Products > Test Equipment
DIY active probe - another approach (BUF802)
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tggzzz:

--- Quote from: nctnico on November 24, 2022, 11:37:06 am ---
--- Quote from: Berni on November 24, 2022, 06:08:40 am ---
--- Quote from: colorado.rob on November 23, 2022, 07:35:31 pm ---
--- Quote from: luudee on November 23, 2022, 12:35:39 pm ---Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?

--- End quote ---
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

--- End quote ---

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.

--- End quote ---
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.

--- End quote ---

One trick would be to have the Vcc and Gnd planes on layers 3 and 4, and the signals on layers 1 and 2.

The minimal distance between Vcc and Gnd would improve the distributed decoupling.

The maximal distance between signals and layer 3 would minimise the consequences of prepreg thickness variation. It would be no worse than a 2-layer board, but still suffer from FR4 weave variations.
IAmBack:

--- Quote from: tggzzz on November 24, 2022, 01:48:06 pm ---
--- Quote from: nctnico on November 24, 2022, 11:37:06 am ---
--- Quote from: Berni on November 24, 2022, 06:08:40 am ---
--- Quote from: colorado.rob on November 23, 2022, 07:35:31 pm ---
--- Quote from: luudee on November 23, 2022, 12:35:39 pm ---Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?

--- End quote ---
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

--- End quote ---

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.

--- End quote ---
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.

--- End quote ---

One trick would be to have the Vcc and Gnd planes on layers 3 and 4, and the signals on layers 1 and 2.

The minimal distance between Vcc and Gnd would improve the distributed decoupling.

The maximal distance between signals and layer 3 would minimise the consequences of prepreg thickness variation. It would be no worse than a 2-layer board, but still suffer from FR4 weave variations.

--- End quote ---
Instead of 4 layer expensive board probably better solution is 2-side assembly.
noisyee:

--- Quote from: IAmBack on November 24, 2022, 02:36:28 pm ---
--- Quote from: tggzzz on November 24, 2022, 01:48:06 pm ---
--- Quote from: nctnico on November 24, 2022, 11:37:06 am ---
--- Quote from: Berni on November 24, 2022, 06:08:40 am ---
--- Quote from: colorado.rob on November 23, 2022, 07:35:31 pm ---
--- Quote from: luudee on November 23, 2022, 12:35:39 pm ---Is this a regular FR4 ?
May be something like a Megtron-6 would be a better choice?

--- End quote ---
Wouldn't this turn a $3 PCB into a $300 PCB (minimum)?

--- End quote ---

Yep FR4 is perfectly fine for it and it is dirt cheep from china. You can do most high speed things on it, especially on 4 layer boards that let you keep a ground plane nice and close.

--- End quote ---
Having a 4 layer board is not always a blessing for RF stuff. First the stackup needs to be controlled as in the PCB needs to be build according to the dielectric thicknesses you specify. Otherwise the impedances may be off. Secondly, with a thinner dielectric, the traces get thinner so variation in width of the trace gives you a larger relative error. For my 1.4GHz differential probe design I have used a 2 layer board; it is good enough and keeps things simple.

--- End quote ---

One trick would be to have the Vcc and Gnd planes on layers 3 and 4, and the signals on layers 1 and 2.

The minimal distance between Vcc and Gnd would improve the distributed decoupling.

The maximal distance between signals and layer 3 would minimise the consequences of prepreg thickness variation. It would be no worse than a 2-layer board, but still suffer from FR4 weave variations.

--- End quote ---
Instead of 4 layer expensive board probably better solution is 2-side assembly.

--- End quote ---

I think 2 layer is enough for BUF802, it's an open loop buffer tend to be quite stable. It's not that sensitive to power decoupling or parasitic parameter on signal trace.
FR-4 is good enough for 2 GHz with short trace. With passive high impedance attenuator, you will have plenty of headroom to compensate for bandwidth or response. Just remember to cut out copper plane under the attenuator and BUF802 input pin to minimize capacitance, or you may have trouble compensating low frequency response.
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