RF voltage using oscilloscope???? - Page 1
Forum section: electronics / beginners

author: Crambone
author_rank: Frequent Contributor
timestamp: June 01, 2024, 12:07:21 am
content: I was asked to measure RF Voltage on a few IRF520 Power Mosfets in. transmitter but don’t have an rf probe. Can I use a Siglent MSO scope? If so what would the set be that’s required?

author: radiolistener
author_rank: Super Contributor
timestamp: June 01, 2024, 01:37:04 am
content: If RF power is less than 0.5 Watt, you can use oscilloscope, but it's frequency response is not flat, so amplitude may be inaccurate, especially close to oscilloscope bandwidth limit.If RF power is more than 0.5 Watt, it can damage oscilloscope or it's probe.Note that oscilloscope probe voltage rating is specified for DC. For RF it rapidly drops down with frequency. Usual 600V 1:10 probe at 20-30 MHz has about 20V maximum rating, see probe manual for details.Usually such 600V 1:10 probe just release magic smoke on attempt to measure powerful RF transmitter output. But be careful, it can damage oscilloscope either.

author: CaptDon
author_rank: Super Contributor
timestamp: June 01, 2024, 02:20:41 am
content: The oscilloscope can measure accurately any voltage of any frequency within its bandwidth limits and voltage input limits. R.F. is like any A.C. waveform. The probe may have its own limits. You need to look at the specs in the owners manual. I have easily measured RF voltages from transmitters of 500 watts @ 50 ohms between 2MHz and 30MHz. Why are you tasked to make these measurements if you are not sure what settings you need? Obviously if possible you would select a sensible vertical input setting to allow a nearly full scale waveform and you would select if possible a horizontal time per division that would allow 2 to 4 complete cycles of the wave to be on screen.

author: Crambone
author_rank: Frequent Contributor
timestamp: June 01, 2024, 02:29:04 am
content: Great thanks guys I’m only looking at near 0 watts of output. I’m trying to troubleshoot a problem transmitter.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 01, 2024, 12:32:18 pm
content: I am with the Captain on this. I use a scope to measure signals in power amp of radios all the time.A 100 Watt Transmitter will have 70 Volts into 50 Ohms. Make sure you have a 50 ohm load on the output.A 50 W transmitter will have 50 Volts into 50 Ohms.A 1000 Watt Transmitter will have 223 Volts into 50 Ohms, Oscilloscopes may not be the most accurate means of measuring output and themselves may not be entirely accurate in measuring voltages but they are plenty good for maintenance purposes.Usually a couple of IRF 520 would make less than 50 Watts output in a HF Radio.If you probe the Drive into the 520s ( Gate Voltage ) the probe will cause a large imbalance and the output will be down quite a bit.With the limited info, if you have no output from a 520 Final Amp: check Drain volts then test your mosFETS. They do blow.No Oscilloscopes were injured or killed in the maintenance of these transmitters.I did break a pencil working out the values using Ohm's Law.

author: 807
author_rank: Frequent Contributor
timestamp: June 01, 2024, 12:41:56 pm
content: Quote from: Crambone on June 01, 2024, 02:29:04 amDo you have a schematic of the transmitter? Is it on a set frequency or variable? When you say "near 0 watts of output", do you mean that is the problem with the transmitter? What should the output normally be?You can only take meaningfull measurements if the transmitter frequency is well inside the bandwidth of the scope, so it would help if we knew the model of the scope too.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: June 01, 2024, 01:04:08 pm
content: [ I did break a pencil working out the values using Ohm's Law.

author: Crambone
author_rank: Frequent Contributor
timestamp: June 01, 2024, 02:04:41 pm
content: Yes schematic can be found here. Testing on 28MHzhttps://cbtricks.org/radios/stryker/sr_955hpc/graphics/Schematics.pdf

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 01, 2024, 02:37:00 pm
content: I thought so, a CB problem......Been there many times,,,,Check voltage on collector of Q 49 in Transmit. This is "pre Driver" Should be 8 Volts. If OK then.....Put your scope on the collector of Q 49 and see if there is a measurable carrier on AM. Should be several volts on scope.Let us know.

author: radiolistener
author_rank: Super Contributor
timestamp: June 01, 2024, 03:55:30 pm
content: Quote from: Wallace Gasiewicz on June 01, 2024, 12:32:18 pmThe problem here is that 600V 1:10 probe has absolute maximum rating just 25V at 100 MHz and 50V at 1 MHz. Just open probe manual and check "Voltage vs Frequency Rating Curve".So, if you're measured 50V RF with 1:10 probe and it still alive, you're lucky. Probably you're testing it more close to 1 MHz, because at 10 MHz it's absolute maximum limit is about 30 V max.If you connect transmitter directly to oscilloscope, the situation is more worse, because if you're using 50 Ω input mode it has 5 Vrms max rating, which is just 5^2/50=0.5 Watt.If you're using high impedance 1MΩ input mode, this is not reliable way to measure RF voltage, because result will be highly dependent on cable length.The second issue with high impedance input is that oscilloscope usually don't have specification for absolute maximum rating for RF power in high impedance mode. But I consulted with Siglent support and they said that absolute maximum rating for RF is about 40V max for SDS1102X.if you're needs to measure transmitter output with the probe, I recommend to use at least 2000V 1:100 probe. It has absolute maximum limit about 200V at 1 MHz, 70V at 10 MHz and 50V at 100 MHz. It allows to use it for 100W measurements at least below 10 MHz.Quote from: Wallace Gasiewicz on June 01, 2024, 12:32:18 pmAttempt to measure 1 kW transmitter output with 1:10 probe just produce magic smoke output from the probe and it don't works anymore after that. It's already proved by many peoples who tried it.If you connect 1000W transmitter directly to oscilloscope it will damage your oscilloscope input and it don't matters which imput mode youre used 50Ω or 1MΩ, in both cases your oscilloscope frontend will be burned out.

author: TimFox
author_rank: Super Contributor
timestamp: June 01, 2024, 04:10:09 pm
content: High-power rated 50-ohm attenuators may be a bit expensive, but allow accurate measurement of high RF voltages across 50-ohm loads.Often, such attenuators are directional: the high power must be connected to one of the two connectors while the output (lower voltage) comes from the other.MCL has a wide range of attenuators up to 100 W.https://www.minicircuits.com/WebStore/Attenuators.html#product_detail_oneBird makes them up to 4 kW input powerhttps://birdrf.com/Products/Test%20and%20Measurement/Attenuators.aspx

author: G0HZU
author_rank: Super Contributor
timestamp: June 01, 2024, 06:11:16 pm
content: I would generally recommend that the 1Meg input of a typical scope should not be driven with more than about 13V rms up at RF frequencies of about 30MHz. Some modern digital scopes will be rated at less than this.I generally try and limit RF voltages to about 5V rms at the scope input, especially up at VHF. This is best achieved using an external attenuator or coupler and an external 50 ohm termination at the 1Meg input.The ideal setup will limit the RF voltage at the scope input to about 2V rms and this will permit the use of the internal 50 ohm termination/path of the scope and this should improve the VSWR at the scope input.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 02, 2024, 08:15:16 am
content: You don't need an oscilloscope to fault find RF circuits in a CB radio, I worked for many years on the service side of the VHF/UHF mobile radio industry and while having an oscilloscope (which at best was 100 MHz b/w analog), we all used a simple RF probe and also the current meter of the bench power supply. Indeed the circuit diagrams of most of the radios we worked on included readings taken with a simple diode probe.The type of probe used is the same as shown on this web page:https://n5ese.com/rfprobe1.htmSJ

author: CaptDon
author_rank: Super Contributor
timestamp: June 02, 2024, 05:41:52 pm
content: I grabbed the book for my Tek SC504 analog scope. It states the following, Maximum input voltage Peak (D.C. + Peak A.C.) 250V D.C. Coupled, 400v A.C. Coupled. Peak-to-peak (A.C. Component) 500v @ 1KHz or less, Derates to 10v @ 100MHz. It is a wonder I have never blown the thing up as I have been way over spec in the 80 through 10 meter ham bands!!

author: radiolistener
author_rank: Super Contributor
timestamp: June 02, 2024, 06:04:08 pm
content: Quote from: CaptDon on June 02, 2024, 05:41:52 pmif it's good quality, it has some reserve, so if you connect it for a short time (to avoid overheating) it can survive, but it is dangerous, because it can partially damage some component and your equipment may show incorrect results after that.I often using 2W resistor as a load for 10-20W RF power, if it's connected for a short time it doesn’t have time to overheat, but sometimes it produce a little smoke if you keep it for 3-5 seconds more...

author: G0HZU
author_rank: Super Contributor
timestamp: June 02, 2024, 08:48:12 pm
content: Quote from: CaptDon on June 02, 2024, 05:41:52 pmUp at 28MHz, the input of a typical old school analogue scope like a Tek 465 will look like about 1800 ohm Rp in parallel with about 20pF. This can be measured with a VNA.A 100W transmitter fed into an external 50R load would show 70.7V rms at the scope input. This would mean about 2.8W Pdiss in the 1800 ohm Rp of the scope at 28MHz.It might survive this for a few seconds, but not much longer... I think that components would be stressed and aged if this was done regularly.A crude equivalent model for a typical 200MHz 1meg scope input is shown in the diagram below. The series resistance R1 is typically made up of a series resistor right at the scope input and this is then in series with the ESR of the rest of the scope input.This resistor at the input can be a tiny 0805 or 0603 SMD resistor in some modern scopes. By contrast, it's an axial leaded resistor in the old Tek 465 scope.

author: radiolistener
author_rank: Super Contributor
timestamp: June 03, 2024, 03:58:10 am
content: There is also dielectric and conductor loss in the components, it raise with frequency.This is why even high voltage capacitors heating a lot when you apply 50-100W RF power from transmitter.