spectrum analyzer and power meter questions

measure power with spectrum analyzer

I have used spectrum analyzer to measure the 900Mhz OSC power output and compared with RF power-meter output, there are always has about 5dbm difference between them, Which one should I trust?

is it possible that my spectrum analyzer needs calibration? or the spectrum analyzer shows only the signal strength of the 900Mhz signal and not the harmonics frequency and power-meter measures all the frequency strength? How do I calibrate the spectrum analyzer ? a pure 900Mhz signal source? is 5dbm is just way too much in this case?.

reference level offset spectrum

probably you yourself can not calibrate the spectrum analyzer and you have to bring it back to the manufacturer for doing that.

power meter questions

Hi, Power meter is designed to measure power. so use this. Spectrum analyzer can't measure power correctly at the same accuracy as your power meter can. Differences of 3 dB are normal.

An oscilloscoop is also no volt meter !!!!.

Paul.

spectrum analyzer with power meter

I have same question.

I was also told that the spectrum analyzer shows only the signal power at the fundamental frequency and the power-meter measures all the frequency strength, includeing the harmonics frequency in the effective range of the power meter.

is that right ?

I would vote for the power meter.

Power meter, assuming it is a thermistor type, is well matched to 50 ohms. The power meter head is small, so you can connect it to the source directly without any cable loss. True, the power meter will also add in any harmonic power, so you have to be careful about odd signals with a lot of out of band power content. And for any sort of pulsed power measurements, you can trust that the thermistor power meter will gather up all the average radiated power. A spectrum analyzer with its sampling and band limiting filters can throw away a lot of actual signal power when it is pulsed or modulated.

A spectrum analyzer front end is basically a broad band mixer. The mixer is not a good 50 ohm match, so you end up reflecting varying amounts of power at its input. There is a ton more circuitry that can drift around before the actual power is measured. And you usually have to compensate for the loss of a cable at the input. So, the minute after all that stuff is calibrated in a cal lab, the spectrum analyzer is probably ok. A few weeks afterward, after the circuitry has drifted a little, and it is less accurate than a power meter.

Hi

What's type of analyzer and power meters are used in above measurements?

Hameg5012 analyzer and HP432A power-meter

Accurate power measuring with spectrum analyzer is possible only if resolution bandwidth can be greater than spectrum width of measured signal. Measuring result is actually weighted by bandpass characteristics of that filter and best accuracy is accheived on pure unmodulated signal.

SA is not supposed to do accurate power measurement. However 5dB error doesn't make sense either...

RF Detector type of SA should also be checked.Because depending on application, some options can be used for measuring of the power.

after looking through old HP test and measurement catalog books, here is what I found:

most SA has amplitude accuracy of +-2dB to +-3dB and the SA for EMI measurement has the worst amplitude accuracy. (they are connected to antenna and long cables so usually need to specify attenuation curves to EMI testing software to auto-correct the amplitude when measuring the EMI radiation level, so the accuracy are calibrated for the whole measurement system in testing software),

The power meter with thermistor sensor is the most accurate one, normally you have to switch in between SA and power-meter to get the accurate power measurement.

For most accurate power and frequency measurement the "measuring receiver" is used and they can be used to calibrated the signal generators and attenuators. they are basically the combination of SA and thermocouple power-meter.

So my SA has 5db difference comparing to my power-meter is normal but a little bit large (may be my SA is design for EMI measurement, it comes with EMI testing software) I should trust my old HP power-meter.

thanks a lot for all the help from you guys,

abc123 said:

after looking through old HP test and measurement catalog books, here is what I found:

most SA has amplitude accuracy of +-2dB to +-3dB and the SA for EMI measurement has the worst amplitude accuracy. (they are connected to antenna and long cables so usually need to specify attenuation curves to EMI testing software to auto-correct the amplitude when measuring the EMI radiation level, so the accuracy are calibrated for the whole measurement system in testing software),

The power meter with thermistor sensor is the most accurate one, normally you have to switch in between SA and power-meter to get the accurate power measurement.

For most accurate power and frequency measurement the "measuring receiver" is used and they can be used to calibrated the signal generators and attenuators. they are basically the combination of SA and thermocouple power-meter.

Click to expand...

Yes you can setting level on signalgenerator with power meter as reference and after 'calibrate' SA from signalgenerator and adjust viewed value with 'offset referense level' (here in hp8594) in SA to same value as generators/power meters view. this calibrate also away loss in cable to SA (if not change this between calibrate and measure).

I see many time people forget/ignore cable loss in measure and 'round' value of external attenuate (ie calculate 30 dB, not real value at 30.8 dB on used frequency) and in the end not understud why measured value not compare to expected and need measure all things again.


'reference offset level' in SA and signal generators is very useful in mathematic way calibrate/compesate for cable loss and extra attenuator so SA (and signalgenerators) always send and view on display 'true' power in ex. dBm on connectors to DUT or rescale to view gain etc.

Suprisely very many, also modern SA (and signalgenerators) have very badly handling of 'offset reference level' with stupids limits as cannot view power higher than ex. +27 dBm in scale (R&S FSH3/6) even if using huge external power attenuator (or external amplifier) and want measure > +50 dBm - is it so idiotic/stupidly I not can belive it.... is a only recalculate of viewed value of power with offset - not more, but both hp/agilent, anritsu and R&S small field SA and signalgenerators cannot do this on right way and many times in the end needs recalculate measured value in head instead (and make possibly to mistakes... ), why, why - is only windows programmers without real RF-measure experience on measure/instrument companys to days???

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If you want calibrate your equipment park with certificate but have (very) small budget, calibrate your power meters in first case (cheap cost compare to calibrate SA) and use this as referense with good signal generator and couple of measured external attenuator (if you not trust accurate on signal generators internal attenuator ladder) to calibrate rest of your equipment , you always need measure and setting offset on generators and SA to compensate cable loss, external attenuators etc. unknow parameters, special in 900 MHz and up.

Simple (cheap or near wear out) 1.5 m measure cable movings or new bending after calibrate can change cable loss value around 1 dB in 900-2400 Mhz... so few dB inaccurate value in SA is a not big problem and in any case always need calibrate/compensate for bigger dB range variation of system loss depend of cable dimensions and length, adapters, attenuators etc.




/Xxargs

Based on HP manual 'referense level offset' is only a quick fix, it only add/substrate some offset value to the SA measured value, it is input manually by the user to compensate the loss of cables, connectors and antenna. it does not take care of the frequency response problem of cables,connectors and antenna. Not all SA has "reference level offset" feature. In my EMI testing software you can input the frequency response curve of cables, connectors, antenna, amplifier connected to the SA input and it will do the postprocesing after reading the level from SA and adjust the value for actual viewing. Some article refer SA output as "Level meter output" comparing to "Power meter output" from power-meter. they said it is because the SA does not have good/uniform input impedence when signals goes through all that up/down mixing, filtering, and therefore it is difficult to measure the actual power. therefore using "reference level offset" is a good and easy quick fix for the problem but for accurate measurement of power output we still have to rely on power-meter. even the power meter itself has problem too, the thermistorand thermocouple sensor is more accurate than the diode type of sensor. it is not easy to match the impedence of diode sensor to the 50 ohms input over broad frequency range

just some result from internet search for the SA and power-meter problem.