noise floor/noise figure
How to eliminate/reduce it in a circuit?
REgards,
Greenhill
If you see the transfer curve of amplifier circuit,
you will see when there is no input to amplifier you will see some output. That Output minus noise floor is noise figure.
(Check Gonzalez's book, microwave transistor amplifiers, You will find very simple & good explanation in chapter 4 I suppose)
Noise figure of the system is decided by 1st stage LNA of the downconversion chain.
IF your Noise figure of 1st stage LNA is low then total noise ratio of system will be small.
NR=10^(NF/10)
As general guides, get an LNA with a low noise figure. Minimize the loss in the front end filter (you will probably will have one before the LNA). Use low loss transmission line between the LNA/filter and the antenna. Put the LNA on the antenna if you can, One that people don't often consider is cooling the LNA. Tolerate no loss in front of the LNA if you can. I was once able to meet a very tight specification by cooling the LNA with a Peltier cooler. It is a geek trick but it worked.
cooling is one of the option & 2nd option is to reduce the bandwidth.
Noise power = k*t*delts(f)
cooling is complicated stuff.
go through gonzalez book a I have suggested. I t will help to clear concept.
hi ALL,
Thanks for the explanation..How if it happens to be a mixer?that connect to noise floor/noise figure?
According to my info, when mixer is concerned you talk about conversion loss.
Say your loss is 15dB
Then you will say NF is 15.
To avoid that you have to provide impedance matching networks at Input or Output.
Noise figure in mixer is defined as the ratio of SNR at the IF port to the SNR of the RF port.
http://www.zen118213.zen.co.uk/Syste...xer_Basics.pdf
I hope this will help & clear your doubt.
Regards
Abhishekabs
P.S: Better post this kind of question on following forum
https://www.edaboard.com/forum63.html
you will get more response from RF masters.
Best of luck
There are at least two ways to work with noise performance: one is the noise optimization and another is the noise reduction (or compensation). Optimization methods are well known and relatively easy to do. The practical realization depends heavily on your application and bandwidth. Noise related issues are different for narrowband and broadband systems. Noise reduction is separate field, there are some very special methods but they are complicated, so I believe that here is not the best place to discuss it. But just for your information: even thermal noise may be compensated to some extent.
In a receiver, at its output you can measure the noise floor generated by thermal noise (Pn = kTB), receiver gain (G) and input noise figure (NF).
Receiver noise figure is determined mainly by its front stage (LNA), there is some noise addition by following stages. In mixers, their noise figure is equal to conversion loss but for the complete receiver, there are two possible noise figures: DSB and SSB, meaning that either both or only one sideband is converted to IF band.
Without rejecting one of the two sidebands, we measure DSB noise figure. If one sideband is rejected, receiver NF is SSB; it is 3 dB higher than the DSB NF as only one "signal" from the passed sideband is utilized.
While noise floor depends on receiver conversion gain (above), noise figure does not.
Jiripolivka, you mean 3dB lower than the DSB NF...
No , I mean exactly as I wrote above: the SSB noise figure is 3 dB higher than the DSB one; you can find a detailed explanation in textbooks.
Mixer or superhet receivers are either designed as
1. radiometers, to indicate a wideband noise like in astronomy; as such mixers convert two sidebands to the IF, their NF is DSB.
2. signal or communication receivers- then a filter is used to reject one of the sidebands; then its NF is SSB, and is 3 dB higher than the DSB NF.
Another pitfall some people run into is that the mixer NF can increase significantly when the LO drive is too high. The majority of mixers require an LO drive with about a 3 dB range (example: +10 to +13 dBm), in this case, the mixer NF should be equal to the conversion loss. However, when mixer are OVER-driven, NF can increase due to extra shot noise generated by the rectified current flowing in the diodes. The only mixer I know of that DOES NOT have this extra penalty is the Marki Microwave T3 mixer, because this mixer self-adjusts the operating point of the diodes to help reduce the shot noise, and hence the mixer NF.
Dear Marxman:
You may be right but if you follow mixer-maker's specification, noise problems due to overdrive would not exist.
In the microwave range, a balanced mixer with two diodes typically need +10 dBm LO power. It is usually expensive to generate more power. For LO input lower than +6...+3 dBm, biased mixers are an option.
Fortunately, mixer front ends are now being replaced with RF low-noise preamplifiers, so mixer cease to be the most important part in a receiver.
A good low-noise receiver now rather needs a good input band-pass filter to reject unwanted response and interference.
Consider working in noise temperature. It is more intuitive, showing noise floors in cascaded stages and noise takeover from earlier stages in comparision to next stage's added noise.
For progression of different stage filtering bandwidth's you can work with dBm/Hz of noise spectrum to see effect of narrow band takeover on wider bandwidth noise of subsequent stages.
Jiri,
You would surprised how many mixer users fail to follow directions. I have seen mixer NF >20 dB when the LO is overdriven, this will certainly affect cascaded NF. There is a secondary, and sometimes more insidious problem that sideband noise on the LO can common mode downconvert to the IF frequency and create additional noise that cannot be filtered. The key in this case is to use a balanced mixer with good common mode rejection (related to isolation). As IF frequency goes to DC, this problem gets even worse due to oscillator phase noise, which the mixer cannot cancel. I am told that Radar people are keenly aware of this issue, this is one reason for the strong push for ultra-low phase noise microwave oscillators.
So, even when the pre-amp and BPF are chosen carefully, one must still be aware that the mixer still can play a role in NF issues.
Dear Marxman:
Yes, I agree! I have worked with many mixers and low-noise receivers, so I learned hard how to make a good low-noise receiver. Mixers are tricky beasts.
Since 1990s, MMICs have become familiar, to make good LNAs and LO multipliers and more. IN the LO chain, however, using a wideband amplifier does increase the thermal-noise floor- and this directly drives mixer noise figure up. Several times I had to add a "sharp" LO cavity filter before mixer LO input, to save receiver noise figure.
Using noise temperature instead of noise figure is just fine, mainly if your noise figures are below ~ 5-6 dB. Also, linear terms must be used in Friis' formula.
My favorite Ka-band mixer receivers have 2.9 dB NF DSB, measured by liquid-nitrogen cooled load. It cannot be better!