CN102735887B - Single-ended active probe circuit of digital oscilloscope - Google Patents

Abstract

The invention discloses a single-ended active probe circuit of a digital oscilloscope, which combines high-speed signal completeness test requirements and combines an existing component based on a broadband testing principle for analyzing a single-ended active probe. Through an impedance conversion module, the requirements of the single-ended active probe in high input impedance, smallest input capacitance and small output impedance are realized; simultaneously, through an output circuit, the single-ended active probe circuit can be matched with the input impedance of 50 ohm of the oscilloscope.

Description

A digital oscilloscope single-ended active probe circuit

technical field

The invention belongs to the technical field of oscilloscopes, and more specifically relates to a digital oscilloscope single-end active probe circuit.

Background technique

A digital oscilloscope probe is an electronic component that connects the circuit under test to the input of a digital oscilloscope, and is widely used in electronic testing. Generally, digital oscilloscope probes are divided into passive probes and active probes.

Passive probes are made with wire and connectors and, when attenuation or compensation is required, resistors and capacitors. Passive probes have no active components, namely transistors or amplifiers, and therefore do not need to power the probe. Usually, passive probes have high input impedance (1MΩ or 1MΩ), but limited bandwidth, generally not exceeding 500MHz (-3dB), and are widely used in low bandwidth testing.

Active probes contain or rely on active devices, such as transistors and FETs, that require an external power supply. Most commonly, the active device is a field-effect transistor (FET), which provides very low input capacitance, and low capacitance ensures high input impedance over a wider frequency band, reducing loss of high-frequency signals.

Active probes are divided into single-ended active probes and differential active probes. The single-ended active probes use the ground as a reference to realize single-point testing of the circuit under test, which can meet most applications; differential active probes can measure floating The signal of the device is essentially composed of two symmetrical voltage probes, which have good insulation and high impedance to the field respectively. The differential signal is referenced to each other, not the signal referenced to the ground, so the differential probe is mainly used for differential signals. Test, can provide high common mode rejection ratio (CMRR) in a wider frequency range.

Domestic digital oscilloscopes started relatively late. In the past few years, they were mainly low-end oscilloscopes with low bandwidth and low sampling rate, and the supporting probes were passive probes. The mid-to-high-end market is monopolized by the three major oscilloscope manufacturers in the United States, and a technical blockade and monopoly has also been formed on the active probes for supporting components. The active probes are very expensive, and the unit price is often several thousand dollars. In recent years, with the continuous progress and accumulation of domestic technology, the bandwidth of domestic digital oscilloscopes has entered the 1GHz era. In order to achieve accurate measurement of higher frequency signals, in addition to improving the input bandwidth of its own analog signal conditioning channel, it is also necessary to provide supporting use. active probe.

Active probe principle:

From the information provided by foreign manufacturers, the basic principle of the single-ended active probe is shown in Figure 1. The front end of the single-ended active probe has a high-bandwidth amplifier. Usually the input impedance of the amplifier is relatively high, so the single-ended active probe It can provide a relatively high input impedance; at the same time, the amplifier output has a strong driving capability, and it can be connected to a 50Ω special impedance transmission line and drive a 50Ω load. Since the 50Ω transmission line can provide very high bandwidth, and the front-end amplifier is a broadband amplifier, the entire single-ended active probe system can provide higher bandwidth than the passive probe. From a theoretical analysis, the key to the high input bandwidth characteristics of a single-ended active probe is to have a front-end amplifier to ensure that this high-bandwidth and high-input impedance amplifier is realized by a foreign oscilloscope manufacturer through a customized integrated circuit, and it must be small in size. It is placed in the limited space of the probe, so the implementation cost is very high, and this customized broadband op amp is not for sale by the manufacturer.

The structural principle of the differential active probe is shown in Figure 2. The difference from the single-ended active probe is that its front end is a differential amplifier. Similarly, this type of broadband differential input operational amplifier in the differential active probe is also customized.

Although there are many related introductions about the principle of oscilloscope active probes, they are all explained from the functional principle, and no specific design circuit is given. However, foreign oscilloscope manufacturers realize it by customizing a dedicated integrated circuit (IC), and Such ICs are not commercially available.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a digital oscilloscope single-ended active probe circuit to reduce the cost of the active probe and improve the cost performance.

In order to realize the foregoing invention object, a kind of digital oscilloscope single-ended active probe circuit of the present invention is characterized in that, comprises:

A high-pass module, which is a high-frequency signal path, is used to isolate the DC and low-frequency parts of the input signal, and the high-frequency part of the input signal passes through it;

A low-pass module is a low-frequency signal path through which the DC and low-frequency parts of the input signal pass; it includes a partial feedback network for adjusting the upper limit frequency of the input signal. In addition, the low-pass module also receives from the DC feedback network. Feedback signal for stabilizing the quiescent operating point of the single-ended active probe circuit;

An impedance transformation module, including a broadband field effect transistor and a voltage follower circuit;

Among them, the broadband field effect transistor has extremely high input impedance, and the output impedance is extremely small, which realizes the impedance transformation of the input signal;

In order to enhance the drive, a voltage follower circuit is connected to the output terminal of the broadband FET, and the voltage follower circuit is designed as an emitter follower with a broadband transistor, or as a voltage follower with a broadband operational amplifier;

A DC feedback network, which feeds back the DC voltage signal output by the voltage follower circuit to the low-pass module to achieve a stable DC operating point;

An output circuit is used for compensating the frequency of the input signal and adjusting the gain, and adjusting the output impedance so as to match the 50Ω input impedance of the post-stage oscilloscope.

The input signal is first input to the impedance transformation module through the high-pass module and the low-pass module for impedance transformation and enhanced driving, then frequency supplementation, gain adjustment and output impedance adjustment are performed in the output circuit, and finally output to the post-stage oscilloscope.

The purpose of the invention of the present invention is achieved like this:

The digital oscilloscope single-end active probe circuit of the present invention is proposed by combining the high-speed signal integrity test requirements, analyzing the wide-band test principle of the single-end active probe, and combining existing components and parts. Through the impedance transformation module, the single-ended active probe has high input impedance, the input capacitance is as small as possible, and the output impedance is small. At the same time, through the output circuit, the single-ended active probe can match the 50Ω input impedance of the oscilloscope.

Description of drawings

Fig. 1 is the structural principle of the oscilloscope single-ended active probe in the prior art;

Fig. 2 is the structural principle of the oscilloscope differential active probe in the prior art

Fig. 3 is a schematic block diagram of a specific embodiment of the digital oscilloscope single-ended active probe circuit of the present invention;

Fig. 4 is the schematic diagram of the digital oscilloscope single-ended active probe circuit shown in Fig. 3;

Fig. 5 is a frequency response curve diagram of a specific embodiment of a digital oscilloscope single-ended active probe circuit.

Detailed ways

Specific embodiments of the present invention will be described below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

Fig. 3 is a functional block diagram of a specific embodiment of the digital oscilloscope single-ended active probe circuit of the present invention.

In this embodiment, as shown in FIG. 3 , the digital oscilloscope single-ended active probe circuit of the present invention includes a high-pass module 1 , a low-pass module 2 , an impedance transformation module 3 , a DC feedback network 4 and an output circuit 5 .

The high-pass module 1 is a high-frequency signal path, including a DC-blocking capacitor for isolating the DC and low-frequency parts of the input signal, through which the high-frequency part of the input signal passes and is sent to the impedance transformation module 3 .

The low-pass module 2 is a low-frequency signal path through which the DC and low-frequency parts of the input signal pass and are sent to the impedance transformation module 3 . The low-pass module 2 includes a partial feedback network, which is used to adjust the upper limit frequency that allows the input signal to pass through. In addition, the low-pass module 2 also receives the feedback signal from the DC feedback network 4, which is used to stabilize the static state of the single-ended active probe circuit. working point.

The impedance transformation module 3 has a broadband field effect transistor and a voltage follower circuit. Among them, the broadband field effect transistor has a very high input impedance, and the output impedance is extremely small, which realizes the impedance transformation of the input signal; in order to enhance the drive, a voltage follower circuit is connected to the output terminal of the broadband field effect transistor, and the voltage follower circuit is designed with a broadband transistor as emitter follower, or designed as a voltage follower with a broadband operational amplifier.

The DC feedback network 4 feeds back the DC voltage signal output by the voltage follower circuit to the low-pass module 3 to realize a stable DC operating point. At the same time, by adjusting the parameters in the DC feedback network, the adjustment of the DC gain of the single-ended active probe circuit can be realized.

The output circuit 5 compensates the frequency of the input signal and adjusts the gain, and adjusts the output impedance so as to match the 50Ω input impedance of the oscilloscope at the subsequent stage.

The input signal is first input to the impedance transformation module 3 through the high-pass module 1 and the low-pass module 2 for impedance transformation and enhanced driving, then frequency supplementation, gain adjustment and output impedance adjustment are performed in the output circuit 5, and finally output to the subsequent oscilloscope.

In the output circuit 5, through its gain adjustment, an attenuation multiple of 20dB (10 times) or an attenuation of 14dB (5 times) can be realized.

Fig. 4 is a schematic diagram of the single-ended active probe circuit of the digital oscilloscope shown in Fig. 3 .

In this embodiment, as shown in Figure 4, the input signal input terminal of the single-ended active probe circuit of the digital oscilloscope of the present invention is also connected with a parallel circuit composed of a small resistor R1 and a small capacitor C1, and the resistance value of the small resistor R1 is between several ~Dozens of ohms, the small capacitor C1 has a capacity of several pF, and the input signal is sent to the high-pass module 1 and low-pass module 2 after passing through the parallel circuit. In the damping test link, due to the distributed capacitance and distributed inductance of lead wires and other factors, the small resistor R1 and small capacitor C1 are used to prevent resonance and play a buffer role.

The high-pass module 1 includes a resistor R2 and a DC blocking capacitor C3. The high frequency part of the input signal is sent to the gate g1 of the broadband field effect transistor Q1 in the impedance transformation module 3 through the resistor R2 and the DC blocking capacitor C3.

The low-pass module 2 includes resistors R3 and R4, low-noise and low-drift JFET operational amplifier U1, resistors R7, R8 and capacitor C4 to form a local feedback network and an output resistor R6.

The sum of resistors R3 and R4 in series is 1MΩ, the input signal is grounded through resistors R3 and R4, and the series connection point of resistors R3 and R4 is connected to the positive terminal of operational amplifier U1, so as to ensure that the input resistance is strictly 1MΩ.

On the one hand, the output terminal of the operational amplifier U1 is connected to the gate g1 of the broadband field effect transistor Q1 through the resistor R6, on the other hand, it is grounded through the resistors R7 and R8, and the connection point of the resistors R7 and R8 is connected to the negative terminal of the operational amplifier U1 through the capacitor C4. Constitute a local feedback network, which is used to adjust the upper limit frequency that the input signal is allowed to pass through, and satisfy:

$\frac{R7}{R8}=\frac{R3}{R4}$ (Formula 1-1)

The impedance transformation module 3 includes an emitter follower composed of a broadband field effect transistor Q1 and its peripheral circuits, and a broadband transistor Q2 and its peripheral circuits as a voltage follower circuit.

The broadband field effect transistor Q1 is a broadband double-gate field effect transistor, and the inputs are gate g1 and g2 respectively, wherein gate g1 is used for inputting the input signal, and gate g2 is used for DC bias voltage input, and the input resistance is very high (GΩ level) and very small input capacitance (several pF).

The peripheral circuit of the broadband FET Q1 includes the resistor R9 connected to the -5V power supply at the source level, the resistors R16, R15, and the capacitor C2. The +5V power supply is connected to the ground through the resistors R16 and R15, and the connection point of the resistors R16 and R15 is connected to the broadband field effect transistor. The gate g2 of the transistor Q1 can make the broadband field effect transistor Q1 obtain the best DC operating point, and the selected voltage division value here is 4V. The capacitor C2 is connected in parallel with the resistor R15 to filter the voltage supplied to the gate g2 of the broadband field effect transistor Q1. The drain of the broadband field effect transistor Q1 is connected to a +5V power supply.

The source of the broadband field effect transistor Q1 is connected to the base of the broadband transistor Q2 through the resistor R11, and the collector and emitter of the broadband transistor Q2 are respectively connected to the +5V power supply and the -5V voltage through the resistor R10 and the resistor R12 to form a voltage follower circuit. The input signal is output through the resistor R13. Wherein, the value of the small resistor R11 connected in series with the source of the broadband field effect transistor Q1 and the base of the broadband transistor Q2 is greater than 1Ω and smaller than 100Ω, which is used to adjust the high frequency response so that the frequency response has better flatness. Resistor R13 is about 1~2Ω resistance, which is used for buffering and avoiding high-frequency oscillation of the broadband transistor. In addition, the collector of the wideband transistor Q2 is connected to the ground through a capacitor, so that the resistor R10 and the capacitor C5 form an RC filter for the +5V power supply, which can reduce the noise of the power supply of the wideband transistor Q2.

The output of the impedance transformation module 3 , that is, the input signal is output to the DC feedback network 4 and the output circuit 5 through the resistor R13 .

The DC feedback network 4 is composed of a resistor R15 and a resistor R5 connected in series to the ground. The output of the impedance transformation module 3 is fed back to the negative terminal of the operational amplifier U1 after being divided by the resistor R15 and the resistor R5, and satisfies:

$\frac{R15}{R5}\≈\frac{R3}{R4}$ (Formula 1-2)

In addition to considering the ratio relationship between resistors R3 and R4, the resistance values of resistors R15 and R5 in formula 1-2 also need to be finely adjusted by considering the insertion loss A of the AC signal through the broadband field effect transistor and transistor. The insertion loss A is usually about 0.5~ 2dB. The exact relationship is as follows:

$20\left(\log \frac{R15}{R5}\right)-A=20\left(\log \frac{R3}{R4}\right)$ (Formula 1-3)

By adjusting the ratio of the resistor R15 and the resistor R5, the DC gain of the circuit can be adjusted and the insertion loss A can be compensated.

The output circuit 5 includes capacitors C6, C7 and resistor R14. The resistor R14 is connected in parallel with the capacitor C7; one end is connected to the input signal, and the other end of the capacitor C6 is grounded, which is used to adjust the high-frequency frequency response and perform frequency compensation on the input signal; the input signal is passed through a parallel Resistor R14 and capacitor C7 output. The output impedance is mainly determined by R14. The output signal of the output circuit is connected to the input terminal of the oscilloscope after passing through the probe transmission line, and forms a high-frequency low-resistance voltage divider network with the internal 50Ω of the oscilloscope to achieve the purpose of attenuation and matching. , Capacitor C7 and resistor R14 are connected in parallel for compensating high-frequency signals. Here the value of resistor R14 (when the attenuation is 20dB) is determined as follows:

$A+20\log \frac{50}{R14+50}=-20$ (Formula 1-4)

Among them, A is the insertion loss of broadband field effect transistor and broadband transistor, which can be obtained according to the accurate model simulation of field effect transistor and transistor, usually about -0.5~-2dB, and the sum of this loss and the attenuation of the resistor network in the subsequent stage is - 20dB, that is, a probe that attenuates 10 times (usually expressed as ×10).

Table 1 is a reference list of components for the digital oscilloscope single-ended active probe circuit in this example.

Table 1

In this example, the frequency response curve is shown in Figure 5 by testing the digital oscilloscope single-ended active probe circuit. The attenuation of the active probe circuit is 10 times (-20dB), and the 3dB bandwidth is better than the frequency response of 1.5GHz.

In this embodiment, the low-noise, low-drift JFET operational amplifier ADA4XXX is selected, because the JFET operational amplifier has a very high input impedance (about 10 ¹² Ω), which can ensure that the input impedance is strictly controlled at 1MΩ; Q1, Q2 Choose tubes with good broadband performance, low noise, and small size. Q1 is BF9XXX, and Q is BFS5XXX. In addition, the PCB uses dielectric plates with low high-frequency loss to help ensure high-frequency bandwidth.

The digital oscilloscope single-ended active probe circuit proposed in the present invention is slightly larger than foreign probes because of the use of separate components, but it is basically acceptable, and the implementation cost is low, and the price is high, which helps to improve domestic production. The comprehensive level of oscilloscope breaks the monopoly of a few foreign oscilloscope manufacturers, greatly saves the cost of broadband testing, and has very good promotion value.

In addition, with the continuous progress of domestic semiconductor technology, this circuit can also be used as a prototype circuit for custom probe ICs in the future, so that the circuit can be realized through integrated circuits, which will help to further reduce the volume and improve product consistency.

Although the illustrative specific embodiments of the present invention have been described above, so that those skilled in the art can understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, As long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are included in the protection list.

Claims (4)

1. A digital oscilloscope single-ended active probe circuit is characterized in that, comprising: A high-pass module, which is a high-frequency signal path, is used to isolate the DC and low-frequency parts of the input signal, and the high-frequency part of the input signal passes through it; A low-pass module is a low-frequency signal path through which the DC and low-frequency parts of the input signal pass; it includes a partial feedback network for adjusting the upper limit frequency of the input signal. In addition, the low-pass module also receives from the DC feedback network. Feedback signal for stabilizing the quiescent operating point of the single-ended active probe circuit; An impedance transformation module, including a broadband field effect transistor and a voltage follower circuit; Among them, the broadband field effect transistor has extremely high input impedance, and the output impedance is extremely small, which realizes the impedance transformation of the input signal; In order to enhance the drive, a voltage follower circuit is connected to the output terminal of the broadband FET, and the voltage follower circuit is designed as an emitter follower with a broadband transistor, or as a voltage follower with a broadband operational amplifier; A DC feedback network, which feeds back the DC voltage signal output by the voltage follower circuit to the low-pass module to achieve a stable DC operating point; An output circuit, which is used to compensate the frequency of the input signal and adjust the gain, and adjust the output impedance to match the 50Ω input impedance of the subsequent oscilloscope; The input signal is first input to the impedance transformation module through the high-pass module and low-pass module for impedance transformation and enhanced driving, then frequency supplementation, gain adjustment and output impedance adjustment are performed in the output circuit, and finally output to the subsequent oscilloscope; The broadband field effect transistor Q1 in the impedance conversion module is a broadband double-gate field effect transistor, and the inputs are grids g1 and g2 respectively, wherein the grid g1 is used for the input of the input signal, and the grid g2 is used for the DC bias voltage Input, with a very high input resistance of GΩ level and a small input capacitance of a few pF; The peripheral circuit of the broadband field effect transistor Q1 includes the resistor R9 connected to the -5V power supply at the source level, and the resistors R16 and R15; The +5V power supply is connected to the ground through resistors R16 and R15, and the connection point of resistors R16 and R15 is connected to the gate g2 of the broadband field effect transistor Q1, so that the broadband field effect transistor Q1 can obtain the best DC operating point; The capacitor C2 is connected in parallel with the resistor R15 to filter the voltage supplied to the gate g2 of the broadband field effect transistor Q1, and the drain of the broadband field effect transistor Q1 is connected to a +5V power supply; The collector and emitter of the broadband transistor Q2 are respectively connected to the +5V power supply and -5V voltage through the resistor R10 and the resistor R12 to form a voltage follower circuit; the source of the broadband field effect transistor Q1 is connected to the base of the broadband transistor Q2 through the resistor R11, The input signal is output through the resistor R13, wherein the value of the small resistor R11 connected in series with the source of the broadband field effect transistor Q1 and the base of the broadband transistor Q2 is greater than 1Ω and smaller than 100Ω, which is used to adjust the high frequency response so that the frequency response has a better Flatness; resistor R13 is about 1~2Ω resistor, which is used for buffering and avoiding high-frequency oscillation of the broadband transistor. 2. The active probe circuit according to claim 1, wherein the high-pass module includes a resistor R2 and a DC blocking capacitor C3, and the high frequency part of the input signal is sent to the impedance transformation through the resistor R2 and the DC blocking capacitor C3 The gate g1 of the broadband field effect transistor Q1 in the module; The low-pass module includes resistors R3 and R4, JFET operational amplifier U1 with low noise and low drift, resistors R7, R8 and capacitor C4 to form a local feedback network and output resistor R6; The sum of resistors R3 and R4 in series is 1MΩ, the input signal is grounded through resistors R3 and R4, and the series connection point of resistors R3 and R4 is connected to the positive terminal of operational amplifier U1; On the one hand, the output terminal of the operational amplifier U1 is connected to the gate g1 of the broadband field effect transistor Q1 through the resistor R6, on the other hand, it is grounded through the resistors R7 and R8, and the connection point of the resistors R7 and R8 is connected to the negative terminal of the operational amplifier U1 through the capacitor C4. Constitute a local feedback network, which is used to adjust the upper limit frequency that the input signal is allowed to pass through, and satisfy: $\frac{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 7</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 8</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; .</span>$ 3. The active probe circuit according to claim 2, wherein the DC feedback network is composed of resistor R15 and resistor R5 connected in series to the ground, and the output of the impedance transformation module is divided by resistor R15 and resistor R5. is fed back to the negative terminal of op amp U1 and satisfies: $\mathrm{<span\; class="notranslate">\; 20</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; log</span>}\frac{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 15</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 20</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; log</span>}\frac{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ;</span>$ Among them, A is the insertion loss of the broadband field effect transistor and the broadband transistor. 4. The active probe circuit according to claim 1, wherein the output circuit includes capacitors C6, C7 and resistor R14, and resistor R14 is connected in parallel with capacitor C7; the input signal end is grounded through capacitor C6 for high frequency Frequency response adjustment, frequency compensation for the input signal; the input signal is output through the parallel resistor R14 and capacitor C7, the output impedance is mainly determined by R14, the output signal of the output circuit is connected to the oscilloscope input terminal after the probe transmission line, and the internal 50Ω of the oscilloscope A high-frequency low-resistance voltage divider network is formed to achieve the purpose of attenuation and matching, wherein the capacitor C7 and the resistor R14 are connected in parallel to compensate high-frequency signals.