LMV225: Multi carrier power measurements

Dear All,

I have made a drawing to explain more clear my problem, hope that somebody can help to explain why I have this problem and how I can resolve it.

Thanks,kind regards, Koen.

LMV225_problem.pdf

Koen,

How are you combining these signals. There's factors like the losses through the combining element and whether the signals are in or out of phase (they can construct or destruct each other). Also a straightforward test just for the power detector is just using that first carrier, then raise that same carrier by 3dB and see if the power detector outputs the change correctly.

Regards,


Brian Wang

Dear Brian

Thanks for reply.

I couple the 2 signals with a resistor 6dB coupler.

both signals are going through an amplifier, at the amplifier output there is a resistor tap, at the tap output the LMV225 is connected, the pass through signal is connected to a spectrum analyzer to verify the power levels of both carriers. 

The 2 signals can't be in phase since they have different frequencies and both generators are not connected to a reference generator...

I use the same system as you describe above to check if there is a difference when I add a second generator...it seems the LMV225 is blind for the second signal as long as it is not higher in power then the first one, seems only the highest signal is determining the output level, but if the second signal is identical or lower then the first one, this is not detected and ignored in the output voltage...

This behaviour is a problem for me, because I want to protect my amplifier from producing to much IM2&3, so if I add more carriers I want to detect this to reduce the gain so that the total output power of my amplifier is constant.

I have ordered from Mouser the LMH2110 to see if this detector is behaving in a similar way, from the moment I received it I will do the same test and post the result...

Unfortunately the LMH2110 is much more expensive then the LMV225, so I will probably not be able to put it in my design...

Kind regards, Koen.

Hi Koen,

The first solution I think of is one LMV225 each for the two different amplifiers. That is the cleanest and direct approach to ensuring each are adjusted specifically. But I also see you are factoring in the costs. Since the two frequencies are not the same, they are probably not combining in power as straightforward like it would in a same frequency case. But perhaps you can use your current method, and knowing that the output voltage of LMV225 is set by the dominant power amplifier, you can adjust one amplifier at a time in a process of elimination approach and know which one needs to be lowered in power.

Regards,

Brian Wang

Dear Brian,

Sorry that I was not clear : both generators are coupled before the amplifier, so there is only one amplifier and only one power detector...

The power detector should increase its output voltage by 3dB if both signals are same level, but as described before, that does not happen...instead the output voltage stay the same with one or two signals...only if both signals have a large frequency difference, the detector is adding both powers together, but if only 10MHz apart, the detector seems only can see one...

Dear Brian,

Today i have tested the LMH2110, this detector is working fine, when I add 2 signals then the output voltage is going up by 3dB, it does not matter if both carriers are near each other or big distance between them, very precise actually. So I still have the problem with the LMV225 that he can't see 2 carriers when they are near each other...

Nobody seems to know why this happen with this detector...

Kind regards, Koen.

Koen,

What is the input power level from each of the signals at the point of entering the power detector. Because looking at both LMV225 and LMH2110 there are different ranges in which the log conformity and error are normal, and they differ at different RF frequency. I am thinking possibly one of the frequencies you are using is in such region, for example RF=1000MHz and at maybe 3dB and looking at Figure 10 of LMV225 datasheet it may not be converting accurately for levels higher than around -2dBm.

Regards,

Brian Wang