Protecting Rf generator - Page 1
Forum section: electronics / rf-microwave

author: Jwillis
author_rank: Super Contributor
timestamp: January 11, 2019, 05:50:17 am
content: From time to time I dabble in the odd tube radio.Depending on availability.Anyway on my last project ,high voltage passed into my BK-2050 and fried it.I followed the the practice of putting a capacitor in line with the output and radio by still burned it up.I tried to fix the BK-2050 but can't find replacement shielded coils.So I bought a brand new MEI LI Mag 450 and I don't want this to happen again.Since I really only need the signal to tune ,would it be OK to put a 1:1 transformer in between the RF generator and radio to keep isolation between the two?I was thinking a small ferrite transformer with AGW 36 to 40 wire with about 100 windings each side.

author: Wirehead
author_rank: Regular Contributor
timestamp: January 11, 2019, 08:21:21 am
content: The transformer will never be broadband / linear enough. Why not put beefy clamping diodes on it (make sure they are made for RF -> low capacitance) + the series cap to block DC?.. Couple that with some extra care before attaching, and you should be fine.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 11, 2019, 06:58:05 pm
content: Why not an inline isolator/circulator? I've seen them used in some high power RF amps for preventing those reflections for damaging the less high power rated parts. Of course you're going to want a DC block as well if there's going to be risk of high DC voltages applied, but if it's applied suddenly, that full DC offset is a single AC pulse before the voltage stabilizes, so even with a DC block, if the switch on is sudden, it can do damage past the capacitor.The lower tech, lower efficiency version could also be a high power inline attenuator. For example, a 10W 10dB attenuator would reduce your generator's output, but would also reduce any reflections by 10dB, giving your generator a lot more reverse protection.Clamping diodes could work great too, but I don't know how low capacitance you can find for high power packages, so they may not be well suited to your application if your frequencies are high enough.

author: tkamiya
author_rank: Super Contributor
timestamp: January 17, 2019, 04:08:35 pm
content: How big was the capacitor? What type? That should have been all you need.If you are dealing with AM broadcast band or HF, I would just put 100pf ceramic with at least 500v rating. Absolute output might be affected but for the purpose intended, I am thinking linearity and level aren't that important?One of the tricks used in Heathkit days were take 2 wires and twist it 10 times. That's your capacitor. Or take a short (few inches) of coax, and make sure one end is NOT and can not short, and wire on the other side. This will make a nice high voltage capacitor. I did something like this for antenna and worked well.You can of course do the isolation transformer but depending on core, 100 winding will be way too much. Also, since robust protection is the theme, I would use thicker wire and less winding, if you are going that route.

author: bd139
author_rank: Super Contributor
timestamp: January 17, 2019, 04:20:25 pm
content: If you're using a sig gen on really old kit, you're probably doing signal injection at the antenna port so I would go the twisted bit of wire approach that tkamiya said. With those generators, just jam some wire in the antenna port and run one from the generator next to it. A foot or so of overlap is enough.You can adjust and debug literally everything from the front end on old radios with this method and a VTVM/voltmeter and RF probe.You don't want to start connecting generators inside old tube kit anywhere if you can avoid it due to the high DC bias you found out about. This applied to the older tube generators as well like the Heath RF-1 etc.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 21, 2019, 06:12:55 pm
content: Quote from: Jwillis on January 11, 2019, 05:50:17 amHere is how the pros do it:- For really high power levels, they use arc arrestors.- For the 10dBm range, you could use diode limiters. (e.g. Mini Circuits). They survive about 30dBm plus some surges.I used them in a high-voltage tube noise to protect the output device if a tube fails, like here:https://electronicprojectsforfun.wordpress.com/making-noise/noise-sources-i-have-built/a-tube-noise-source-using-the-2d3b-tube/

author: AF6LJ
author_rank: Unknown
timestamp: January 21, 2019, 10:47:35 pm
content: Quote from: Wolfgang on January 21, 2019, 06:12:55 pmYupThere ya go