lna attenuation
i want to ask about one thing which is related to rf front end of a receiver.I found in an exemple that the first element after the entennantenna
The RF input signal first passes RF attenuation that helps to prevent overload and distortion.
but in usual receiver, the first bloc may be an LNA low noise amplifier which enhance the gain of the signal without bringing a considerable noise: in that case and working in the suitable region for linearity and noise figure, the circuit may prevent distorsion in this case.
So is it really possible to replace an attenuator with a LNA?
If they have an Attenuator before the LNA it is most likely there for a
reason. Like it's a WiFi Rx that might be sitting 12 inches from the Tx, and
the Dynamic range of the Rx is much better than the standard...
Or "The Attenuator was designed in to the Link Budget"
If you did replsce the Attenuator with an LNA it would most likely distort or
destroy an Amp in the Rx Chain.
sorry a part of the phrase was not sent; the first element is an attenuator and then there is a mixer.
the problem that for the same application and the same requirements many other receivers that are proposed include a LNA and not an attenuator: that what makes me wondering about the choice of the attenuator which is not an usual choice.
an other point, that even in the standard requirements, attenuators aren't mentionned but only LNA. that's why i asked about the possibility to change this element by a LNA. have i right now ?
thanks Element_115 for your fast response.
regards
imar
I am totally confused.
Will you send us the complete flow , external antenna attenuator mixer etc?
Attenuators in RF front is to increase the upper limit of the desired signal. As signal strength is too high no issue on the NF.
Is about an LNA with bypass-switch. An external FET in parallel with the LNA operate as a floating switch. There could be 3 possible states: high-gain, low-gain (bypass), disabled.
http://www.freescale.com/files/rf_if...et/MC13821.pdf
this the entire flow that i found.
[url=http://images.elektroda.net/28_1234944164.gif]
the problem that :
- there is no explanation about the choice of the attenuator
- other researchs for the same requirements and for the same standard used LNAs and not attenuators.
Generally Before LNA we placeing not att but filter!
"The RF input signal first passes RF attenuation that helps to prevent overload and
distortion."
In this application they need att, but generaly we not placing ATT before LNA
This is related to system design.
I do not agree, the reason is in a cascaded design it is same if you interchange the blocks. The thing you need to be care full is the IP3 and the select the one configuration having higher IP3
To know the attenuator details you need to have all other details like ADS nominal range. AGC threshold IF stage gain RF gain etc.. In simple words it is called system
budget.
you need to have the system requirements.
if you add/ substrate from the system requirements with cascaded gain at extream two positions then you will get the difference as the attenuator range.
So it could be possible now to interchange the two blocs if i take in account all the requirements envolved in the standard specifications especially noise figure, IP3 ang gain this part of the receiver?
You can do what you want once you take care all parameters as:
NF, G, IP3 etc...
dear friends
Imar is right.some times we have some attenuator.to be accurate we have some limiter ofcourse.and the reason we use is that we are going to prevent LNA.because LNA usually can not recieve powers more than a special values for example 20 dBm.so if some power greater than the threshold power of LNA delivered to the LNA it would be damaged.and limiter in that region have to be choosed n a manner that prevent the LNA and to do that it have to attenuate the input signal.so it is completly true to use attenuator(limiter accuratly) befor LNA
hope i could convey myself
and an other point is that such a limiters always used in RADARS specially army ones
Hi Imar
I just saw the picture. This picture does not imply that it is coming from
the Antenna! It is just at high Freq's. The picture is just showing that
when the RF signal is being sent to get mixed, that it is Smart to have a
way of giving the mixer a proper signal (Not too big and not too small if possible).
So:
>--Ant--BPF--LNA--AGC-- {RF Signal} --> [Your picture]
Cheers
Limiters can be used in case of constant envelop modulations like GMSK etc.
Limiters before LNA are not common practices in commercial communication systems.
I agee that LNA selection is based on the maximum input power of the system.