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Design >> RF Design >> overall NF
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Message started by aaron_do on Jun 12^th, 2007, 11:44pm

Title: overall NF
Post by aaron_do on Jun 12^th, 2007, 11:44pm

Hi all,

I am designing a low IF receiver, and i'm interested in finding the overall NF. I found that the spot NF is around 16 dB at 2 MHz, but if i integrate the total output noise from 10 kHz to 100 MHz, the NF is around 20.5 dB (although i think i can improve this somewhat). My system BW is only 1 MHz so is it necessary to integrate the noise over all frequency? I am using FSK with a simple demodulator at the output - will an ADC be better?

thanks,
Aaron

Title: Re: overall NF
Post by ACWWong on Jun 13^th, 2007, 2:44am

you just need to consider the system BW (your channel BW).

Title: Re: overall NF
Post by didac on Jun 16^th, 2007, 12:53am

Hi,
Usually this is forgiven(and usually I didn't make use of it also) but the noise bandwidth is usually bigger that the nominal filter bandwidth(channel filter bandwidth at -3dB), technically you should computer the equivalent noise bandwidth of the filter(B_N) and integrate the noise over this bandwidth(that give you an equivalent rectangular filter) but usually if the filter is sharp is less than 1.05-1.1 time your bandwidth.

Title: Re: overall NF
Post by aaron_do on Jun 16^th, 2007, 2:41am

I have a third order fitler, and i integrated the output noise of the system, and it was signifantly higher for all frequencies than just the filter bandwidth...

Aaron

Title: Re: overall NF
Post by didac on Jun 16^th, 2007, 4:00am

Hi Aaron,
Equivalent noise bandwidth must be calculated via integration of the transfer function of the filter(basically you found the equivalent rectangular shape that contains the area of the filter). Depending of the shape of your filter you will see more or less noise. As I said usually is a good approximation to integer over your -3dB bandwidth.

Title: Re: overall NF
Post by aaron_do on Jun 16^th, 2007, 9:34am

HI didac,

that doesn't seem to be what ACWWong was saying...he said to only consider the system BW. I originally thought I should integrate the output noise over all frequencies but now i'm not so sure......

Aaron

Title: Re: overall NF
Post by didac on Jun 16^th, 2007, 9:49am

Hi Aaron,
I suppose that ACWWong said that you should just integrate over your channel bandwidth. I think I only mess up things with a technical detail about the definition of noise bandwidth,let me see if I found a reference for you, but I think if you integer over your channel bandwidth it's a good estimation of the noise you will be fighting.
PS:sorry if this detail caused you doubts,I will search a reference and try to post it but my undergraduate courses were all in Catalan or Spanish so I must dig a little bit to find a good reference.

Title: Re: overall NF
Post by didac on Jun 21^st, 2007, 12:09am

Hi Aaron,
Sorry for the delay in promised references, I've been performing measurements and I didn't have many free time. I've found two references in English that maybe will be useful:
1)"Design of CMOS RF Integrated Circuits" of Thomas.H Lee,Chapter 11,pages 334-337 explains and derives the equivalent Noise Bandwidth.
2)"Design of Analog CMOS Integrated Circuits",of Behzad Razavi,Chapter 7,page 239, says very straightforward the definition.
PS:As was talked before and ACWWong said I think that integrating over the channel BW it's good. The references are only if you are a curious about this definition.
Good luck with your design.

Title: Re: overall NF
Post by aaron_do on Jun 21^st, 2007, 8:57am

thanks for the help. I have the books so i'll check out the definition

Aaron