# Spurious Measurement - RF Cafe Forums

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hwrd69
Post subject: Spurious Measurement
Unread postPosted: Thu Sep 16, 2004 12:32 pm

I maintain software for a station that tests RF modules. However, I am new to RF, so I have a question regarding a calculation. If I have a known carrier (f) and a known spurious signal that is f+x away, what is the proper calculation for dBc between these two? Can I just subtract the dBm value of each from each other? I have searched but have been unable to find the info I am looking for.

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Itay
Post subject:
Unread postPosted: Thu Sep 16, 2004 1:53 pm

Greetings,

You can subtract the dBm values of each as you mentioned. There is another important term in RF related to spuriouses and especially to intermodulations, it is called IP3 and stands for 3rd order Intercept Point. This parameter is especially measured in non-linear devices such as Mixers and Amplifiers and is calculated as:

IP3=P1+Delta/2, where

P1= the power level of the fundamental (desired) signal (in dBm units);
Delta= the difference between the level of the third order product: 2f2-f1 or 2f1-f2 to the level of the fundamental signal (in dB);

The results is in dBm value.

If your station supports PLLs' measurements then this term is not relevant, and you just have to subtract the values of the two signal levels.

Hope this helps,
Itay

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Hwrd69
Post subject:
Unread postPosted: Thu Sep 16, 2004 4:27 pm

Thanks... That is what I tried, but differing results than a different test that written by someone else. His may because instead of two known signals, he is doing his test across a "region", or spectrum. On this information he is performing his calculations.

Thanks again for your help and if I need to know more (obviously I do)I will be back....

Howard

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Kirt Blattenberger
Post subject:
Unread postPosted: Fri Sep 17, 2004 1:45 pm
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings hwrd69:

I might be a little late to the game here, but for what it's worth

* Itay's remarks on IP3 product calculation are correct when the tones generating the intermods are well below the compression point of the system, but begins breaking down when within about 10 dB or so of the P1dB (for just two tones - lower as more tones are added).

* Since you did not specifically address 3rd-order intermods as being the cause of the spurious signal, then here are a couple possible reasons for disagreement of your results with your predecessor's:
- If the two signals are not CW (have some bandwidth), then the dBc value measured previously might be the difference between the integrated powers of the two signals.
- The previous measurement might account for any attenuation in the system (a filter, maybe) that introduces a difference in the two signal levels other than simply the amplitude delta displayed on a wideband spectrum analyzer.
- Your measurement system could be generating or adding power to the spurious signal. Add 10 dB at the input of the spectrum analyzer and see if both signals decrease by 10 dB (they should).

What is a typical discrepency between your measurement and the other guy's?

- Kirt Blattenberger :smt024

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hwrd69
Post subject:
Unread postPosted: Tue Sep 21, 2004 7:07 am

Kirt,

In the readings I have seen so far, the delta, typically, has been about 10dB. One thing I noticed about his algorithm is that because he doing interpolation across a spectrum region (e.g., 4MHz to 9MHz above & below the carrier) he is adding in a linear factor to help with his calculations.

In my case, I am just looking at the carrier vs. a particular spurious signal at a particular frequency (a test requirement). This may possibly be the explanation for the delta between his readings and my readings. I just would not expect such a large delta. Also, to answer your inquiry, I am using a wideband spectrum analyzer (Agilent E4440a).

Again, for lack of a better expression, I am a newbie to RF signal analysis. But the math should work (I think).

Howard Rosenbloom

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KC2SHO
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Unread postPosted: Tue Sep 21, 2004 12:14 pm
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Joined: Mon Apr 12, 2004 4:50 pm
Posts: 20
Location: South Florida
Hwrd69, Are you making your measurements though a directional coupler?

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