# input power of spectrum analyzer

• posted

I have a spectrum analyzer with 50ohm input. It is marked the max.

30dBm/50VDC at the input. The 30dBm roughly calculates to be 136dBuV at 50ohm impedance. The signal i am observing has frequency components at f1,f2,...fn with magnitude close to 110dBuV. I did not understand what the max.30dBm/50VDC at the input of analyzer stands for. Is it the maximum signal level at any frequency level or is it the total power of the signal at 50ohm should be less than 30dBm? if its the power, how can i know how much power in the spectrum as the spectrum may have infinite frequency components. Someone please clearify me this doubt. thanks kristo
• posted

The marked value refers to the maximum total power allowable to avoid any thermal damage at the input front-end (the DC voltage is just the input coupling capacitor maximum voltage). The thermal conversion of EM energies no matter of the frequency components of the signal, therefore any harmonic components should be considered and summed to compute the total thermal energy. Usually, the harmonic components are not considered indeed, because they are quite low in energy if compared to the fundamental signal energy. E.g. whether a very "dirty" signal has it's third harmonic component

10db under the fundamental, the fundamental participates to the thermal effect ten times more than its third harmonic, and we are talking about a very "dirty" signal (for signals having harmonic components 20dB under the fundamental, those harmonics participate just for 1% of the total). Anyway, if you need to measure that kind of distorted signals, or other composite signals, take care of the total power to avoid damages at the input attenuator, or worse, at the 1st mixer. Have a nice day. Massimo.
• posted

"Maximum input" means just that. The total *power* (30dBm = 1 Watt) which may be applied to the input without damage to the input attenuator or mixer, irrespective of its spectral makeup.

More importantly, you need to know the input level for a specified degree of compression, (usually 1dB), which should be specified in the manual. That will be some way below maximum permissible, (I'd guess +10dBm). Exceeding this will result in intermodulation products occurring in the instrument, and what you see won't be what you really have.

You need to know the total (broadband) power of your signal. Use some sort of broad(ish) band power meter (thermocouple, bolometer, etc.) first, to get an idea.

Then use additional attenuator(s) at the analyzer input. to get the total power down to (say) -10dBm, at which the analyzer will be happy. Power attenuators can be expensive, especially ones with flat characteristics extending up into the high UHF range.

Be careful, a zapped analyzer frontend can cost several months' salary to get fixed, not to mention the nose bleed, if it belongs to someone else :-)

There was (probably still is), a good application note on the use of the spectrum analyzer on the Agilent site. It has, among other things, one of the best explanations I've seen about why the need to apply corrections to noise measurements when using video (VBW) filtering.

```--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more```
• posted

```--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more```
• posted

The 50V input refers to an impulse or static voltage that can come from ESD or a (long) charged up cable. Always dicharge yourself and the connecting cable (center-conductor) before connecting up to the sensitive front-end.

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