Author Topic: Homebrew spectrum analyser high-impedance probe  (Read 36833 times)

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Offline jaxbird

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #25 on: December 26, 2014, 06:19:46 pm »
There's a high speed probe on page 96 of this Jim Williams LT app note- http://cds.linear.com/docs/en/application-note/an47fa.pdf

The honorable Jim Williams was such a casual, maybe have a look at the work of Bob:

http://www.ti.com/ww/en/bobpease/assets/www-national-com_rap.pdf

Analog Discovery Projects: http://www.thestuffmade.com
Youtube random project videos: https://www.youtube.com/user/TheStuffMade
 

Offline dadler

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #26 on: April 27, 2015, 05:37:26 pm »
Maybe the "Poor Man's 1GHz Active Probe" project by Elektor magazine would help you out...

www.elektor.com/magazines/2004/october/poor-man-s-1-ghz-active-probe.57219.lynkx

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

This link is dead.

I found a similar link-is this an archive of the same article?

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

Offline fpliuzzi

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #27 on: April 27, 2015, 10:26:13 pm »
@dadler

Sorry about the late reply, but yes the article in your link is the same Elektor article that I was referring to in my post above from 2013.
 

Offline signality

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #28 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

:)

 

Offline T3sl4co1l

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #29 on: September 17, 2015, 04:58:32 pm »
What are you probing?

I'm building a tube radio right now.  I've been doing stage-by-stage tests by terminating their outputs into 50 ohms directly.  This has two advantages: the dynamic range is more than enough to get a useful signal out (typical tube currents are around 10mA, giving on the order of 4dBm; my HP 8590A has about -70dBm noise floor at typical settings), and I don't have to worry about the output parameters of the stage (compared to the resistance of kohms+ and reactance of ~300 ohms, typical of the output terminal at this frequency).

If you're testing tuning/matching networks, it can be very useful to build them to match to 50 ohms.  For example, you might have an impedance of ~1kohm from a transistor collector/drain, which obviously cannot be observed with a 10x resistor probe, nor is it practical to use a scope probe (with ~10pF of capacitance that will completely mess up your frequency response) -- even if you had the 1M to 50 ohm amplifier box to use such a probe on the spec.

If you're matching one stage into another, with absolutely no need to touch 50 ohms inbetween (it would be a waste of materials and time, and you'd have to re-tune it anyway to optimize out the 50 ohm link), it might be better to wire up the following stage as normal, except leaving its output shorted into 50 ohms.  What better probe than the device itself?  And you get gain for free, besides!

In general, transistors are quite happy being shorted into a low impedance.  The output characteristic is largely capacitive, so you only need to check that the baseband bandwidth is sufficient (Fo = 1 / (2*pi*(50 ohms)*Cout) for the frequency you're monitoring.

That said, because transistors operate with voltages much lower, and have much higher gain, than for my case with vacuum tubes, you won't be able to assume so easily that the output is acting like a true short.  Miller effect and all that will still be relevant.  But with it being so easy to build grounded-base/gate or cascode stages, you can use those to your advantage, for this reason and others.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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Online Marco

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #30 on: September 17, 2015, 05:26:53 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

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.
 

Offline T3sl4co1l

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #31 on: September 17, 2015, 07:07:09 pm »
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
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline xzswq21

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Re: Homebrew spectrum analyser high
« Reply #32 on: May 05, 2017, 03:50:28 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.
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

:)


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.

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
« Last Edit: May 05, 2017, 05:02:17 pm by xzswq21 »
❤ ❤
 

Offline harrimansat

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Re: Homebrew spectrum analyser high-impedance probe
« Reply #33 on: December 15, 2019, 08:20:35 pm »
My DIY BF998 fet probe, I will do more tests with tracking and pulse generator.

 


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