PC Scope Probe

Circuit : Rodney Byne, UK
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This simple pc scope probe is nothing more than a FET follower. It has a high input impedance and a low output impedance to match a mic or line input socket on a pc or laptop. Personal computer oscilloscope probe circuit
FET Pinouts

WARNING: Please be careful when applying any signal into a sound card. See also this sites General Disclaimer.

Circuit Notes
Its useful feature is that the fet input is effectively attenuated by a factor of 30 at the output, when the mono plug is inserted in the mic socket.

This means it is then practical to examine waveforms under test, for example logic square waves of 5v peak-to-peak, without loading the circuit source point.

The resulting terminated output will thus be within a safe mV range as can be accepted by the sensitive sound card input. Installed oscilloscope/spectrum analyser software will display essential elements of the wanted waveform.

Proviso: Sine waves and square waves of 50:50 duty cycle to 20khz are no problem to check. But examination of fast rate clock and transient pulses is not possible due to the sound card's poor risetime response and not by limitations of the probe itself. The probe will faithfully transfer exactly what it receives.

Component notes and final build suggestion
Both 47mfd capacitors pass down to low frequencies. The first 47mfd cap blocks dc from the fet source pin and the second 47mfd cap blocks the phantom +3v dc outputted from the pc mic socket.

If a simple open-plan construction will suffice, the nine components will fit on a piece of veroboard cut to 43mm by 16mm. Close-crop the component legs after soldering. Maplin board pins of code FL24, can be used for all external wiring connections.

The finished board will then fit comfortably within the triangle of three mounting holes on the back of a plastic PP3 battery carrier of Maplin code CK65V.

To secure the board trackside to the plastic, cut to size double-sided sticky tape, apply then press the two surfaces carefully and firmly together. Solder the red and black battery wires correctly to the board.

Red/Black spring loaded probe clip leads of Maplin code N21CB or miniature insulated crocodile clips, are suggested for circuit probing.

Using the PC Scope Probe
One of the best freeware Oscilloscope programs available to download for lab and hobby use on a pc is TrueRTA, as shown in the screenshot.
(This program also has by default, a 1 Octave spectrum analyser which is unsuitable for serious work, as lacks definition with its wide block display)

Download TrueRTA here: http://www.trueaudio.com/ which runs in Windows and Wine in Linux.

Calibrating the TrueRTA millivoltmeter for first time use
Ensure the pc software mic boost is switched off. Set the pc mic fader up 1/3rd from the bottom.

The small audio test set as described in CXI Test Gear Item 21, is suitable for the following experiment and for training purposes.

Set an external calibrated AF signal generator to 80mV output. An accurately calibrated rms level meter must also be parallel-connected with the sig gen output. Then plug this pair into the pc mic socket. The loaded level will be seen to drop slightly, which will require minor adjustment upwards on the sig gen output.

A sine wave of uncalibrated size will be displayed on the scope screen.

On the top task bar under Metering dropdown, select "Input level in mV"

Also on the top task bar is Audio I/O. The dropdown box takes you to Line Input Calibration. Click to open. Ignore steps 1 to 4.

Only act on step 5 as instructed, "Enter the actual input voltage as read from your AC voltmeter" So enter in the little white box: 0.08
Click ok.

TrueRTA will compute to compare with 80mV going in and display 80mV in the top left corner of the scope screen. The previously uncalibrated sine wave will have become correctly sized.

Provided the mic fader is left undisturbed, this input calibration remains as set. But it is prudent to measure as above again after exiting and re-launching TrueRTA. When exiting the program, usually select No when asked "Save changes to untitled project".

Following calibration, the screenshot shows the scope probe in action with my laptop running TrueRTA. Using clip-on hooks, it is examining the square wave on a transistor collector of a multivibrator oscillator.

As shown, a representative level of 52.8mV is displayed on the scope. This is because TrueRTA calculates the square waveform's rms equivalent to a sine wave as best it can.

However what is important is that the actual level being measured at the test point is 1584mV (ie. 1.584V), or thirty times the figure displayed.

This larger figure is too large to input directly to the pc mic socket for safety, so must be reduced significantly and is thus achieved by the attenuation factor 30 of the probe circuit.

As said at the top of the page, it is safe for the pc and probe to analyse any waveform up to 5V peak-to-peak.

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