Impedance Adapter

I built the Impedance Adapter in early 2015. I really love using a Spectrum Analyzer to look at signals in the RF path of a circuit, there is so much more information about the signal and what's going on in the circuit with a spectrum analyzer. The problem is Spectrum Analyzers all have a 50 ohm input impedance which would severely load the circuit when sticking a probe in there. I needed something that had a high impedance input so as not to load the circuit being tested and provide a 50 ohm impedance output for connection to the spectrum analyzer. I decided to use a tube as tubes have very high input impedance and if I used a cathode follower circuit I could get the low output impedance of 50 ohms I desired. I used Grayson Evans KJ7UM book Hollow-State Design as a guide to designing the tube circuit. This is an excellent reference for designing tube circuits, I highly recommend the book.


Here's the final design scat
I started with a 6C4 triode. Evan's in his book used this tube as a cathode follower design example and in the end he determined that the 6C4 is not a good tube to use for a cathode follower and sure enough it did not work well. But it was a good exercise to use the formulas and prototype something before finalizing a design. I went on the hunt for a better tube to use as a cathode follower. I finally settled on the 955 acorn tube triode. I chose this tube because it was a high frequency tube in the day and it could possibly give me a greater useful frequency range I could use the adapter with. I had the specs and curves for the tube and it worked out really well using the formulas so I prototyped it and finalized the design.

I built the adapter in a 5x7x2 Aluminum project box.  I enclosed the RF circuitry a shield made of copper clad printed circuit board. Plate voltage and filaments are fed into the enclosed circuit through feed thru caps. I used dry transfer decals to label the front panel and then put a coat of clear acrylic on it to protect the dry transfer decals. It really does not get that warm so I didn't make any ventilation holes. I left it on for a week and the heat was just fine. Looks really cool with the acorn tube.

Performance was pretty good. I plotted frequency against output level from 100KHz to 1GHz. I determined that usable frequency was from 100KHz to 450MHz.  I do attribute the usability to 450MHz to the acorn tube I used. It does have an average insertion loss of -17db. Here's the full Excel data sheet if you want the full raw data.

Here's front and rear view of the finished project Here it taking it's place on the bench. Sitting next to it is an HP8447A Dual 20db Amp which I can use to bring the signal back up to compensate for the 17db thru loss. I primarily use it with the HP E4411B Spec An below it.

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