LNA gain increase question?

[super]

Dear all:
I have a question qbout LNA gain question.
In receiver, if LNA gain increase 10dB, will the min sensitivity increase 10dB or not?
According Pmin=-174+BW+NF+SNR
nf=nf0+nf1/gain1+...
Pmin and NF will be improved but not 10dB
But from linkbudget LNA+MIXER+AGC will increase 10dB.
Someone have any idea or can explaination for me?

Thanks,
Best regards.

 

[vfone]

According to Friis formula if you increase the gain of the first stage (LNA - G1) with 10dB, always you get less than 1dB receiver system NF improvement.
Receiver system NF and SNR will affect 1 to 1 (in dB) the sensitivity level (Pmin).
For example if receiver NF is 5dB, and sensitivity (Pmin) is -100dBm, if receiver NF goes to 6dB the sensitivity drops to -99dBm.

 

[super]

vfone,
from the Friis formula I think the Pmin won't improve 10dB if Gain increase 10dB.
But from general receiver linkbudget it should be improve 10dB in Pmin,right?
Pmin is that I concern.

Thanks.

 

[vfone]

No, you don't get 10dB improvement in reference sensitivity level (Pmin) if increase the gain of the LNA with 10dB.
As I already mentioned, you will get less than 1dB improvement in system NF, resulting less than 1dB improvement in Pmin (when increase the gain of the LNA with 10dB).

 

[tony_lth]

Super, you confused sensitivity and link budget power.
The sensitivity is measured as fowllowing:
Say in telecomm system, 1st setup system with almost BER=0, then increase the link insertion loss until to get BER=1E-6. Then sensitivity is (TX Power)-(Link Loss) in dBm.

 

[super]

Vfone, tony_lth
Thanks for your explanation I got it that increase gain 10dB won't improve Pmin 10dB.

As I mention, linkbudget will be improved 10dB from the formula calculation,
but I still confused why linkbudget will improve 10dB, what's the difference between linkbudget and Pmin, ?


Thanks,

 

[tony_lth]

Pmin, i.e. sensitivity describe the receiver performance, that is min detectable power which can be tolerated.

Link budget is the system budget for power. Say, in PTP systems, the sensitivity can be -90dBm, but the link budget power can be -40~-60dBm, there should be about at least 20dB margin for rain, temperature etc.

 

[RCinFLA]

Improvement in sensitivity with LNA gain depends on NF of subsequent stages and NF of LNA.

You might consider using noise temperature instead of noise figure. Each stage is modelled as a two input summation with an idea amp (0 db noise figure). The input summation is noise temp of device and noise power of prior stage(s). You can see the concept of noise takeover where delivered noise power from prior stages mask the noise temperature of device.

For input, the summation is noise temperature from antenna (or source) added to noise temp of LNA. This noise power is then amplified by the LNA gain and becomes the input noise power to subsequent stage. If device is use for space communications the noise temp from antenna may be very low. (not 300 deg K, which is generally used for earth noise temp).

 

[super]

tony_lth,
In your case PTP system "sensitivity can be -90dBm,link budget power can be -40~-60dBm".
I think linkbudget total gain should be at least than 90dB to detect -90dBm signal, but you write
-40~-60dBm , can the receiver detect -90dBm signal?
It should be total gain larger than Pmin, if yes then increase 10dB gain Pmin should be increase 10dB.
But from the Frii formula it's depend on NF.
This point is that I confused.

Thanks.

 

[tony_lth]

yes, the receiver can detect. even ignore the receive antenna gain, the LNA should have say 25dB gain, then IF gain should be about less than 70dB. The receive level is determined by the receive gain and finally by the de-modulator sensitivity. Say de-modulator can detect -40dBm, so the gain should be 50dB if input -90dBm.

 

[super]

tony_lth,
From your analysis,
LNA(25dB)+IF(70dB)=95dB gain --->linkbudget (for detect -90dBm signal)
Receiver(50dB)+demod(40dB)---> Pmin( to detect -90dBm signal)

It means when have demod function, the receiver only need 50dB gain and the rest gain is job of demod to operation.
but when only receiver(without demod), to detect -90dBm signal it at least need 90dB gain, right?
Is that the difference between linkbudget and pmin?

Thanks,

 

[tony_lth]

I don't understand why you said " -90dBm signal need 90dB gain".
Linkbudget is for system design, to design every component specifications in a system.
And Pmin is only one specification of a receiver.

 

[super]

OK, I see.
"Linkbudget is for system design, to design every component specifications in a system."
Pmin is only one specification of a receiver that need meet our BER

But linkbudget total gain must large than Pmin, right?

Thanks.

 

[tony_lth]

OK, I see.
"Linkbudget is for system design, to design every component specifications in a system."
Pmin is only one specification of a receiver that need meet our BER

But linkbudget total gain must large than Pmin, right?

Thanks.

But linkbudget finally receive level should be larger than Pmin, not gain.

 

[Abao.lin]

Here one example shows how the sensitivity could be improved with external LNA!
https://www.maximintegrated.com/app-notes/index.mvp/id/1836