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Poor Man's 1GHz Differential Probe

This is project 040108 published in Elektor magazine in October 2004. I just added a few goodies.

donnie-agemaDonnie Agema
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Elektor project 040108 is a low cost active differential probe for signals up to 1 GHz based on FET BF998. I simply added an SMA connector, pogo pins instead of steel needles, LED power on indicator, a 9V battery connector cannabalized from an old 9V battery, and a power switch.

Details can be found in the links provided.

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 replacin the originally specified BF998 with a CF739, which had better specs in the datasheet as 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.

20191028_153843[1].jpg

Finished CF739 version.

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

Preview

20191028_141005[1].jpg

Test setup for CF739 version.

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

Preview

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

Preview

20191021_192715[1].jpg

Here is my test setup.

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

Preview

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

Preview

  • 1 × R2 = 4kOhm 7 0805
  • 1 × IC1 = 78L05 in SO-8 case
  • 1 × SMA female jack
  • 1 × 9-Volt battery connector Can be cannabalized from an old 9-Volt battery
  • 1 × Push-button ON/OFF switch

View all 18 components

  • New version using CF739 instead of BF998

    Donnie Agema • 10/28/2019 at 14:50 • 0 comments

    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 replacin the originally specified BF998 with a CF739, which had better specs in the datasheet as 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.

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mh-nexus wrote 10/12/2023 at 13:38 • point

Since you updated the Elektor design, can you publish your updated PCB layout?


What probes could you connect to the SMA connector instead of the pogopins? Any examples?

  Are you sure? yes | no

Donnie Agema wrote 10/12/2023 at 18:26 • point

I did not make any changes to the original PCB layout. I only changed the FET and added an SMD connector for the scope leads, switch and battery connector.  i did not have any success making any custom probes to replace the pogopins.  The problem there is the inherent increase in capacitance which reduces the input bandwidth.  I might try to redesign the input layout of the PCB to accommodate for that, but that is not on my priority list right now.

  Are you sure? yes | no

Donnie Agema wrote 02/04/2020 at 23:28 • point

If I publish the schematics, I would be infringing on Elektor's copyrights, i.e. I would be doing the stealing.

No, I have not measured the output impedance, but I will give that some thought.

  Are you sure? yes | no

MS-BOSS wrote 02/05/2020 at 10:34 • point

Sorry for that, I thought you were the author of the Elektor article.

  Are you sure? yes | no

MS-BOSS wrote 02/04/2020 at 16:45 • point

Hi, you might want to publish the schematics for this project. Otherwise, people have to either buy the Elektor article or steal it.

Have you measured the output impedance of the probe? When looking at it, I would expect it to have about 20-40 Ohm output impedance and dependent on what is the input connected to.

  Are you sure? yes | no

Donnie Agema wrote 02/05/2020 at 16:39 • point

Not sure if this is the best method to measure the output impedance, but I measured the p-p voltage at 300MHz on the 50 Ohm input  of my oscilloscope and again on the 75 Ohm input. Then, using the formula (R1-(R1*(V1/V2)))/((V1/V2)-(R1/R2)) I get an ouput impedance of 42.1875 Ohms.

  Are you sure? yes | no

MS-BOSS wrote 02/05/2020 at 16:52 • point

It's better to measure RMS voltage, because p-p measurements are sensitive to noise. Otherwise it is an OK method for single frequency measurement.

  Are you sure? yes | no

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