20191028_153843[1].jpg

Finished CF739 version.

JPEG Image - 2.08 MB - 10/28/2019 at 15:12

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20191028_141005[1].jpg

Test setup for CF739 version.

JPEG Image - 1.83 MB - 10/28/2019 at 15:10

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CF739 Version.jpg

After looking back at the original Elektor project, I became not so satisfied with my results, seeing the original specs claiming a bandwidth of 100 kHz to 1.5 GHz +-2.5dB, which I was not getting even close to. Since I still had 2 unused PCBs from OSHpark, I decided to build another one, keeping as close to the original design as I could. But that did not bring any significant improvement. The Bode plot I saw on the spectrum analyzer suggested to me that the problem part was the FET, so I tried replacing the originally specified BF998 with a CF739, which had better specs in it's datasheet than those of the BF998. That proved to be the right move, giving me a bandwidth of 95 kHz to 1.94 GHz at +-2.5 dB. NOW I am satisfied! Oh, yeah, I also used an SMA connector instead of the pogo pins, allowing me to easily configure it with any probe tip I might end up needing.

JPEG Image - 210.87 kB - 10/28/2019 at 15:08

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20191021_192715[1].jpg

Here is my test setup.

JPEG Image - 1.94 MB - 10/21/2019 at 17:59

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PoorMansProbe_BODE.jpg

I was curious whether or not I had introduced problems with my modifications, so I did a test on my new Siglent SAA3032. Frequency response is somewhat better than what I had expected, being practically flat from 100KHz to 300MHz, going about +3dBm at about 500MHz, peaking at 1.2GHz and then dropping to -3dBm at about 1.8GHz. I am quite satisfied!

JPEG Image - 207.52 kB - 10/21/2019 at 17:50

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