Sampling non 50 ohm HV RF Signals - Page 1
Forum section: electronics / rf-microwave

author: W2DML
author_rank: Contributor
timestamp: November 27, 2024, 03:26:41 pm
content: Hello,I'm wondering if anyone has some suggestions on how to sample an RF signal that is not on a 50 ohm line. This is for a high power RF Amplifier I'm working on.I would like to sample the RF on the Grid, in previous designs a Capacitor divider was used but those are frequency dependent. I decided to implement a voltage divider coupled with a capacitor as a "RF Sample" measurement. This will work well with a Hi-Z input like a scope but if I would like to use a 50 ohm RF device, the measurement will not be accurate. Is there a better way to do this? I would like to also do this on the output of the amplifier which swings around 16kV & drives a 1kohm impedance. The other issue is the design needs to be passive as this is a radiation environment, with no low voltage busses supplied.

author: TimFox
author_rank: Super Contributor
timestamp: November 27, 2024, 04:37:15 pm
content: Why not use a capacitive divider, such as 1 nF output to ground and 10 pF from grid to output? That would be good into 50 ohms for frequencies above 3.2 MHz (at -3 dB "bass response").Feel free to re-calculate for whatever frequency range you need.Note that this forms a high-pass filter to 50 ohm load, dominated by the larger capacitor (to ground) and 50 ohms for the low-frequency cut-off.If 10 pF is too high for the grid circuit, you can reduce it and increase the voltage division ratio to use a reasonable capacitor at the output.If such a capacitive divider, with a voltage ratio of 1:nis connected across a tuned circuit, at frequencies above the low-frequency cut-off the effective load on the circuit will be a capacitor equal to the series combination of the two capacitors (9.9 pF for my example) in parallel with (n2) x (50) = 500 k(again, for that example).

author: W2DML
author_rank: Contributor
timestamp: November 27, 2024, 07:29:33 pm
content: I was trying to avoid the capacitive divider over concerns of bandwidth vs divider ratio. I was wrong & your suggestion should work well. The 10pF total capacitance is fine because we have a tuning cap we can adjust to compensate for any mismatch. Ty for your help.

author: mtwieg
author_rank: Frequent Contributor
timestamp: November 28, 2024, 02:31:57 pm
content: So you want to measure RF on the grid (guessing that means AC mains, low impedance at 100-240VAC), but you also mention an RFPA with 16kV of swing driving a 1K load impedance...? Seems to be some fundamental contradictions here...Any means of coupling will have a lower cutoff frequency. Not sure what to make of your concerns wrt accuracy or bandwidth without actual specs.You might consider directional couplers as an option, though for such high voltages I doubt you'll find a suitable one off the shelf. No idea if directional measurements would be useful for your application.

author: TimFox
author_rank: Super Contributor
timestamp: November 28, 2024, 02:58:11 pm
content: He's talking about radio-frequency voltage on thegrid of a vacuum tube.MHz, not 10s of Hz.(His other use is on the plate, with the higher voltage swing.)A directional coupler is good for sampling a transmission line, such as 50 ohm coax, but not useful directly at the high impedance nodes such as grid and plate.

author: W2DML
author_rank: Contributor
timestamp: September 13, 2025, 05:27:25 am
content: Yes exactly, I am looking to measure on a tube. I ended up going with a standard vacuum cap divider with a door knob cap on the bottom of the divider on the anode. I'm currently talking to one of my vendors of designing a custom 30 ohm air coax DDC, so we'll see how that goes.

author: radiolistener
author_rank: Super Contributor
timestamp: September 13, 2025, 06:48:59 am
content: I think you don’t really have many options here: either use a capacitive divider with vacuum capacitors (but you’ll still need to deal with matching to 50 Ω), or design a directional coupler. In practice, the latter seems like the only reasonably reliable solution for this kind of setup.

author: geggi1
author_rank: Frequent Contributor
timestamp: September 14, 2025, 08:44:23 am
content: You should look in to transformer based samplers like these.You will get galvanic separation between the test gear and your amplifier.https://www.qsl.net/on7eq/projects/rf_sampler.htmhttps://www.n4ga.com/50db-hf-rf-sampler/

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 15, 2025, 10:42:54 pm
content: Quote from: W2DML on September 13, 2025, 05:27:25 amOkay, do you want to measure current, voltage, or directional power flow...?What frequency range are you operating in?QuoteAre you referring to a DDC with coax connectors for its ports, or a DDC made out of coax?No idea why you would want it to be 30 ohms, especially if it's meant for measuring a high-impedance node....

author: W2DML
author_rank: Contributor
timestamp: September 16, 2025, 01:58:27 am
content: Quote from: geggi1 on September 14, 2025, 08:44:23 amThe transformer based method looks interesting but for a 100kW, 16kV RF signal that will be tricky.Quote from: mtwieg on September 15, 2025, 10:42:54 pmI ended up going with a traditional Cap divider which will give me voltage measurements into our logging system. The only drawback is we won't get real power measurements with a VNA even if we find a way to trick a calibration.Similar to a waveguide directional coupler, we could have one made using air coax which is what the output of the amplifier is. This RF system is in the 24-28MHz range.

author: coppercone2
author_rank: Super Contributor
timestamp: September 16, 2025, 05:03:01 am
content: and I am wondering something, if its 16kV, and your resistor is 500k, v=ir, 16000/500000 = 32mA32mA is alot @ 16000Vis that not a rather substantial load? and high power dissipation?A 50mega ohm resistor would dissipate only 5W, making it a rather beefy power resistor.. it leaves the following series that claim low inductance :Super-moxSlim-moxMaxi-moxHVLRHTEI would love to know how low the inductance/rf behavior is, cheapest one I found ishttps://www.digikey.com/en/products/detail/resi/HVLR7609F50M0K9/16717626rated at 7.5w/22kVAre they even feasible to have a single resistor working like that at MHz? Or are these types of solutions more complicated with series/parallel resistors?

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2025, 02:53:13 pm
content: I assume the associated circuits at the vacuum-tube grid and anode are 24-28 MHz resonant tuned circuits.A capacitive divider at each position presents a small capacitance (approximately the “upper” capacitor) in parallel with a high resistance (approximately the load resistance times the square of the voltage ratio, which equals the reciprocal of the capacitance ratio), at the resonant frequency.The capacitance is absorbed into the tuned circuit; with a large enough ratio, one could drive 50\$\Omega\$ directly, or through a resistive divider.Such circuits will measure the grid voltage and anode voltage.

author: W2DML
author_rank: Contributor
timestamp: September 20, 2025, 01:00:22 am
content: Quote from: TimFox on September 16, 2025, 02:53:13 pmYes, the input is tuned using a variable vacuum cap to make an RLC circuit. The Output(anode) is directly driving an RF cavity. An RF cavity is basically an RLC load, with ferrite tuners to adjust the L. We are able to tune with the parasitics of the tube amp as part of that circuit. The output divider I ended up using is 2.5pF on the top, 2.5nF on the bottom. The reactance at 24.6MHz is <<< 50ohm so it should be fine as a voltage measurement on our 50 ohm RF samplers. With some simple math we can find the power V^2/(2(R/n)) where R is the shunt impedance of the cavity, n is the coupler ratio, and V is peak voltage measured.