Splitter, Amplifier, & coupler Noise figure

[hilmanhalimra]

Dear All,

Is there anyone know about noise figure at splitter, amplifier, or coupler. Is it stated on the product technical description or we just simply calculate it with the noise thermal-input power/noise thermal-(output power--> input - loss). if we calculate it like that the noise figure is the same as the loss --> like the theory of noise figure for feeder. Please help me i am newbie in radio.
Thanks before.

Regards,
-hh-

 

[N_Jay]

http://edocs.soco.agilent.com/display/genesys200807/Cascaded+Noise+Figure+Equation+Based

 

[rbm]

A short answer:
You can use this calculator:
Overall Noise Figure of Two Stage Amplifier
http://www.minicircuits.com/pages/mcl_nf_calc.html

The first stage 'pretty much' sets the overall system noise figure.

The losses 'BEFORE' an amplifier will directly add to the system noise figure. (The losses AFTER will have little effect.)
(ie. 7.5 dB loss through a splitter = 7.5 dB noise figure)

If your amplifier was the first stage after the antenna (no coax loss, antenna connected right at the amplifier), and the amplifier gain is 10 dB with a 2.0 dB noise figure.

You can make the second stage be the splitter. So if it's a three-way splitter with 7.5 dB of loss, then in the calculator the gain would be -7.5 dB and the noise figure would be 7.5. Then it would calculate the overall system noise figure. (For this case, assuming no coax or other losses.)

Of course the scanner noise figure, if you know it, will have some effect also. But for hobby purposes you can ignore that and still get an idea of the effect of the amplifier and splitter.

You'll see that after the amplifier, the system noise figure won't change drastically.

If you have the amplifier near your scanner with coax losses to an outside antenna, that could be considered to be your first stage. So, if the coax loss is 6.0 dB at 900 mHz, the noise figure of your first stage is 6.0 dB.

Then there's the other problem of front-end overload, intermod, adjacent channel problems etc. if the gain is too high.

There's a lot more to it but this should get you in the ball park.

(The second image will give you an idea of what happens if you have NO pre-amp, 6 dB of coax loss, and then go through a three-way splitter.)

Rich

 

[prcguy]

This is not good info, losses after the first amplifier have very little effect on system noise figure. Use a more appropriate calculator for system noise figure in a receive system, or better yet calculate the G/T of the system, which is the bottom line.
Here is a link to a good G/T system calculator for a high performance 2m SSB receive application. You can download the Excel spreadsheet and change antenna gain, feedline loss, pre amp gain/NF, etc.
http://www.vk1od.net/gt/index.htm
prcguy

A short answer:
You can use this calculator:
Overall Noise Figure of Two Stage Amplifier
http://www.minicircuits.com/pages/mcl_nf_calc.html

The first stage 'pretty much' sets the overall system noise figure.

The losses 'after' an amplifier will directly add to the system noise figure.
(ie. 7.5 dB loss through a splitter = 7.5 dB noise figure)

So, if your amplifier was the first stage after the antenna (no coax loss, antenna connected right at the amplifier), and the amplifier gain is 10 dB with a 2.0 dB noise figure.

You can make the second stage be the splitter. So if it's a three-way splitter with 7.5 dB of loss, then in the calculator the gain would be -7.5 dB and the noise figure would be 7.5. Then it would calculate the overall system noise figure. (For this case, assuming no coax or other losses.)

Of course the scanner noise figure, if you know it, will have some effect also. But for hobby purposes you can ignore that and still get an idea of the effect of the amplifier and splitter.

You'll see that after the amplifier, the system noise figure won't change drastically.

If you have the amplifier near your scanner with coax losses to an outside antenna, that could be considered to be your first stage. So, if the coax loss is 6.0 dB at 900 mHz, the noise figure of your first stage is 6.0 dB.

Then there's the other problem of front-end overload, intermod, adjacent channel problems etc. if the gain is too high.

There's a lot more to it but this should get you in the ball park.

(The second image will give you an idea of what happens if you have NO pre-amp, 6 dB of coax loss, and then go through a three-way splitter.)

Rich

 

[rbm]

This is not good info, losses after the first amplifier have very little effect on system noise figure.
prcguy

That's what happens when I try to reply from work with all the interruptions. I edited my prior post from AFTER to BEFORE.

This statement is incorrect.
"The losses 'after' (should be: BEFORE) an amplifier will directly add to the system noise figure."

The noise figure of the splitter (coax etc.) is equal to it's loss. (As shown in the second image.)

The fact that losses after the first amplifier have very little effect is EXACTLY what I was indicating. (As shown in the first image.)

In the calculator, the amplifier is not necessarily the first stage.

Rich

 

[prcguy]

Ok, your forgiven...
In addition to using the G/T calculator to characterize your receive system there are other factors that will determine if its even worth placing a pre amp in your system due to surrounding noise levels.
If you have access to a calibrated signal generator and a directional coupler for your frequency range of interest (20dB is good) you can find out what your local noise contribution is.
With the signal generator feeding the coupled input, your receiver connected to the OUT and a 50ohm load on the INPUT connector of the coupler, find what RF level it takes to get 12dB SINAD or 20dB quieting.
Then disconnect the load and connect your antenna system. Since the signal generator is still connected, increase the level until you reach the same 12dB SINAD or 20dB quieting level. The difference between the levels with the load and antenna are the degradation in dB that your surrounding noise is causing.
This measurement assumes you have chosen a frequency clear of transmissions and making the measurement at a few frequencies in the same band will give you a better idea of the problem.
For most people in urban areas the degradation will be at least several dB and in this case a super low noise figure amplifier will be wasted.
prcguy

That's what happens when I try to reply from work with all the interruptions. I edited my prior post from AFTER to BEFORE.

This statement is incorrect.
"The losses 'after' (should be: BEFORE) an amplifier will directly add to the system noise figure."

The noise figure of the splitter (coax etc.) is equal to it's loss. (As shown in the second image.)

The fact that losses after the first amplifier have very little effect is EXACTLY what I was indicating. (As shown in the first image.)

In the calculator, the amplifier is not necessarily the first stage.

Rich