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Homebrew spectrum analyser high-impedance probe

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Marco:

--- Quote from: T3sl4co1l on September 17, 2015, 04:58:32 pm ---For example, you might have an impedance of ~1kohm from a transistor collector/drain, which obviously cannot be observed with a 10x resistor probe

--- End quote ---

If you are willing to decouple your circuit from subsequent stages for the coax probe then why not do the same for a resistor probe? In this case a 20x resistor probe.

T3sl4co1l:
That's an option too.  Actually, I even did that the other day: testing the local oscillator output, I used a 51 ohm series resistor into the 50 ohm load, to represent the 100 ohm mixer load in my circuit.  Just a matter of adjusting 6dB mentally when measuring oscillator power.

For interstage measurements, you have two problems: 1. the saturation level may be low to begin with, so you don't have much room to work with before driving it into clipping, or not being much above the spec's noise floor.  (Of course, if its noise floor is that bad, a LNA can be used to help.)  2. The following stage probably has reactance too, so you can't simply disconnect it anyway.

This stage-wise process obviously works better in design and construction than in troubleshooting...

Tim

xzswq21:

--- Quote from: philpem on December 25, 2014, 11:53:06 pm ---Should probably update this...

I eventually did build one of the probes -- the Elektor original. The EAGLE CAD files are here: https://bitbucket.org/philpem/rf_probe

Enjoy.

--- End quote ---

--- Quote from: signality on September 17, 2015, 02:32:43 pm ---If anyone is still looking for a design for a DIY high impedance probe, there's a version of Bob Pease's JFET active probe here:

https://easyeda.com/andyfierman/Bob_Peases_High_Impedance_Active_Scope_Probe_PCB_Design-g2QNnljhf

with simulations:

https://easyeda.com/andyfierman/Bob_Peases_High_Impedance_Active_Scope_Probe_simulations-gsmEyQ8qj

:)



--- End quote ---

--- Quote from: fpliuzzi on April 03, 2013, 12:26:25 pm ---Maybe the "Poor Man's 1GHz Active Probe" project by Elektor magazine would help you out...

http://elektrotanya.com/files/forum/2009/10/e04a036.pdf

This high impedance, active probe is on my to do list (need it for my RF Explorer-3G analyzer).
I have the components already, but I need to make the PCB for it.

Specs listed in article for the DIY probe:
Input-Z: 0.75pf//10Mohm
Output-Z: 50 ohms
Bandwidth: 100KHz - 1.5GHz (+-2.5dB)
Gain: -20dB nominal
Noise figure: 1dB (using SOT143 BF998 MOSFET)

Regards,
Frank

Edit: Updated the broken link.

--- End quote ---

Dear
I have a small signal within 50mVpp and 10KHz to 20MHz.
I want to measure the signal with a high-impedance probe and with a gain of 10V/V (20dB). the amplifier should has an high linearity.
you uploaded some useful circuits inthe below link:
https://bitbucket.org/philpem/rf_probe
what about linearity and second and third haronic distortions?
Please help me to build a good amplifier.

thanks

harrimansat:
My DIY BF998 fet probe, I will do more tests with tracking and pulse generator.

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