EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: W2DML on November 27, 2024, 03:26:41 pm
-
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.
-
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:n is 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 \$\Omega\$) = 500 k \$\Omega\$ (again, for that example).
-
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.
-
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.
-
He's talking about radio-frequency voltage on the grid 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.