SDS2000X Plus Measuring RF Power - Page 1
Forum section: electronics / testgear

author: eloso
author_rank: Regular Contributor
timestamp: May 08, 2022, 09:35:10 am
content: Hi,I have set up a a x10 probe to view on Channel 1 the waveform of a 3.5Mhz steady carrier from a transmitter as applied to a 50 ohms load for which the transmitter is designed.I use the Measurement menu to display RMS of Channel 1.I can now calculate the RF Power in Watts that is being dissipated in the 50 ohm load. W = (Vrms * Vrms)/50Rather than using the calculator every time, I would like to think that I can display this calculated Watts value directly on screen using some kind of Math function but am struggling a little to see how to do it.I thought I could setup a function, say F1 in the Math menu and use the RMS of Channel 1 as a source then just do the algebra on it. But the only sources available as inputs to any Math function are the Channels C1-C4, Equivalent Zoom functions, the other function F2 and the reference waveforms. I kind of thought I would see Vrms of Channel one as a possible input.I have a feeling I am missing something really obvious but have had a good look , tried finding something on the web and am not making progress.I wonder if someone could point me in the right direction to get a nice clear Watts indication directly on the screen?ThanksEloso

author: 2N3055
author_rank: Super Contributor
timestamp: May 08, 2022, 02:44:12 pm
content: Quote from: eloso on May 08, 2022, 09:35:10 amHow much power ??!!!!You cannot apply more than 5V RMS which is 0.5W to scope input directly when in 50Ω mode...You might need an attenuator..Hi impedance probes are not really best way. Even if you insist to use Hi Impedance probe, you need to terminate source with 50 Ω load. Probes also have frequency derating factor.. And 15-20pF capacitance...That being said first, if you put attenuator in the front of input set to 50 Ω, then make sure to set it as a probe factor in the channel.Also, measurements are applied to channels (real and virtual, like Math and REF) not the other way arround.You can enable FFT, set it in dBm mode and read power directly in decibels from marker table.

author: eloso
author_rank: Regular Contributor
timestamp: May 08, 2022, 03:28:19 pm
content: Thanks for taking time out to reply.I should perhaps confirm, and stress for anyone reading - do not try and apply 100W directly to scope input when the input is configured to 50ohm mode. There is no doubt - it WILL damage it.In my own case I am using an external 50 ohm load which is itself rated for 1Kw . What I am doing with the scope is measuring the peak to peak or alternatively the RMS voltage with a x10 probe and the scope input set to 1Mohm. Cannot stress enough that anyone trying this should make sure they get this right.It is a method used by many and that I have used for years to get good CW PEP and also SSB measurements. It just requires some trivial math to arrive a the final result in W PEP.Nice tip about the FFT - I must admit that having also posession of a nice Spectrum Analyser I have never tried the FFT on the scope, so I will look a that and see if I can get good results that way.And since I made the posting and stopped thinking about the problem, the answer came to me later and it comes from the basic understanding of measurements and channels as pointed out by yourself.I had it the wrong way around. I have solved the "problem" by setting up a math function to evaluate (C1*0.707)^2 / 50This gives me a trace on the screen of the instantaneous power corresponding to every point on the X axis.Then I apply a measurement to determine the peak to peak of this instantaneous power Function. This gives me a readout of PEP in Watts. It would be nice if I could make it say the word "Watts" alongside it but that would be being rather demanding !I think I had muddled thinking because I was trying to get the scope to do it the way that I do it on a calculator - take a measurement then apply the math. But as you have pointed out, on the scope we do the math and then take a measurement. Simples !Thanks againAnd please everyone do remember to be careful with high power RF and your 'scope.RegardsEloso

author: bob91343
author_rank: Super Contributor
timestamp: May 08, 2022, 04:31:16 pm
content: This does not measure power. You want to calculate the power by squaring the voltage, assuming the load is pure resistance of 50 Ohms.The proper way to measure power is to multiply voltage by current and power factor. So your system is only an approximation.It's common to do it your way; in fact that's what I do in my ham station by means of an rf voltmeter and mental calculation. It's close enough for me; I can measure over a kilowatt and feel sure it's as good an approximation as I need.Just a sanity check here.

author: rf-loop
author_rank: Super Contributor
timestamp: May 08, 2022, 05:23:21 pm
content: No one denies to do like this for some special cases. But this have also weaknesses...FFT is quite nicew for measure power and also display scale is log.But yes with my generator at this time this is max. Dummy load is external.F1 display 6.2(W) (yes there read V2)ETA: when I keep this answer open for get image etc.. and some cofee break.. I have not seen @eloso last msg.

author: eloso
author_rank: Regular Contributor
timestamp: May 08, 2022, 05:57:50 pm
content: Wow - yes, great contributions everyone and yes, I must learn that when discussing stuff in a forum to be read by many that unwritten assumptions can mislead folk.Bob therefore - absolutely bang on of course about my "method" being an approximation. My method works (and please do jump in again and tell me if I'm wrong) for a specific use case where:1. I am generating a continuous sine wave and delivering it to a 50 ohm resistive load (power factor = 1).2. I am interested in Peak Envelope Power (i.e. the average power over a single RF cycle at the crest of the modulation) which because the source is a continuous unmodulated sine wave, in my particular case is the same as average power over several cycles.3. One reason I am interested in PEP is that the regulator for radio licensing in my country uses this figure in the regulations. The other is that the radio manufacturer uses this figure in its specifications sheets and service manuals.4. The 'scope formula that both myself and bob are using is valid in this case because of the continuous nature of the signal.Many methods for measuring PEP are very sensitive to error and even things like the venerated Bird Thruline Wattmeters have a very loose tolerance for accuracy. I find the 'scope method to be the most consistent when coupled with a short patch cable and a good quality 50 ohm load. In recent times I have invested in a nice SA and this has a good stated level of precision but I can't use the SA for high power measurement with only a dummy load.Good quality dummy loads are much easier to come by than RF samplers. For the SA I use a home made RF sampler which sniffs a signal 40db down. So the generator passes the signal through the sampler to the high power dummy load and the SA can measure the sample. But this then adds error owing to the dummy load in addition to error of the sampler. Even a 0.5db error in sampling makes for a huge change in the final measurement and one of the reasons for the whole exercise as described in these posts is for me to figure out if, despite having the use of the SA, I might continue to use the scope for more accurate RF power measurement. In reality they're pretty close so I will use whichever is handy at the time.Thanks again everyone !Eloso