pm:

i partially agree with you, but partially not. For example, suppose 20 dB gain of LNA is sufficient for contribution to NF from the following stages would be rather small. With poor matching LNA gain would be 20 db, NF suppose to be 1.5 dB; matched LNA would have >20 dB gain, but NF suppose to be 2 dB. So the receiver's NF would be, for example, 2 dB with poor matched LNA and 2.4 dB with well-matched. I think that the first case is more attractive, am i wrong?

Title: Re: Negative effects of poor matching
Post by rfcooltools.com on May 24^th, 2011, 10:10am

Pashtet,

Loss before the LNA is dB for dB equivalent to a noise figure hit, by not being matched the system might be loosing the potential to be more optimum. I say might because an example can be brought up that implies the contrary, but usually that is the case. But a more subtle issue in the example you provide is what is the noise figure after the LNA can be exaggerated further, If that noise figure is around 20dB then a low noise figure of the LNA and a gain of 20dB is roughly an additional 3dB noise penalty when referred to the input of the LNA so a 2dB NF will equate to approximately 5dB input referred. Where as a 2.5 dB NF and a gain of 26dB will be around 3.5dB NF.

http://rfcooltools.com

Title: Re: Negative effects of poor matching
Post by Pashtet on May 26^th, 2011, 8:47am

rfcooltools.com wrote on May 24^th, 2011, 10:10am:

Loss before the LNA is dB for dB equivalent to a noise figure hit, by not being matched the system might be loosing the potential to be more optimum. I say might because an example can be brought up that implies the contrary, but usually that is the case. But a more subtle issue in the example you provide is what is the noise figure after the LNA can be exaggerated further, If that noise figure is around 20dB then a low noise figure of the LNA and a gain of 20dB is roughly an additional 3dB noise penalty when referred to the input of the LNA so a 2dB NF will equate to approximately 5dB input referred. Where as a 2.5 dB NF and a gain of 26dB will be around 3.5dB NF.
http://rfcooltools.com

Your calculations seems to be right. Also - as you said - the situation would be opposite if NF of circuitry after LNA would be, for example, 8 dB.
Taking matched LNA gain and NF as 26 and 2 dB respectively, unmatched - 20 and 1.5 dB we get 1.66 dB input referred NF in case of unmatched and 2.04 dB in case of matched LNA.
From our discussion i grasped that even if LNA is not well matched to passive antenna, but still having enough gain and low NF, we wouldn't notice any degradation in receiver's performance.

Title: Re: Negative effects of poor matching
Post by baohulu on Sep 22^nd, 2011, 8:31pm

Pashtet wrote on May 16^th, 2011, 5:30am:

for your question, I think there are two case:
1. in your case, your receiver is a wireless receiver. then, the reflect power what you say is another description of not max power transmission. but that not max power transition does not affect others, because your receiver antenna radiation power is low, others can't receive it
2. if in cable transmission, LNA input impedance mismatch will affect cable transmission character impedance. thus it will affect transmission efficiency, that means the transmitter should have higher driver capability.

so , in the first case, you may get a NF without matching to 50ohm. however, in the second case, you may loose antenna gain to get LNA better, the total NF way not be good.

the above is just my thinking, just for reference, I am not quite sure about it