How to estimate the linearity of RF transformer??

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fnx7

Joined: 19 Apr 2006
Posts: 25

31 Dec 2008 11:49 linearity test of transformer

If adding a RF transformer as TC1.5-1(mini-circuits) at the output of a amplifier,
how to estimate the linearity of it?

Supose the output is 12channel +2dBm/channel

Specs of TC1.5-1 maximum RF power 0.25W
DC current 30mA

Does the transformer degrade the linearity of output?

Thanks

FvM

Joined: 22 Jan 2008
Posts: 5154
Helped: 766
Location: Bochum, Germany

31 Dec 2008 12:09 linearity of transformer

Personally, I would expect measurable non-linearities (if any) only at the lower end of transformer frequency band. But as Mini-Circuits doesn't specify it, you have to measure by yourself. The linearity impact of load impedance changes due to the transformer, as seen at the amplifier output, is most likely larger.

Mazz

Joined: 03 Nov 2001
Posts: 592
Helped: 80

31 Dec 2008 12:18 transformer frequency linearity

Since the transformer is a passive one, it has a very high linearity, you can assume that its impact is negligible.

The spec given is intended as max DC and AC current that can degrade the performance or even damage the coils.

By the way, with that output power you should be safe (0.25W-> 24 dBm).

I hope it can help.

Mazz

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kspalla

Joined: 19 Aug 2008
Posts: 848
Helped: 140

31 Dec 2008 13:23 How to estimate the linearity of RF transformer??

From Mazz,
It is just in the break point. SO I suggest find the alternative.
One point is though it is passive circuit there could be chance of PIM.
BTW what is he frequency of operation?

Mazz

Joined: 03 Nov 2001
Posts: 592
Helped: 80

31 Dec 2008 16:20 How to estimate the linearity of RF transformer??

kspalla

be aware: if "12channel +2dBm/channel" means that he has 12 times +2dBm (it seems strange but if it like this...), the total max power is less than +13 dbm.

Mazz

FvM

Joined: 22 Jan 2008
Posts: 5154
Helped: 766
Location: Bochum, Germany

31 Dec 2008 16:39 How to estimate the linearity of RF transformer??

Main source of possible nonlinearities is the magnetic core, also affected by DC bias current, BTW. The influence of the non-linear permeability is mainly present at low frequencies.

fnx7

Joined: 19 Apr 2006
Posts: 25

04 Jan 2009 2:56 Re: How to estimate the linearity of RF transformer??

Thanks

The frequency is 54~860MHz.My understanding is main source of possible nonlinearities is the magnetic core as FvM said.
For I have read a patent about it which can increase the transformer linearity.

And the specs of MINi only indicate the maximum power. I think it is the limited power that will not damage the core.

byteptr

Joined: 29 Apr 2008
Posts: 112
Helped: 11
Location: Europe

04 Jan 2009 10:48 Re: How to estimate the linearity of RF transformer??

Hi,

I think you can measure the nonlinearity by the classic two tone test or triple beat method. First I start with pure tone and superposed DC Bias for magnetization curve and extracting the tranformer parameters.

Because two or more tone test is difficult if not high isolation (the sum of two tones allways produces intermodulation distortion in both signal generators/amplifiers) you must use moderate power amplifiers in very low levels and/or circulators/isolators.
Briefly, to use the known techniques for RF power amplifiers.

BR

FvM

Joined: 22 Jan 2008
Posts: 5154
Helped: 766
Location: Bochum, Germany

04 Jan 2009 15:04 How to estimate the linearity of RF transformer??

I think, it's easier in this case, cause the PA already exists and is part of the application. So the test would be for additional IM caused by the transformer. It should be carried out at minimal operation frequency. As I mentioned previously, you may want to check for PA IM caused by load impedance variation rather than the transformer itself.

Alternatively, you can use a different PA to measure transformer nonlinearity at a considerable higher power level and/or lower operation frequency to have a guaranteed margin.