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Post subject: Spurious Measurement
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.
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);
the difference between the level of the third order product: 2f2-f1
or 2f1-f2 to the level of the fundamental signal (in dB);
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,
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....
Unread postPosted: Fri Sep 17, 2004
Aug 03, 2003 2:02 pm
Location: Erie, PA
I might be a little late to the game here, but for what
* 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
Unread postPosted: Tue Sep 21, 2004 7:07 am
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
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
Again, for lack of a better expression, I am a newbie
to RF signal analysis. But the math should work (I think).
postPosted: Tue Sep 21, 2004 12:14 pm
Joined: Mon Apr 12, 2004 4:50 pm
Hwrd69, Are you making your measurements though a