measuring transmitted  power with an osciloscope and a dummy load - Page 1
Forum section: electronics / rf-microwave

author: conander
author_rank: Contributor
timestamp: September 22, 2016, 01:57:03 pm
content: Hello all,I recently bought a second hand MFJ tuner that includes a 300w dummy load. It also has a cross SWR meter. The power meter seems not to measure very well because in the different scales and it has some resistor for adjustment on the PCB board. I was thinking about using the dummy load 50 Ohm and measure the Vpp there with my osciloscope to adjust the potenciometers properly.I have been researching in internet but always when instruments are used to measure in transmision lanes like frecuencimeters or osciloscopes..they use a contactless probe just measure the presence of the signal.I was wondering if I am wrong if I measure directly in the dummy load with low power like 10w on CW mode for example and then calculate and adjust the transmitted power with.V=Vpp/(2*SQR(2)) and P=V^2/RAm I right or I miss something?For 50w I should find like 141,42Vpp. Is that ok for my Siglent SDS1202xRegards.

author: voltz
author_rank: Frequent Contributor
timestamp: September 22, 2016, 07:56:19 pm
content: Hi,Yes you can measure power fairly accurately using a 50ohm load and scope.However, there are few things to watch out for:The scope Y amp bandwidth needs to be able to handle the frequency of your transmitter. Otherwise the reading will be low.Best to test at lower frequencies rather than higher ones. But depends on your scope. Some cheaper probes also tail off at around 20Mhz.If the voltage across the load is higher than the scope Y channel input limit, simply use either a x10 or x100 probe to attenuate the signal to a safe level.Your scope should have this information printed right next the input BNC. Its usually less than 100v, some digital scopes as low as 30v.Using the formula Vrms = Vpp/2*0.707 and P=Vrms^2/R will give you the power at the load, remembering to factor in the multiplication factor of the probe you use for Vpp.Edit: more information,Your scope is good up to 200Mhz bandwidthYour Y amp input limit is stated as "1Mohm 400Vpk(DC + Peak AC <=10kHz), 50ohm 5Vrms"So do NOT select 50 ohms input! make sure its 1M only.

author: conander
author_rank: Contributor
timestamp: September 23, 2016, 08:20:13 am
content: Thanks a lot voltz! Just one concern I have....I also saw what you wrote. My osciloscope input spec says 1MOhm "1Mohm 400Vpk(DC + Peak AC<=10kHz)"What does <=10KHz means?I am trying to measure at 10MHz for examples on my Kenwood ts450s.Also...what kind of power a MFJ-949E is measuring??? The one with Vrms?or the one With Vpp?When do we press the button of the power meter that says PEAK is it holding the Prms for a while??? or is it measuring the P of Vpp?Sorry if this questions are silly....Regards.

author: tggzzz
author_rank: Super Contributor
timestamp: September 23, 2016, 11:14:31 am
content: Quote from: conander on September 23, 2016, 08:20:13 amWell spotted; good to see someone that readsallof the spec.If you look at *10 high impedance scopes' specifictions, they will typically show a graph of max voltage vs frequency - and at 100s of MHz the max voltage is often ~30V.Personally, if I wanted to measure a high power output, I would use an appropriately rated 20dB directional coupler and a high power dummy load.

author: voltz
author_rank: Frequent Contributor
timestamp: September 23, 2016, 12:37:49 pm
content: Standard power meters like the MFJ read in RMS power. So you are using the calculations to determine power using Vrms.If your meter has a Peak button, i dont think there will be a separate adjustment for that, its should be correct once the rms power is set. (please correct me if i'm wrong).I have the MFJ 949C, it does not have a peak button.As tggzzz points out, that <=10Khz indicates that as your input frequency increases, the input impedance decreases. That reduces the maximum input voltage because currents within the scope would be higher. And yes, there should be a graph showing that in your manual. I think at 10Mhz, you're probably fine if you use a x10 or x100 probe.

author: tggzzz
author_rank: Super Contributor
timestamp: September 23, 2016, 02:07:53 pm
content: Quote from: voltz on September 23, 2016, 12:37:49 pmNo. The scope probe's spec gives the maximum allowable voltage the probe can withstand at different frequencies. That voltage is independent of the circuit under test, and is true whether the source impedance is 50ohm, 1kohm, 1ohm, etc.The effect you mention (i.e. impedance decrease) is independent of level, and is the same at 1mV or 1V etc.

author: voltz
author_rank: Frequent Contributor
timestamp: September 23, 2016, 04:11:27 pm
content: You've misunderstood my post. I was referring to the scope input, not the probe.The scope input will reduce input impedance as the frequency increases (its basically capacitively reactive). And the rated value of 1Mohm is tested at <=10Mhz as stated in the spec. If the frequency being injected is too high, the impedance will be low, and higher than allowable currents could flow. Possibly causing damage. Thats what limits the input voltage Vs frequency. Basic ohms law after that for max current @ f.The probe is another matter, it too has its own highest rating voltage. And it too has its own capacitive reactance to take into account.

author: w2aew
author_rank: Super Contributor
timestamp: September 23, 2016, 04:34:34 pm
content: Also remember that the coax cable connected to the scope will add to the scope's input capacitance - so the resulting capacitance loading presented by the scope and the coaxial lead can easily be over 100pF (which is only 160 ohms at 10MHz). Not really the way to go...You'd be much better off using a 10x probe, and a probe socket to minimize the ground lead length. This will dramatically reduce the capacitive loading on the signal at the dummy load.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 24, 2016, 07:52:44 am
content: Quote from: conander on September 23, 2016, 08:20:13 amThe power meter will read the same in the average ("AVG") or Peak ("PEAK") positions,if you are feeding it with a "key down" CW signal.The "PEAK" position is an attempt to show the "Peak Envelope Power" (PEP) of an SSB or AM waveform.PEP is the average power which would exist if the signal was a CW one with the same power level as the "Peak" one reached by the modulated signal.These meters by their nature,read average volts,but are calibrated in power in a 50 Ohm load.In"AVG",if you are using SSB,or AM,the meter will try to follow the modulation,but will be limited by the mechanical inertia of the meter movement.In "PEAK",the rectified RF signal charges the capacitor of an RC network.The RC network charging time constant is very short,so the capacitor charges on the briefest of modulation peaks.Its discharge time constant ,via the meter is quite long,allowing it to give a reading for these brief peaks.This is often called a "peak & hold circuit".

author: conander
author_rank: Contributor
timestamp: September 24, 2016, 08:27:17 am
content: Quote from: w2aew on September 23, 2016, 04:34:34 pmThank you all for your interesting points of views and comments. They are well noted an appreciated.w2aew I follow your channel in YouTube what i can say is part of my favourites where I have learnt a lot! Good sack you have!Would be interesting for me to see how I can calibrate a Swr meter or power meter without having a bird 43.Thank you all again!Enviado desde mi iPhone utilizando Tapatalk