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Noise figure degradation to chip to module - RF Cafe Forums

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smanoha
Post subject: Noise figure degradation to chip to module Posted: Tue Nov 07, 2006 4:40 am

Captain

Joined: Tue Apr 25, 2006 2:01 am
Posts: 7
Hello,
I am noticing a 0.75 dB difference (degradation) between the noise figure measured on-chip and the noise figure measured in module (after assembling the same chip in said module with input and output microstrip transmission lines). The chip is designed to work at 10 GHz and the length of the 50 ohm input microstrip line (the loss of which, I would assume, would add to the noise figure) is about 1.5 mm. The subtrate used is alumina. It appeared to me that the NF deterioration was on the higher end and I am presently looking at ways to minimize this. I am guessing that the problem might be in the transitions (coaxial-microstrip and microstrip line to chip). It would be helpful if someone could suggest practical guidelines and tips that could help mitigate this problem.

-smanoha


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IR
Post subject: Posted: Tue Nov 07, 2006 7:01 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

When you add any length of transmission lines (And especially in such high frequenices), you can expect mismatches. I don't know what is the function of your chip, but let's assume that this is an LNA. If this is an LNA for example, then you have to match it to a slightly different input impedance caused by the addition of transmission line. If the LNA was designed to Gamma (opt) which resutls in the best NF, the now the LNA is matched to another Gamma which degrades the NF. I suggest you to simulate the chip together with the added transmission line and change the dimensions of the transmission line if possible.

Anyway, from the simulation you will get an insight of the problen and the needed changes to be done in order to decrease the degradation.

_________________
Best regards,

- IR


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nubbage
Post subject: Posted: Tue Nov 07, 2006 12:24 pm

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
You might have some error in measurement due to mismatch of the testgear.

If you have an amplifier VSWR of say 2:1 and a testgear VSWR of say 1.6:1, with a modest loss for the connectors plus 1.5mm alumina strip line, this would account for about 0.7dB extra loss and therefore extra noise factor (according to my software)

However I cannot simulate random lengths of the interconnecting line to see what the max and min combination of VSWR would be. Nonetheless you can see the order of magnitude. IR is right: you need to simulate every component transition as accurately as possible, then work a combination of inter-element spacings to see if that equates to 0.75dB. If it does, then all you can do is use very good connectors (which anyway degrade with use) and perhaps match the testgear with a stub tuner or an isolator

It is notoriously difficult (as you probably already know) to measure such small losses to an accuracy of 0.1dB

In addition I agree with IR re the difference between MAG matching conditions and Best Noise Factor matching conditions. They are different and many workers in the field have noticed this, particularly at the very low noise temperatures currently attainable.


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IR
Post subject: Posted: Tue Nov 07, 2006 12:49 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

Just to emphasize what nubbage wrote:

You should perform simulation first before attempting to do measurements. Doing measurements first will cost you in a lot of time and effort. Simulation is the scientific and insightful way to gain understaning over the root-cause of the problem and to be able to quantize it.

When you get some results and hints from the simulation, then you can do some tests and with the existing interface between the simulation tools to test equipment you can see the differences between the measured and simulated results. So at the end you can converge into the final solution.

Good luck, if you need more help please let me know!

_________________
Best regards,

- IR


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smanoha
Post subject: ThanksPosted: Tue Nov 21, 2006 2:23 am

Captain

Joined: Tue Apr 25, 2006 2:01 am
Posts: 7
Thanks IR and nubbage for your help



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