How to calculate the degrade of a QAM signal'SNR?

[fnx7]

qam snr

If a QAM signal which power level is -30dBm is attenuated 10dB, what is the degrade of SNR?

Thanks

 

[radiohead]

qam signal bandwidth

It depends on the SNR at the input. The 10 dB attenuator itself also injects noise (unless it is cryogenically cooled). But you have to relate this attenuator noise power to the noise power at the input.

So there's many factors involved. Unless you provide more details, it's impossible to give a detailed answer.

 

[fnx7]

snr db calculation

Thanks radiohead !

Suposed the SNR of input signal is 45dB, how to calculate the output SNR?

 

[kspalla]

qam calculation

Otput SNR will be less than 35dB

 

[Mazz]

qam calculater snr

To make the proper calculation, you need to know your system bandwidth also.

You can proceed like this:
signal: -30 dBm->att 10 dB ->-40 dBm
noise: -75 dBm. The attenuator reduces input noise as well but adds it's own noise (that has to be integrated into system bandwidth).
For example, if you have B=20 MHz bandwidth, the output noise of the attenuator (assuming it is matched to the load and chatacteristic impedance is 50 Ohm) will be -101dBm(-174+10LOG(B)) that is negligible respect to -85 dBm.
So your SNR is still 45 dB.

Mazz

 

[fnx7]

snrdb qam

Hi kspalla , the signal power level is -30dBm. So the SNR will not be degrade 10dB.

Added after 2 minutes:

Thanks Mazz

 

[kspalla]

qam signal to noise ratio

With the assumptions given above it comes that even after adding 20dB pad it still maintain SNR of 45.
I think there could be wrong calculation.
I am surprise that SNR will not affect in a channel of loss 10 dB per say.

Put me in other way,
What value of attenuation pad will make the above system differ from 45dB SNR?
is it more than 20dB atennuator?

 

[Mazz]

typical qam snr

kspalla

with 25 dB attenuation, for example, output signal will be -55 dBm and noise (-100dBm + -101dBm due to floor) goes roughly to -97 dBm, so SNR starts to be reduced.
If attenuator is higher than 25 dB it starts to limit the SNR.

I hope it helps.

Max

 

[kspalla]

calculate 45 db = -10log10

Hi Mazz,
Thanks for the explanation.
Now I understood how the NF will change wrt BW and pad attenuator.

 

[vfone]

cual es el snr de qam

If the input_level(dBm) is higher than sensitivity(dBm) + front_attenuator(dB), the SNR(dB) will not be affected by the attenuator.

 

[kspalla]

how to calculate power of noise for a signal

Hi Mazz and Vfone
I have another question.
The SNR remain unchanged with 10dB attenuator / amplifier as long as it is above noise floor.
Is above statement is true?

Thanks in advance

 

[Mazz]

qam signal details

It is not clear which noise floor you are referring to.
I think the correct statement is the one given by vfone.

Mazz