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Step 1: 50 Ohm Termination	Step 2: Add Central Post	Step 3: Add Tip Resistor
Step 4: Add Heat Shrink Tubing	Step 5: Add Frequency Compensation	Step 6: The Final Probe

Construction:

The probe shown above is a home built DC to 1GHz probe that works as well commercial probes costing hundreds of dollars. This probe and its properties are discussed at length in my seminars and in talks at various IEEE EMC Society presentations. To build one, follow these steps:

• Step 1: Build a 50 Ohm termination using four 1206 surface mount 200 Ohm 1% resistors soldered into a BNC barrel adapter as shown. Glue the resistors to an insulating washer first to make mounting easier. To view a method using an insulating washer to make construction of the 50 Ohm termination easier see the Technical Tidbit for March 2000, Improved Construction Technique for a 50 Ohm Termination.
  
• Step 2: Add a central post of about 3.5 cm. Build out its diameter with copper EMI tape until the tape just touches the resistors when the post is inserted into the BNC. Solder the resistors to the EMI tape. Use 16 gauge bus bar wire for the post.
• Step 3: Add a tip resistor. This resistor determines the probe factor and input impedance. A 976 Ohm 1% 1206 surface mount resistor works well and results in a 40:1 probe with a 1000 Ohm input resistance.
• Step 4: Add heat shrink tubing over the central post to insulate it.
• Step 5: Cover the tubing with copper EMI tape, extending it over the BNC barrel. This forms a distributed capacitor to compensate for the parasitic capacitance of the tip resistor. The copper tape is trimmed until the probe has flat frequency response (falling off a dB or so at 1 GHz is OK and actually desirable). The probe response can be measured on a spectrum analyzer that has a tracking generator. If this is not available, leave the copper tape at its full length to insure the probe does not have increasing gain at 1GHz.
• Step 6: Add a ground lead and overall heat shrink tubing for insulation and you are done! The probe shown above in the Step 6 picture was built by one of my seminar attendees.

Theory of Operation:

The probe is a voltage divider comprised of the 976 Ohm tip resistor and a 25 Ohm load (the 50 Ohm termination in parallel with the 50 Ohm characteristic impedance of the coaxial cable). However, at 1 GHz the parasitic capacitance of the resistors causes the divider to become an RC voltage divider as shown in Figure 1 below. R1C1 is the 976 Ohm tip resistor with its parasitic capacitance while R2C2 is comprised of the 25 Ohm load and the parasitic capacitance of the 200 Ohm resistors.
 

Figure 1. RC Voltage Divider

The transfer function for the RC voltage divider is given by:
 

Note that when R1C1 equals R2C2 all frequency dependence drops out of the transfer function and the divider behaves like a simple resistive divider. Since R1, 976 Ohms, is about 39 times the value of R2, 25 Ohms, then C2 must be about 39 times the value of C1. It is very unlikely that the four 200 Ohm resistors will have that much capacitance so we must add extra capacitance across the 200 Ohm resistors. If the parasitic capacitance of the tip resistor is 0.1 pF, then a total of 3.9 pF of capacitance across the 200 Ohm resistors, including their parasitic capacitance, will be needed for the frequency response to be flat.

The foil serves to add the required capacitance. By trimming its length, just the right amount of capacitance can be added to give the probe a flat frequency response. Without the foil, typical surface mount resistors will give the probe about 6 dB of unwanted gain at 1 GHz.

At 1 GHz, the input impedance of the probe is dropping due to the parasitic capacitance of the tip resistor. An unwanted gain of 6 dB at 1 GHz implies that the parasitic capacitance of the tip resistor is becoming comparable to its resistance of 976 Ohms. Lowered input impedance at high frequencies becomes the main limiting factor for the useful frequency range of this probe. To build a probe useable to higher frequencies, resistors with lower parasitic capacitance must be used. Resistors used in microwave circuits are typically adjusted by grinding down the thickness of the film rather than making a cut in the film and so have lower parasitic capacitance. These resistors should allow higher frequency response for this probe. Because of the reduced tip resistor capacitance, less foil will be needed.
 
An in-depth audio format discussion of this article covering background as well as more technical details is available at: http://emcesd-p.com

If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems.

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