Mr Carlson's Lab Spectrum Analyzer's Protection Circuit

[Smoky]

Hey Folks!

I'm looking to align older radios in the future and I want to use my spectrum analyzer during the process. Paul's spectrum analyzer is an HP3585B and it has an optional 1MOhm input. I bought a good/used HP3588A not long ago and it has a switchable 50/1M Ohm input. The difference is, the max voltage on my spectrum analyzer's input is 4 volts Pk:



I want to make a protection circuit just like his but mine will require a different number of diodes and capacitors and I was hoping you can let me know if I'm figuring things right, primarily, the amount of capacitance. The first thing different that I want to do is just to make a dedicated cable, I don't want to attach BNC's to the circuit's project box and all of that. One other thing too, I plan to use a small Pomona 3753 aluminum project box so I may not want it near the end of the probe's cable either. Also, I'd like to have the circuit and the cable shielded from the BNC attached to the spectrum analyzer all the way to the tip of the probe so I'd plan to use RG316.

As for most of the stuff I do, it's for fun and practice, and in the end, I'm hoping to use this spectrum analyzer and a Heathkit VTVM for radio alignments



What parts I have for the circuit so far are 62pf/500v capacitors and 1N914 100v/200mA diodes. I'll have to make the circuit the point-to-point way.

[Smoky]

I wanted to tackle the probe side of the circuit to see if I could get shielding up to the point of attachment to the radio's circuit. I searched around and I found these Kelvin clips:



They turned out to be really cool. The tip's blade can be un-screwed and soldered. The blade is thick and the tip has serrations that look like they'd grip really good to the component under test. The clips cost $2.50 each.

I started by stripping-back the RG316 to expose the center conductor and shielding braid. I'm using an E-Z Micro Hook for the ground lead. I bought a 12" one (204XM-12BLK, ~$5) and cut it in half. Btw, that dinky thing is rated for 1kV!

I slipped the cables through the handle of the Kelvin clip and soldered the ground lead and the center conductor in place:



It really turned out nice. The Kelvin clip is small but its mouth can open to 1" too



Here's where the project stands at the moment ready for some figuring and measuring:

[coppercone2]

I  think you can shore it up with a GDT but I think you would need some ballasting because of trigger time. I read something interesting just now where someone claims that a coaxial cable separating it from the diodes of some length might be enough to do it, where you might want local protection on the antenna followed by this circuit at the input of the SA if its remote enough