Re: Measuring high impedance filters
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Hi Jim and Nick,
It has been my experience that an L network works a lot better than a transformer. However, I have never tried to build one for 50:2000 ohms. The transformers introduce more loss than an L network. I typically use ZMAT a software program which comes with the book, “Experimental Methods in RF Design” to design the network. I then wind the toroid and use a combination of a fixed and a variable cap. I temporarily mount the L network on a piece of copper clad and use a resistor as a temporary load. I then connect my RF generator with my Return Loss Bridge to the 50 ohm side. I set the frequency on the RF Generator correctly and adjust the variable capacitor for the best return loss (typically -25-30 db). Then, without changing the variable cap, I mount the L network in place. I learned this method from my good Aussie friend Kerry.
I am not sure if a 50:2000 ohm “L” network would work well. The Q might be too high. You might try a pair of L networks in series. Maybe a 50: 350 ohm and then a 350:2000 ohm.
Nick, as far as going down to 455 KHz, the SSNA has not been tested below 1 MHz. You may have to modify the amp or substitute a MMIC for the amp.
"building something without experimenting is just solder practice"
From: PHSNA@... [mailto:PHSNA@...] On Behalf Of n5ib@...
Sent: Saturday, October 05, 2013 3:34 PM
Subject: Re: [PHSNA] Measuring high impedance filters
I'd think you could use a broadband transformer. It ought to do OK over the fairly limited bandwidth of the filters.
For 50:2000 you'd need about a 1:6 turns ratio. And the reactance of the high-turns winding ought to be around 2000 ohms or better. That's around 70 uH at 5 MHz.
Ferrite cores should serve.
Or maybe a combination of techniques... 4:1 quadrafilar transformer to give 50:800 then attenuator pad to handle the remaining transformation. Wouldn't have to introduce so much loss, so maybe the level would be high enough for decent dynamic reange.