I am measuring intermodulation using spectrum analyzer. How does the input attenuation affect the power of intermodulation products?
Electronic – Input Attenuation and Intermodulation Products
attenuationRFspectrum analyzer
Related Solutions
- Is the signal level that is shown in a spectrum analyzer power of the signal being measured. If yes then what power is it: average power, true power or peak power?
Typically, a spectrum analyser (SA) cannot be classified as a true power meter, because it is intended to display the IF envelope voltage. Power can be derived then assuming a 50 Ohm reference impedance. (Some SAs offer to choose among less frequent reference impedances for instance of 75 Ohm).
Modern SAs however include a power detector circuitry for accurate power readings. The detector mode setting enables several measurement options such as RMS power or peak power... If the trace is set to (power) detector mode, and the latter is for instance set to RMS, it is possible to use this info to obtain a more or less reliable RMS power reading!
The SA marker indicates the total power (on the vertical axis) contained within the frequency band set by the resolution bandwidth (RBW) filter, and centered around the selected marker's frequency (horizontal axis). The power reading is usually expressed in dBm unit.
Example: If the RBW is set to 10 kHz and the marker reading is say -6dBm at 1.95GHz, it means that -6dBm/10kHz with the 10kHz centred around 1.95 GHz.
Ho much power in 10kHz when expressed in mW?
P_10kHz_mW = pow(10,-6/10) = 0.25 mW
Assuming that this level is uniform over 3.84MHz, how much in 3.84MHz?
P_3.84MHz_mW = 0.25*(3.84MHz/10kHz) = 96.45 mW
P_3.84MHz_dBm = 10*log(96.45) = 19.84 dBm
If you know your signal is only located in this band, (in the example above that corresponds to one UMTS channel), it is possible then to calculate the power that corresponds to that band. Some SAs include the ability to automatically perform the power reading of a desired band (e.g. 3.84MHz) by integrating the readings of the RBW filters over that band. This should lead to the same answer above. In both cases, the power/Hz (power carried by exactly one 1Hz e.g. at exactly 1.95GHz) can be derived as well.
- Also when using a power meter for measurement of power, what is difference in its measurement of power with respect to that of a spectrum analyzer? I mean, what is the requirement of a power meter if a spectrum analyzer is capable of measuring the power itself (assuming same level of signal being measured by both)?
The power meter (PM) is an instrument dedicated for power measurements. Theoretically, a signal's power could be measured at highest accuracy using a PM. No integration is required since the PM has a very low frequency selectivity (i.e. has a wideband nature). Roughly speaking, the power reading thus corresponds to "everything being fed" to the PMs probe. If only the desired signal is present at its input, this results in a very accurate reading. However if there's an "interferer" along, or if one is interested in the power reading carried by a certain frequency band, then this constitutes a drawback for the PM.
An example is that a PM is not suitable for measuring adjacent channel power (in communication signals) as an absolute value or relative to the transmit channel power. The power in the adjacent channels can only be measured with a "selective" power meter. The SA is the solution in this case.
There are essentially two mechanisms:
- Internal nonlinearities, arising from non-ideal behaviour of mixers and amplifiers.
When passed through a nonlinear device, a modulating signal containing F_1 & F_2 can produce an output signal with side bands located at: $$\{F_1, F_2,\\ (F_1+F_2), (F_1-F_2),\\ 2*F_1, 2*F_2,\\ 2*F_1+F_2, 2*F_2+F_1,\\ 2*F_1-F_2, 2*F_2-F_1,\\ 3*F_1, 3*F_2\\ ...\}$$ Even though we want mixers that are multipliers they're often closer to Taylor series, and only approach being multipliers for a given signal domain. We also want linear amplifiers, but we also want them to be efficient and cheap... that is a tricky, tricky mix.
- Effects due to externally applied RF.
Power amplifiers are sensitive to energy applied to the output and particularly when operated close to or in saturation, they can look an awful lot like a switching mixer if you squint just right, so again you can get sum and difference series produced. This is an especially good party trick on a shared mast with multiple co-located services where adding a transmitter can cause one that YOU do NOT own, to interfere with another receiver that you ALSO do not own... That can get amusingly political. I have had LOTS of fun with this when co-locating low band VHF kit.
Power amplifiers are sensitive to energy applied to the output.
To expand on this detail (from discussion in comments): It all boils down to the non-linear curve that describes the input-output relationship of an amplifier. When a voltage is introduced at the output, you're modifying that relationship. Thus, the resulting inter-modulation products observed at the output contain the sum & difference of every multiple of the frequencies introduced at either the input or output.
Best Answer
So, based on your comment, you want to know what happens when you change the "attenuation" setting on your spectrum analyzer's front panel.
Exactly how a spectrum analyzer's front panel controls affect intermod distortion depends on the analyzer's construction. Good analyzers should come with manuals showing their internal arrangement for just this reason.
In general, you can expect a spectrum analyzer to be laid out like so:
(Copied from https://www.keysight.com/main/redirector.jspx?action=ref&lc=fre&cc=FR&nfr=-536900402.536880437.00&ckey=2010290&cname=EDITORIAL)
There's not much between the front panel plug and the mixer, and mixers tend to suffer from intermodulation distortion.
So, in general, if you're seeing a distortion product in the output of your spectrum analyzer, it's either coming from the system your testing, or its being generated in the spectrum analyzer itself. (Or both, just to make your life extra fun).
Also in general, if you decrease the input voltage to the mixer by a factor of \$N\$ (I'm going to try to keep voltage and power separated here), you would expect to see all of the signal component voltages drop by a factor of \$N\$.
If there's \$k^{th}\$ order intermodulation inside the spectrum analyzer, however, then you would expect to see the \$k^{th}\$-order product's voltage to go down by a factor of \$N^k\$ -- so if you dropped the input voltage to the mixer by a factor of 2 (6dB), and there were 2IM products, you'd expect the output voltage of that product to drop by a factor of 4 (12dB).
Take as an example that you're putting what ought to be two signals into your spectrum analyzer; one at 100MHz and one at 101MHz. In the output, you're seeing signals at 1Mhz, 99MHz, 100MHz, 101MHz, 102MHz, 200MHz, 201MHz, and 202MHz.
Six of the signals you're seeing are spurious, and are the result of intermodulation distortion or plain old harmonic generation (the 200MHz and 202MHz signals). The products at 99MHz and 102MHz are the result of 3rd-order intermod; the products at 1MHz and 201MHz are the result of 2nd-order intermod (i.e., plain old mixing), and the products at 200 and 202MHz are simple 2nd-harmonics of the input tones.
If these spurious signals are being generated in the external circuitry, then if you click in 6dB more attenuation, they'll all go down by 6dB.
If the signals at 1, 200, 201 and 202MHz all drop by 12dB, then they were from distortion inside the spectrum analyzer. If the signals at 99MHz and 102MHz drop by 18dB (3 * 6dB = 18dB), then they were also from distortion inside the spectrum analyzer.