Spurious Measurement - RF Cafe Forums
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Post subject: Spurious Measurement
Unread postPosted: Thu Sep 16, 2004 12:32 pm
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.
Unread postPosted: Thu Sep 16, 2004 1:53 pm
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:
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
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,
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
Thanks again for your help and if I need to know more (obviously I do)I will be back....
Unread postPosted: Fri Sep 17, 2004
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
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
Unread postPosted: Tue Sep 21, 2004 7:07 am
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).
Unread postPosted: Tue Sep 21, 2004 12:14 pm
Joined: Mon Apr 12, 2004 4:50 pm
Location: South Florida
Hwrd69, Are you making your
measurements though a directional coupler?