# Receiver's Noice Input =K*T*BWeq ?

Discussion in 'Wireless & RF Design' started by Fovakis, Jun 2, 2013.

1. ### Fovakis Thread Starter New Member

May 14, 2013
23
0
Hi!

My transmitter send a signal with BWsignal=17KHz..

The receiver has an antenna with BWantenna=30MHz...

I work in a laboratory so T~300k.What is the Input Noice to receiver?

P=KTBWeq=>-174(dBm/Hz) + 10logBWeq

BWeq? what is this?

2. ### mikeleeson New Member

Aug 22, 2012
26
4
BWeq is the equivalent noise bandwidth of your receiver. It defines how much of the received noise power spectrum is measured by your receiver. You can find it by measuring the gain of your receiver as a function of frequency - this gives you the bandwidth directly.

Are you trying to find the actual noise going into your receiver or the effective noise of your receiver? They are not the same.

The effective noise is the noise generated at the output of your receiver but referred back to the input. This can be represented by a noise source at the input of a perfect (noiseless) receiver.

The actual noise going into the receiver will be a combination of the noise generated by your transmitter and noise generated by losses in the signal path. If you have a high powered signal, the noise generated by the transmitter will be negligible: the noise from the signal path losses will dominate.

The value of T used in P=kTB is the physical temperature of a black body radiator which would generate the same noise power (spectral density). You cannot assume all of your noise sources are at 300K - it is not as simple as that. I have an antenna looking at the sky and the sky has an effective noise temperature of 40K (not 300K).

Are your antennas both in the laboratory or are they mounted outside?

3. ### Fovakis Thread Starter New Member

May 14, 2013
23
0

Hi!

So the equivalent noice bandwidth is defined from the last filter of the receiver right?My signal has BW~20KHz but the last filter has BW~30KHz. The eq bandw noice is 30khz?

What is this with the gain you said? Please can you explain that.?

Also i did the experiments in the Lab both transmit and receiver and the distance between them is 3 to 5 meters... here is a photo for more details and questions..http://i1284.photobucket.com/albums/a578/fovos1/P6131098_zps37d09fcd.jpg

4. ### mikeleeson New Member

Aug 22, 2012
26
4
You have a picture of a spectrum analyser display and you can use it to measure the gain of your system: measure the input power and the peak of the output signal. If you vary the frequency of the input signal, the peak will move but will also change its amplitude. The change in amplitude is caused by the system bandwidth. So if you measure the gain as a function of frequency, you can determine the bandwidth.

This bandwidth is usually the smallest bandwidth of your receiver - so in your case it will be 30kHz.

You need to be careful when you use a spectrum analyser to measure noise. The level of noise (as is seen on the display) depends upon the RBW setting of the spectrum analyser. Try changing the RBW setting and see how the noise 'level' changes. You should always record the RBW value when you make a noise measurement on the spectrum analyser (or you can normalise the measurement to a 1 Hz bandwidth which gives a spectral density).