Measuring RF Power with a Spec Analyser  - Page 1
Forum section: electronics / rf-microwave

author: jlake
author_rank: Contributor
timestamp: May 14, 2019, 03:28:44 pm
content: Hi folks,Measuring an E4400B signal generator's CW output at 250 kHz-3 GHz with an N9320A spectrum analyser. The SA is using the external 10 MHz reference from the sig gen.So, I output 500 kHz at -25 dBm on my sig gen. I keep ACM enabled on the E4400B.I set my center freq to 500 kHz on my SA.I set the RBW to 3 kHz (changing this changes the magnitude).I set my span to zero and my VBW to 1 Hz.I start a 1 second sweep.My SA now shows -26.21 dBm, give or take fluctations of 0.1 dBm.If I change the output power on my sig gen., then the SA shows an equivelant change, but is still approx 1.5 dBms lower than the output value on my signal generator. This difference seems to scale with frequency, i.e. at higher frequency, the difference is multiple dBms, while 250kHz-1MHz the difference is less than 2 dBms, and occasionally less than 1 dBm.Why is my power not closer to -25 dBm? Is there really 1 dBm of attenuation in my cable?Why does my power change dramatically when I change RBW?Is the only *true* way to measure RF power accurately, with an RF power meter?Yes, I have read Keysight's application note RE: SA and noise!ThanksJ

author: tkamiya
author_rank: Super Contributor
timestamp: May 14, 2019, 08:02:44 pm
content: I'm guessing calibration issue on signal generator and/or spectrum analyzer combined with cable/connector loss.Mine are spot-on but they are both calibrated by certified lab. I also use very high quality cable/connector purchased on eBay. (can't afford them new). Dumb thing like how tight connectors are, how sharp your cable bends, etc, starts to take measurable difference above 1GHz and gets worse from there.Your spectrum analyzer should have a calibration output. Have you tried that?

author: jlake
author_rank: Contributor
timestamp: May 14, 2019, 09:14:28 pm
content: Quote from: tkamiya on May 14, 2019, 08:02:44 pmThanks for your reply. The E4400B only goes up to 1GHz, so no worries there. I will try the calibration output, though the SA seems fine, since other people are routinely using it for measurements. I'm going to try a different spectrum analyser too.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 15, 2019, 11:27:57 am
content: A spectrum analyzer is never great at measuring output power precisely. And the output power level can also vary depending on the performance of the generator.In addition, your cables will introduce some loss (the fact that your loss increases with frequency makes sense - but the amount it seems to increase does not, unless you have bad and/or long cables).The only way to verify is with a power meter and then check what the power level is at the end of the cable (and at the output of the generator). This should allow you to figure out where the issue is.Quick note: how long did you leave the instruments on for before doing this test? I know at my lab we have to leave the PSGs on for hours before they really perform as well as they can.

author: Bud
author_rank: Super Contributor
timestamp: May 15, 2019, 12:18:15 pm
content: QuoteThat should not be happening with RBW changing and may indicate the SA cal is not good, or one of the instruments is out of frequency.Impedance matching of siggen and SA also will influence readings.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 15, 2019, 01:12:32 pm
content: Quote from: Bud on May 15, 2019, 12:18:15 pmThat can happen depending on the phase noise and actual noise floor of the system. If you increase the RBW you will be getting more of the thermal noise (unless you are measuring signals with low SNR this should have a very small effect) but also more of the 'ramp' around the peak. Ofcourse, you shouldn't see a change of 10's of dB, but even a few dB can happen if you go to really low RBW. It would be interesting if the OP could comment on the magnitude of this change.Keep in mind a spectrum analyzer usually displays dBm-in-a-bandwidth-equal-to-the-resolution-bandwidth, and not dBm/Hz or such.

author: Bud
author_rank: Super Contributor
timestamp: May 15, 2019, 01:21:28 pm
content: No, few dB change on the peak when you change resolution bandwidth is insane and certainly indicates a fault somewhere. Around the peak at the floor yes but not on the peak.The SA service manual could be consulted for magnitude adjustment procedure across RBW settings. It may be user level calibration.

author: _Wim_
author_rank: Super Contributor
timestamp: May 15, 2019, 05:15:06 pm
content: At only 0.5Mhz cable losses should be insignificant.When RBW is increased, does the level keep increasing by each step increase of RBW, or does it remain almost constant from a certain bandwidth on?Do you have identical dbm devaition with a signal level that is much higher (-10 dbm or 0 dbm) and much lower (-40 dbm)? If yes, this could indeed be some user calibration as suggested above?Can you set the spectrum analyser to peak hold, and slowly sweep the signal generator over its entire range? Is the resulting max hold spectrum slowly falling, slowly rising or wavy?

author: jlake
author_rank: Contributor
timestamp: May 15, 2019, 09:09:43 pm
content: Hi all,Problem now resolved! I tried using the alignment feature with the calibration out into the SA's input, but I kept getting an error "Signal power too low, alignment couldn't begin". So, I factory reset, and the same thing happened. I changed my cable setup a number of times too. In the system menu, under alignment, there's a "Load defaults" button. I enabled the calibration output, tried to align, and it worked.I don't know what was quite wrong with it.All seems fine now!Thanks for your help, everybody

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 18, 2019, 08:51:59 am
content: Quote from: Bud on May 15, 2019, 01:21:28 pmYet three just-back-from-calibration spectrum analyzers show as much as 10 dB of variation in level when playing around with RBW when a colleague of mine was measuring a THz signal source.Granted, in a lot of cases, with low-phase-noise sources, it shouldn't cause too much issues, but if you are looking at stuff with a tiny bit of modulation or decent phase noise, it can really have a big impact.

author: jlake
author_rank: Contributor
timestamp: May 18, 2019, 10:26:35 pm
content: Quote from: TheUnnamedNewbie on May 18, 2019, 08:51:59 amHave you any reading material on why this occurs? Why would changing your resolution bandwidth change the measured power if your source is noisy in phase?

author: ogden
author_rank: Super Contributor
timestamp: May 18, 2019, 11:01:52 pm
content: Quote from: jlake on May 18, 2019, 10:26:35 pmIt's because RF power is spread into (band of) frequencies. Imagine you have 1W transmitter which is hopping between two frequencies at 50% duty cycle. When you look at just one of two frequencies with infinitely small RBW, you measure 1/2W. You measure 1W power only when you include both frequencies in the measurement - either by using narrow RBW, sweeping over range that includes both freq's and doing math (power in band/freq_range) or using RBW/VBW that covers both frequencies and hopping/sweeping rate.Further reading:http://literature.cdn.keysight.com/litweb/pdf/5965-7009E.pdf

author: Bud
author_rank: Super Contributor
timestamp: May 18, 2019, 11:12:42 pm
content: Quote from: TheUnnamedNewbie on May 18, 2019, 08:51:59 amThat is because the characteristics of the signal being measured, not because of the spectrum analysers.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 20, 2019, 05:59:49 am
content: Quote from: Bud on May 18, 2019, 11:12:42 pmThat is just a matter of perspective. You can just as easily turn it around - because of the characteristics of spectrum analyzers and their method of operation, this effect can occur.Again, this doesn't happen in the OPs case because he is measuring a lab source (I read over that the first time, so I get why my comment might seem a bit odd n ow - my bad).