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CONSTRUCTION OF A PROBE

FREQUENCY COMPENSATED

· WHAT IS A PROBE?

A probe or just a tip, is a device that allows a physical connection between a signal source or test point and a measuring instrument mail, as example oscilloscope.

A variety of probes, from simple devices and robust as the tips direct BNC-alligator, to more sophisticated, expensive and fragile as the tips offset frequency.

· WHAT IS AN IDEAL PROBE?

An ideal probe has the following characteristics:

• Easy connection to the test point
• absolute fidelity of the signal measured

or zero load presented to the signal source "load referred to a resistor, put it in a virtual form, is not physical "

• complete immunity to noise ·
  

WHAT IS A TEST PROBE FOR OSCILLOSCOPE ?

Due to the high frequencies that come into play, oscilloscopes not commonly use simple cables to connect the test point, but are used probes oscilloscope. These probes use a coaxial cable to transmit the signal from the tip of the tip to the oscilloscope input, while maintaining those high frequencies important for the correct operation of this instrument.

The oscilloscope probes are classified into two groups: passive and active, which are explained below.

PASSIVE POINTS

When there is a need for compensation and mitigation. Necessary part of cables and connectors, the use of resistors and capacitors. For that contain no active electronic components such as transistors or amplifiers, and therefore do not require you to provide power.

The most common design is inserted resistor that the values \u200b\u200bof Mega Ohms in series with the tip of the tip. The resistor serves to minimize the load capacitance of the cable would introduce into the test point. This in series with the impedance the oscilloscope, the resistor creates a voltage divider x10, so these points are known as capacitance or low points X10 tips

Hot Spots

This kind of tip contain or rely on active electronic components such as transistors, for its operation. In most cases, the active element is a field-effect transistor (FET) in the form of a small amplifier, built from a FET mounted directly within the tip of the probe. This will get low capacitances between the order of pico farads to less than a farad, without compromising the value of high resistance in direct current.

· WHAT IS A PROBE OFFSET FREQUENCY?

Since the entry into an oscilloscope has parasitic capacitances in parallel with a resistor of about 1 Mega Ohm (approximately), such capacitor must also be in parallel with a capacitor, capacitor called bridge, so to avoid the formation of a low pass RC filter with the parasitic capacitance of the oscilloscope as shown in Figure 2. The value of this capacitor bridge (C _TRIM in Figure 2) should be chosen so that, combined with the oscilloscope input capacitor, forming a voltage divider also x10. So the tip provides a uniform attenuation x10 from direct current through the attenuation resistors to provide enough high frequency alternating current, the attenuation provided by the capacitors.

most modern designs on the end points have a thick film electronic circuit that combines laser-trimmed resistor of the order of Mega ohms with bypass capacitor value fixed. Additionally, add a small capacitor called adjustable compensation capacitor in parallel with the input capacitance of the oscilloscope.

In both cases, the tip should be adjusted so as to ensure uniform attenuation across frequency. This process is known as compensation frequency probe , this process is usually carried out by measuring a square wave and adjusting the compensation capacitor until the oscilloscope displays a signal as square as possible. If you notice that the edges are sharp or rounded, it is said that the tip is over-compensated or undercompensation , respectively.

· than market value "ExSite FREQUENCY OFFSET IN BITS?

The table below contains some of the commercial values \u200b\u200bthat you can find on the market.

band width	Attenuation	Maximum input voltage
60MHz	x1 and x10	600VDC
100MHz	x1 and x10	600VDC
60MHz / 150MHz	x1 and x10	600VDC
60MHz / 250MHz	x1 and x10	600VDC

AS BUILD A PROBE OFFSET FREQUENCY

ASSEMBLY BNC-ALLIGATOR LEADS

To assemble the tips BNC-Cayman, you will need approximately five feet of RG-58 coaxial cable A / U, with central cable (not wire, remember that the wire is only one pole). We also require a BNC connector male RG-58 coaxial cable nut (Figure 1.1). Consider purchasing 60 cm red wire and 60 cm preferably black cable terminated with alligator, otherwise buy the 4 alligators, 2 of each color for each cable, as shown in Figure 1.

Preset mentioned in the diagram in Figure 2, is a pr eset horizontal "De preference."

Figure 1

Remember that this part is optional, if you want to gain experience in the building of direct tips BNC-Cayman because you can buy at any electronics store. and are inexpensive. If you make the decision to buy the tip, you must purchase 30 centimeters still red and black wire, and alligator for each color. And that will be used to connect the circuits.

Figure 1.1

DESCRIPTION OF CABLE RG58 A / U

Cable RG58 A / U, also known as the core of twisted wire or coaxial cable. It is widely used because it ensures easy handling, plus it is cheap compared to other cables used for connections. This cable consists of a center that is known as the core, this is where it derives its name.

This core is composed of several tightly twisted copper wires, the core is surrounded by an insulating material, which is surrounded a cylindrical conductor, all this is to prevent any short circuit. If that were the case would damage the transmission allowing the entry of noise. (Figure 1.2).

Figure 1.2

ATTENUATION CIRCUIT ASSEMBLY OF COMPENSATION IN FREQUENCY

The electronic material to build your probe is as follows:

• de Resistencia a 3.3 mW (Mega Ohms), Ra de la figure 2.
• de Resistencia a 4.7 mW (Mega Ohms), Rb de la figure 2.
• 1 Resistencia de 4.7 Ω (Ohms), Rd de la figure 2.
• 1 Resistencia de 1 mW (Mega Ohms), Ri de la figure 2.
• 1 Resistencia de 39 Ω (Ohms), Rc de la figure 2.
• 2.2-Horizontal 1 Preset nominal (Mega Ohms), corresponding to Figure 2.
• 1 15 pF capacitor, C1 in Figure 2.
• 1 of 36 pF Capacitor, Ca in Figure 2.
• 1 Capacitor 25 pF, Ci in Figure 2.
• 1 6PF variable capacitor to 26 pF for C _TRIM in Figure 2.
1

two-position switch

Figure 2

Circuit Dimming Frequency Compensator

According to Figure 2, build only the part of paragraph "a" or it is the part that is inside the red rectangle of the circuit, because that section only provides the frequency compensation circuit. Tips will have to build or to buy as BNC-direct Cayman, these are represented as the capacitor das subsection "b".

Subsection "c" represents the input of the oscilloscope, as discussed above, this is considered to compensating circuit calculations.

To calibrate the probe offset in frequency, will have to set a square signal of 12 Vpp at a frequency of 1 kHz in the signal generator (Figura2.1) of laboratory or your computer. Observe on the oscilloscope for a signal channel directly, the point X1 And put the oscilloscope channel to 2 Volts / Division and the other channel to observe the attenuated signal is the X10, it is recommended to put this channel in 200mVolts / Division.

Calibration is performed by means of preset and _TRIM C until the signal of both channels are equal (ie is both a perfect square wave) to the extent that it put on, for that you will have to decrease or for decades, to display the two signals, as shown in the following figures.

GENERATOR

Figure 2.1

Below are the simulated signals, using the software TINA Demo, which shows the channel X1 (Figure 2.2), X10 (Figure 2.3), and an extreme case (Figure 2.4) in which it was observed that the tips are not set correctly.

Graphic output properly attenuated

Figure 2.2

X1 Attenuation

Figure 2.3 Attenuation X10

In Figure 2.3, the graphical output properly attenuated (noting that the signal only goes up to 0.6V because the tip attenuates the signal 10 times to recover the original signal must have a scope that has the option of dimming X10 )

extreme or bad Attenuated Tips

Figure 2.4

Tips ill-tempered. One can see that not maintained throughout the upper or lower which means that components are lost high or low frequency signal.

once already armed the circuit of Figure 2, paragraph "a", connect the BNC-Cayman tip parallel to the variable capacitor C _TRIM the alligator red top and black alligator to the bottom of Figure 2. Also connect the oscilloscope to the BNC connector. So get your circuits.

BIBLIOGRAPHY

· http://es.wikipedia.org/wiki/Punta_de_prueba

· http:// www.diotronic.com

· Know Electronic

Article Authors:

ILLESCAS NIETO Lysanias OSVELIA

EMMANUEL MARTINEZ SANCHEZ

ROJAS CARLOS EDUARDO PEREZ