[SOLVED] Acceptable SNR for radio transceiver ICs

Hi!
Radio transceiver ICs mention their receiver sensitivity and transmitter power level. Example -120db/20db.
However they do not mention the acceptable SNR. My concern is that if I were to make use of LNA to increase the overall receiver sensitivity, then to what extent would the noise injected by the LNA degrade performance?
For example if I insert a LNA with 20dB gain and 3dB noise figure, then although it will increase the sensitivity, nevertheless it will also inject 3dB noise. Will this effectively improve the range of communication (due to added gain) or will it reduce the range (due to added noise)?
To what extent can noise be added? The transceiver ICs do not mention acceptable SNR. Or is there some indirect method to evaluate the SNR?

Thank you

If you add a preamplifier in fron of the receiver, you will have three possible solutions
- LNA has a noise figure lower than the receiver : In this case you will increase the sensitivity of the receiver
- LNA has the same noise figure of the receiver : In this case you will decrease slightly the sensitivity of the receiver
- LNA has a noise figure higher than the receiver : In this case you will decrease considerably the sensitivity of the receiver.

Also think the following two things:
1) The company made the receiver usually try as more as possible to reduce the noise figure of the receiver and usually they have instruments and tools much better than the ones we have
2) Putting the preamplifier in front, you will have a signal strenght indication that is not true (due to the gain of the preamplifier).

Personally, I think that the only place where is necessary to put a preamplifier is when the antenna is far from the receiver and you have to compensate for the cable losses.

Mandi

mrinalmani said:

Radio transceiver ICs mention their receiver sensitivity and transmitter power level. Example -120db/20db.
However they do not mention the acceptable SNR.

Click to expand...

Your numbers don't mean anything without units. -120dB what? 20dB what? As for acceptable SNR, US standards state a minimum SNR of -106dBm at 12 dB SINAD for an analog signal.

mrinalmani said:

For example if I insert a LNA with 20dB gain and 3dB noise figure, then although it will increase the sensitivity, nevertheless it will also inject 3dB noise. Will this effectively improve the range of communication (due to added gain) or will it reduce the range (due to added noise)?
To what extent can noise be added? The transceiver ICs do not mention acceptable SNR. Or is there some indirect method to evaluate the SNR?

Thank you

Click to expand...

A LNA with 20dB gain and 3 dB of noise will probably hurt you - a lot. First of all, 20 dB of gain is very hard to keep from oscillating (I'm assuming that this gain is coming from a single device). Second, I don't really consider 3dB of noise to be "low noise". In a receiver lineup, all noise is added dB for dB from the input to just past the first amplifying device. So, if your line up consists of a harmonic filter with 0.5 dB of IL, then a T/R switch with 0.5 dB of IL, then a preselecter filter with a 0.7 dB IL, then a LNA with 1.3 dB of noise, you will START with an output noise of 0.5 dB + 0.5 dB + 0.7 dB + 1.3 dB, or 3 db. In a radio design, great time and effort is expended trying to reduce the ILs of these first four parts, because they have the biggest bang for the buck. From the LNA on, each successive part's noise is reduced by the preceeding parts gain. Overall, any reduction in SNR will result in shorter range.

Thanks for the replies.
I am talking particularly about AX5043 which is a radio transceiver chip by ON Semiconductor intended to for <1GHz radio link. My frequency of interest is around 200MHz. (High frequency is being avoided because the signal must cross nearly 3 feet through water)
The transmitter power is 16dBm and receiver sensitivity is -137dbm @ 0.1kbps and -111dmb @ 50kbps.
At 50kbps the total link budget is (111+16) = 117dbm. However rough calculations show that I would require a link budget of approx 140dbm.
I do not know how to achieve this. What would be a good starting point?
Also from the comments above it appears that adding a LNA is not a very easy task. What then could be a solution?

Also if LNA does not guarantee increase in performance, then why use them at all in any circuit? How to determine if the noise added by the LNA is acceptable or not?

(I don't have a lot of knowledge in this field. please bear...)

Thanks

The preamplifier is used to kill the noise figure of the first mixer and filter for noise image rejection, otherwise you will have a receiver with roughly 10 - 11 dB of noise figure, with preamplifier it will be reduced to 2-3 dB, as stated before.
About the link budget, you have 127 db of balance , to reach 140 dB ( but with which fade margin ? ) you need only to amplify, to have 13 dB of amplification is not so difficult, considering to have roughly 1 W output power.

Mandi

to achieve 140 dB link budget, two ways:1. increase your transmitter power by adding a PA to 30 dBm, then your link budget is 30+111=141 dB. 2. improve your receiver sensitivity, this is not easy to improve and depends on the NF of current receiver. Basically, sensitivity improves with NF dB by dB. Ideally, the best NF can be achieved by adding 3 dB LNA is 3 dB, so if original receiver NF is 26 dB (obviously impossible), you can improve sensitivity by 26-3 = 23 dB and meet your requirement. After above analysis, the only feasible solution is adding PA.