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source impedance

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tomas632

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Hi, i am using an IC that takes an rf input of 2017MHz, but the datasheet says that to give the best tradeoff between noise figure, gain etc I should put a source impedance of 31+j26 ohms on the input. Can somebody help me relate this figure to the actual components I should use?

my understanding is that a +j figure means inductive, but if I had a 31ohm resistor followed by a j26 inductor in series i.e X=2*pi*f*L then L = 2nH. However does a series inductor not block AC? My other thought is that some sort of series resistor/parallel inductor could create this 31+j26 figure, but how would I work this out?

Hope someone can help...
Thanks
Tom
 
Hi, i am using an IC that takes an rf input of 2017MHz, but the datasheet says that to give the best tradeoff between noise figure, gain etc I should put a source impedance of 31+j26 ohms on the input. Can somebody help me relate this figure to the actual components I should use?

my understanding is that a +j figure means inductive, but if I had a 31ohm resistor followed by a j26 inductor in series i.e X=2*pi*f*L then L = 2nH. However does a series inductor not block AC? My other thought is that some sort of series resistor/parallel inductor could create this 31+j26 figure, but how would I work this out?

Hope someone can help...
Thanks
Tom

I think that you misunderstand what "source impedance" means. This is the impedance presented to this input by all of the circuitry connected to the input, so it is the output impedance of the "source". There is no point in arranging only some passive components to present this impedance to this input, because then the input is useless, since there is no "source" attached. Usually the most important part of that source is the output impedance of the preceding stage, be it an amplifier, an antenna, a signal generator or whatever. Ideally, the preceding stage should have an output impedance of 31+j26 ohms when feeding this input, for best performance, so the data sheet says. If you are certain that the output impedance of the source has this impedance, there is no need to add resistors and capacitors and inductors. However, we often do include such things in the form of a matching network, whose job it is to transform the actual output impedance of the source to the desired 31+j26. However, with many ICs of this kind the compromise in performance may be very modest if you provide an input from a 50 ohm source without bothering with impedance matching. This is often acceptable.
 
I just quickly converted it to Polar expecting it to come out close to 50 ohms. No quite.
~40.5 ohms (~40 degrees).
I was then going to calculate the mismatch to a 50 ohm load... but couldn't remember the formula for the complex impedance. (I'm getting slack).
This page took the fun out of it.
https://chemandy.com/calculators/return-loss-and-mismatch-calculator.htm

So a return loss of 8.44 is a little high, but given it's just a receiver you can live with that. Part of that trade off with antenna efficiency versus ease of construction.
It isn't much effort just to change the physical length of your antenna etc to get a better match if you so desired.
 
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