Why the reading of the power meter(R&S NRVS) is unstable - RF Cafe Forums

 Post subject: Why the reading of the power meter(R&S NRVS) is unstable

Posted: Thu Feb 17, 2005 9:47 pm 

Dear friends,

I got somthing interesting, when I calibrate the offset of a coupler+20dB att+ SMA cable by power meter in oreder to set this value into the power meter. I set the 0dBm for Sigen output, the reading of the power meter should be ~-21.5dBm, but this reading is unstable vary from -21.4dBm and stop to -21.9dBm(0.4dB range) . Another thing is this problem just happened on 1785MHz, ok for 1710MHz.

Would anyone like to share your valuable experience on this?

Thanks a bunch!
Simon

 Post subject: Re: Why the reading of the power meter(R&S NRVS) is unst

Posted: Thu Feb 17, 2005 11:04 pm 

Greetings Simon:

Since the instability only occurs in one frequency band, I suspect the signal generator first. Have you tried measuring the signal generator directly without the coupler (use a 20 dB pad if power is too high) to see if it is stable at 1785 MHz? Put the power meter as close to the generator output as possible to bypass all cables and connectors in-between.

If the signal generator power is stable without the coupler and cables, then next thing to suspect would be a bad cable or connector (or connection). It is possible for a pinched or partially broken cable (intermittent strand contact) to cause problems in a narrow band. Poor connector contacts can do the same. Connect your power meter at the end of the cable where the coupler normally goes, and use the attenuator if necessary, and check for instability. If it is stable, shake and twist the cables a little bit to see if it becomes unstable.

If the signal is still stable, then the only thing left is that the couple is bad. If it was hit with too much power at some time in the past, there could be a burn inside that is selectively unstable. One way a power coupler can be over-powered is to crank a lot of power into it with the output port either open (more likely) or shorted. Doing so causes the coupler structure to have to dissipate much more power than it is designed to handle.

Let me know the outcome of these test, if you try them, please.


- Kirt Blattenberger

 Post subject:

Posted: Fri Feb 18, 2005 5:17 am 

Hi Kirt,

Thank you for your reply. today I did some experiment again, I eliminated the coupler because I don't need to test any Isolation power. what I mean is just use cable(2)and att(3dB+20dB), but the result is still unstable. There is one more thing need to be noted, the problem happen on the both two frequency (1710MHz and 1785MHz). Sorry for the misunderstanding. Anyway, I 'll try to changed the other components as you said. (cables and att )next week see if I can find the root cause.

Thanks again!
Simon

 Post subject:

Posted: Fri Feb 18, 2005 11:56 am 

Power heads have a power range. If your operating near the low end of the power head then the reading will be unstable due to noise. Look at your power head. There should be a range. If it days that the low end is -20dBm, then the head is not sensitive enough to make the measurement.

 Post subject:

Posted: Fri Feb 18, 2005 10:43 pm 

Yes ,you are right. but the unstable reading will fix at a specific value, the time from unstable to stable is so long ,5 minutes, I think, in this 5 minutes, the reading differ 0.4~0.5 dBm from the beginning even though change another new power meter(NRVS).