Remote probe for an oscilloscope diagram. Active probes with low input capacitance. Probes with frequency response correction

Regardless of the class of devices, to analyze certain signals, it is necessary to bring the signals under study to the inputs of the devices. It is very rarely possible to bring their sources very close to the inputs of oscilloscopes and analyzers. They are often located at a distance of a fraction of a meter to several meters. This means that special matching devices, connected between signal sources and inputs of an oscilloscope and analyzers.
Typically, probes are used for the following important purposes:

  • remote connection oscilloscope to the object of study;
  • reducing the sensitivity of vertical (sometimes horizontal) deflection channels and studying high-level signals (passive probes);
  • decoupling of measuring circuits from oscilloscope units (optical probes);
  • high signal attenuation and signal research in high-voltage circuits (high-voltage probes);
  • increasing the input resistance and decreasing the input capacitance (compensated dividers and repeater probes);
  • correction of the amplitude-frequency response of the probe-oscilloscope system;
  • obtaining current oscillograms (current probes);
  • selection of antiphase signals and suppression of common mode signals (differential probes);
  • increasing the sensitivity of oscilloscopes (active probes);
  • special purposes (for example, matching the outputs of wideband signal sources with the 50-Ohm input of an oscilloscope).

It is quite obvious that the role of probes is very important and sometimes is in no way inferior to the importance of the oscilloscopes and analyzers themselves. But, often, the role of probes is underestimated and this is a serious mistake for novice users of these devices. Below are the main types of probes and other accessories for oscilloscopes, spectrum analyzers, signal analyzers, and logic analyzers.

Probes based on compensated divider

The simplest and long-used type of probes are passive probes with a compensated voltage divider - Fig. 5.1. The voltage divider is built on resistors R1 and R2, and R2 can simply be the input resistance of the oscilloscope.

Rice. 5.1. Compensated divider circuit

The DC divider parameters are calculated using the formulas:

For example, if R2 = 1 MOhm and R1 = 9 MOhm, then it has RВХ = 10 MOhm and KD = 1/10. Thus, the input resistance is increased by 10 times, but the voltage level supplied to the input of the oscilloscope also drops by 10 times.

In the general case (on alternating current) for the transfer coefficient of the divider, you can write the expression (τ1= R1C1 and τ2= C2R2):


. (5.3)

Thus, if the time constants τ1 and τ2 are equal, the transfer coefficient of the divider ceases to depend on frequency and is equal to its value at direct current. Such a divisor is called compensated. Capacitance C2 is the total capacitance of the cable, mounting, and input capacitance of the oscilloscope. In practice, to achieve the compensation condition, capacitance C1 (or C2) must be adjusted, for example, using a variable capacitor trimmer - trimmer (see Fig. 5.2.). Adjustment is performed with a special plastic screwdriver included in the probe accessory kit. It includes various tips, adapters, colored stickers and other useful little things.

Rice. 5.2. HP-9250 Standard Passive Probe Design Based on a Frequency Compensated Divider

When compensated, there is no distortion of the rectangular pulse (meander), usually created by the calibrator built into the oscilloscope (see Fig. 5.3). When the peak of the pulse decreases, undercompensation is observed, and when it rises, overcompensation is observed. The nature of the oscillograms is also shown in Fig. 3 (taken with a TDS 2024 oscilloscope with a P2200 probe). It is recommended to carry out compensation at maximum large image oscillograms of the corresponding channel.

Rice. 5.3. Oscillograms of Tektronix TDS 2024 oscilloscope calibrator pulses at different degrees of compensation (top to bottom): normal compensation, overcompensation and undercompensation

When working with a multichannel oscilloscope, you should use probes individually for each channel. To do this, they need to be marked (if this has not already been done at the factory) with stickers of different colors, usually corresponding to the colors of the oscillogram lines. If you do not adhere to this rule, then due to the inevitable variation in the input capacitances of each channel, compensation will be inaccurate.

For a 1:10 divider, resistor R1 should be equal to 9R2. This means that the capacitance C1 must be 9 times less than the input capacitance C2. The input capacitance of the divider is determined serial connection C1 and C2:


(5.4)

The approximate value is valid for KD»1 and C1«C2. At KD =10, the input capacitance of the divider is almost 10 times less than the input capacitance of the oscilloscope. It should be remembered that C2 includes not only the true input capacitance of the oscilloscope, but also the capacitance of C1 is increased by the amount of the mounting capacitance. Therefore, in fact, the decrease in the input capacitance of the divider compared to the input capacitance of the oscilloscope will not be so noticeable. Nevertheless, this is precisely what explains the significant reduction in distortion of pulse fronts when working with a divider.

Increasing the active component of the input resistance of the divider is not always useful, since it also leads to a change in the load on the device under test and different results are obtained in the absence of a divider and when using it. Therefore, dividers are often designed so that the input impedance of the oscilloscope remains unchanged both when working without a divider and when working with it. In this case, the divider does not increase the input impedance of the oscilloscope, but still reduces the input capacitance.

Increasing the level of the studied signals

The maximum voltage at the oscilloscope input is determined by the product of the number of divisions of its scale grid by the vertical deviation coefficient. For example, if the number of graticule divisions is 10, and the deviation factor is 5 V/div, then the total voltage swing at the input is 50 V. This is often not enough to study signals of even moderately high levels - above tens of volts.

Most probes allow you to increase the maximum test voltage at direct current and low frequency from tens of V to 500-600 V. However, high frequencies ah reactive power (and active power, released at the loss resistance of the probe capacitors) increases sharply and it is necessary to reduce the maximum voltage at the probe input - Fig. 5.4. If you do not take this circumstance into account, you can simply burn the sample!

Rice. 5.4. Dependence of the maximum voltage at the probe input on frequency

The probe's maximum input voltage should never be exceeded at high signal frequencies. This may cause the probe to overheat and fail.

A type of passive probe is high-voltage probe. They typically have a division ratio of 1/100 or 1/1000 and an input impedance of 10 or 100 MΩ. Low-power probe divider resistors can usually withstand voltages of up to 500-600 V without breakdown. Therefore, in high-voltage probes, resistor R1 (and capacitor C1) must be made using series-connected components. This increases the size of the probe's measuring head.

A view of the Tektronix P6015A high-voltage probe is shown in Fig. 5.5. The probe has a well-insulated body with a protruding ring that prevents fingers from slipping into the circuit whose voltage waveform is being recorded. The probe can be used at voltages up to 20 kV at DC and up to 40 kV at high duty cycle pulses. The frequency range of an oscilloscope with such a probe is limited to 75 MHz, which is more than sufficient for measurements in high-voltage circuits.

Rice. 5.5. Appearance Tektronix P6015A High Voltage Probe

When working with high-voltage probes, the greatest possible precautions must be taken. First connect the ground wire, and only then connect the probe needle to the point at which you want to obtain a voltage waveform. It is recommended to secure the probe and generally remove your hands from it when taking measurements.

High-voltage probes are available for both digital and analog oscilloscopes. For example, the HV-P30 probe is available for the unique ACK7000/8000 series wideband analog oscilloscopes with up to 50 MHz bandwidth, 1/100 split ratio, 30 kV peak-to-peak peak sine wave voltage, and up to 40 kV peak pulse voltage. Probe input impedance 100 MΩ, input capacitance 7 pF, cable length 4 m, BNC output connector. Another probe, the HV-P60, has a 1/2000 division ratio and can be used at maximum voltages up to 60 kV for sine wave and up to 80 kV for pulse signal. The probe's input impedance is 1000 MΩ, and the input capacitance is 5 pF. The seriousness of these products is eloquently indicated by their high price - about 66,000 and 124,000 rubles (according to the Elix company price list).

Probes with frequency response correction

Passive probes are often used to correct the frequency response of oscilloscopes. Sometimes this is a correction designed to expand the frequency band, but more often the inverse problem is solved - narrowing the frequency band to reduce the influence of noise when observing low-level signals and eliminating fast spikes on the edges of pulsed signals.
These probes (P2200) are included with the Tektronix TDS 1000B/2000B series commercial oscilloscopes. Their appearance is shown in Fig. 5.6.

The main parameters of the probes are given in table. 5.1.

Table 5.1. Basic Parameters of P2200 Passive Probes

Rice. 5.6. P2200 Passive Probe with Built-in Low Pass Filter in 1/10 Divide Switch Position

From the table 5.1 clearly shows that the use of a probe with a division ratio of 1/1 is advisable only when studying low-frequency devices, when a frequency band of up to 6.5 MHz is sufficient. In all other cases, it is advisable to work with the probe at a division ratio of 1/10. In this case, the input capacitance is reduced from 110 pF to approximately 15 pF, and the frequency band is expanded from 6.5 MHz to 200 MHz. Oscillograms of a square wave with a frequency of 10 MHz, shown in Fig. 5.7, well illustrate the degree of distortion of oscillograms at division ratios of 1/10 and 1/1. In both cases, a standard probe connection with an interlocking tip and a long ground wire (10 cm) with an alligator clip was used. A square wave with a rise time of 5 ns was obtained from a Tektronix AFG 3101 generator.

Rice. 5.7. Waveforms of 10 MHz square waves using a 200 MHz Tektronix TDS 2024B oscilloscope with P2200 probes at 1/10 (upper waveform) and 1/1 (lower waveform) division ratios.

It is easy to notice that in both cases the oscillograms of the observed signal (and for the AFG 3101 generators at a frequency of 10 MHz it is close to ideal and has smooth peaks without a hint of “ringing”) are greatly distorted. However, the nature of the distortion is different. With a divider position of 1/10, the signal shape is close to a meander and has short-duration fronts, but is distorted by damped oscillations arising due to the inductance of a long grounding wire - Fig. 8. And in the 1/1 divider position, the damped oscillations disappeared, but a significant increase in the time constant of the probe-oscilloscope system was clearly noticeable. As a result, instead of a meander, sawtooth pulses with exponential rise and fall are observed.

Rice. 5.8. Scheme for connecting the probe to the RL load

Probes with built-in correction must be used strictly for their intended purpose, taking into account the strong difference in frequency characteristics at different positions of the voltage divider.

Accounting for Probe Parameters

We present typical data of the circuit in Fig. 5.8: internal resistance of the signal source Ri=50 Ohm, load resistance RL>>Ri, input resistance of the probe RP=10 MOhm, input capacitance of the probe CP=15 pF. Given such elements of the circuit, it degenerates into a sequential oscillatory circuit, containing resistance R≈Ri, inductance of the ground wire L≈LG (about 100-120 nH) and capacitance C≈CP.

If an ideal voltage drop E is applied to the input of such a circuit, then the time dependence of the voltage at C (and the oscilloscope input) will look like:


(5.5)

Calculations show that this dependence can have a significant overshoot at large L and small R, which is observed in the upper oscillogram in Fig. 5.7. At α/δ=1, this surge is no more than 4% of the amplitude of the difference, which is a completely satisfactory indicator. To do this, the value L=LG must be chosen equal to:

For example, if C=15 pF and R=50 Ohm, then L=19 nH. To reduce L to such a value (from the typical order of 100-120 nH for a ground wire 10 cm long), it is necessary to shorten the ground (possibly signal) wire to a length of less than 2 cm. To do this, remove the nozzle from the probe head and abandon the use of a standard ground wires. The beginning of the probe in this case will be represented by a contact needle and a cylindrical ground strip (Fig. 5.9) with low inductance.

Rice. 5.9. Probe head with tip removed (left) and adapter to coaxial connector (right)

The effectiveness of the measures used to combat ringing is illustrated in Fig. 5.10. It shows waveforms of a 10 MHz square wave when the probe is turned on normally and when the probe is turned on with the tip removed and without the long ground wire. One can clearly see the almost complete elimination of obvious damping oscillatory processes on the lower oscillogram. Small fluctuations at the top are associated with wave processes in the connecting coaxial cable, which in such probes operates without matching at the output, which gives rise to signal reflections.

Rice. 5.10. Oscillograms of a 10-MHz square wave when the probe is turned on normally (upper waveform) and turned on with the nozzle removed and without a long ground wire (lower waveform)

To obtain oscillograms with extremely short rise times and ringing, measures should be taken to minimize the inductance of the measured circuit: removing the probe tip and connecting the probe using a needle and a cylindrical ground insert. All possible measures should be taken to reduce the inductance of the circuit in which the signal is observed.
Important parameters of the probe-oscilloscope system are the system rise time (at the 0.1 and 0.9 levels) and the bandwidth or maximum frequency (at the 3 dB sensitivity roll-off level). If we use the known value of the resonant frequency of the circuit

, (5.7)
then we can express the value of R through the resonant frequency of the circuit, which determines the limiting frequency of the deflection system path:

. (5.8)
It is easy to prove that the time the voltage u(t) reaches the value E of the drop amplitude will be equal to:


. (5.10)

This value is usually taken as the probe's settling time with optimal transient response. The total rise time of an oscilloscope with a probe can be estimated as:

, (5.11)
where tosc is the rise time of the oscilloscope (when a signal is applied directly to the input of the corresponding channel). The upper limit frequency fmax (which is also the frequency band) is defined as

. (5.12).
For example, an oscilloscope with t0=1 ns has fmax=350 MHz. Sometimes the multiplier of 0.35 is increased to 0.4-0.45, since the frequency response of many modern oscilloscopes with fmax>1 GHz differs from Gaussian, which is characterized by a multiplier of 0.35.

Do not forget about another important parameter of probes - signal delay time tз. This time is determined, first of all, by the linear delay time (per 1 m of cable length) and the length of the cable. It usually ranges from units to tens of ns. To prevent the delay from affecting the relative position of oscillograms on the screen of a multichannel oscilloscope, you must use probes of the same type with cables of the same length in all channels.

Connecting Probes to Signal Sources

Connecting probes to the desired points of the devices under study can be done using various tips, nozzles, hooks and “micro-crocodiles”, which are often included in the probe accessory kit. However, most often the most accurate measurements are made when connecting using the primary probe needle - see fig. 5.11 or two needles. When developing high-frequency and pulsed devices on a printed circuit board, special contact pads or metallized holes are provided for this purpose.

Rice. 5.11. Connecting the probe to the pads printed circuit board device under study

It is especially important in our time to connect probes to the contact pads of miniature printed circuit boards, hybrid and monolithic. integrated circuits }