【Prize】Small signal measurement method of real-time oscilloscope

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As the most common instrument in the electronics industry, the oscilloscope has been expanding and deepening its application. In previous issues, we introduced some common or traditional functions and applications of real-time oscilloscopes. In this article, we will talk about small signal testing, which has become increasingly common in the past decade.

According to the above definitions, typical power consumption tests of various portable and wearable devices involve current in various states, including standby, burst text messages, calls and network data modes, etc. Various IoT devices also face similar challenges in order to achieve low power consumption and long battery life.

Another typical small signal in digital circuits is power supply ripple and noise. The increasing bandwidth and high-integration power consumption requirements of backbone network systems drive signals toward lower rising edges, lower swings, and higher-order modulation. At the same time, the amplitude of the signal at the receiving end of the transmission link is greatly reduced, and equalization must be used to open the eye diagram, resulting in higher and higher requirements for the signal-to-noise ratio of the most typical PAM-4 signal.

The requirements for the ripple and noise of the power supply voltage of the circuit system are becoming increasingly stringent. If the ripple noise of the DC power supply is too large, the integrity of the signal of the associated high-speed chip may be affected due to the power supply noise problem during signal transmission.

Figure 1 Power supply requirements for different circuit units of a Broadcom SerDes chip

In addition, in the RF field, many RF microwave signals with very small amplitudes are also required, including EMI/EMC diagnosis.

In this article, we will introduce the testing methods of small signals.

Figure 2 MXR oscilloscope measuring 200uV small signal

Without connecting any probe or cable to the oscilloscope channel, set the vertical scale of CH1 to 1mV/div, without limiting the bandwidth, and using the full bandwidth of 6.3GHz, we can see that the tested ACVrms = 198uV. This noise floor value is very small and is consistent with the noise floor value given in the MXR oscilloscope manual.

By viewing it in the frequency domain through FFT conversion, the measured power spectral density is -160dbm. This value is very good, equivalent to the indicator of the current mainstream spectrum analyzer, and is very beneficial for measuring some small signals.

Figure 3: Time and frequency domain noise floor at 6.3GHz full bandwidth of MXR

The number of ADC bits of the oscilloscope It is directly related to the quantization level of the sampling process and the final quantization noise, so it is also very important for small signal testing. Of course, the ENOB (Effective Number of Bits) indicator is more telling. To learn more about the oscilloscope sampling system and horizontal ADC indicators, please refer to the article "Where is the Accurate Measurement of Keysight Oscilloscopes Derived from" in the third stop of the MXR Cloud Marathon .

Figure 4 MXR series ADC Hi-Res mode can reach up to 16 bits.

The vertical sensitivity of an oscilloscope generally refers to the vertical scale. For small signal measurement, if the vertical scale can be adjusted to the minimum, the small signal can be adjusted to the full screen as much as possible, thereby fully utilizing the linear range of the ADC and minimizing the quantization noise of the ADC. The vertical sensitivity of the MXR series mid-range oscilloscope can reach a minimum of 1mV/DIV, so it will have a very good representation of signals with an amplitude below 10mV. In particular, under the following 1 MΩ input amplitude, the maximum 5 V/div does not include the vertical sensitivity of the external probe.

Figure 5 MXR series oscilloscope vertical sensitivity

First of all, of course, as discussed in the first part, choosing an oscilloscope with sufficient performance is the most important thing.

Figure 6 Multi-channel waveforms are placed on different grids

For an explanation of the high-resolution mode, please refer to the article "What is the origin of the accurate measurement of Keysight oscilloscopes" in the third stop of the MXR Cloud Marathon . The disadvantage of Hi-Res is the reduction in bandwidth, so it is necessary to ensure that the sampling rate and available bandwidth of the oscilloscope are sufficient for the measured signal to avoid aliasing.

In average mode, you can directly filter out random noise by averaging several waveforms, but the signal must be repeated, and the data collected in average mode cannot support FFT analysis.

A bandpass or lowpass filter can be used to reduce the oscilloscope's noise floor.

Usually, mid-range oscilloscopes are equipped with 20MHz and 200MHz hardware filters as standard. These two filters are sufficient for general power supply ripple testing.

Figure 7 Hardware 20/200MHz bandwidth limitation

For some small signal tests with special requirements, if you need to flexibly customize the bandwidth limit, you can select Low Pass / High Pass and other filters in Math operations.

Figure 8 Mathematical filters supported in Math operations

In some cases, an external probe must be used to connect the signal. Considering that the probe has an attenuation factor, the oscilloscope will automatically identify the probe model and attenuation factor and amplify the signal by that many times according to the attenuation factor of the external connection accessories. This will amplify the background noise of the oscilloscope synchronously and drown out the small signal. Of course, when using the probe, a short ground loop is also necessary to avoid coupling more noise.

For some small signals with DC bias, the most accurate measurement results can be obtained by adjusting the vertical scale to expand the signal to the maximum display linear range after unloading the DC bias voltage. Therefore, the vertical offset setting capability of the oscilloscope is sometimes very important for small signal testing. Generally, when the vertical scale of the oscilloscope is set to a smaller value, its vertical offset will also be reduced. Therefore, you can consider using the vertical offset setting of the external probe at this time.

As shown below, select the Normal mode and adjust the oscilloscope channel offset knob to control the vertical offset of the oscilloscope channel itself. Select this option when measuring differential signals. Select Probe Control to directly remove the V DC at the front input of the probe amplifier . Select this option when using a differential probe to measure a single-ended signal. By using the probe's own offset, adjusting the channel offset knob now controls the vertical offset of the probe, allowing the offset DC voltage to be subtracted from the input signal before the signal reaches the amplifier.

Figure 9: Using the vertical offset of the probe to remove the DC component from the signal

Taking Keysight's classic differential probe 113xB series as an example, the probe itself can support ±12V vertical offset, providing flexible testing capabilities for small signals with DC bias.

Power supply ripple and noise are typical small signals. The importance and significance of their testing have been discussed in many articles and are also mentioned in the preface of this article.

In fact, based on the large-scale improvement of circuit integration and the widespread application of SoC today, the ultimate root cause of many problems in high-speed serial signal integrity testing is power and clock problems, because the main task of hardware research and development of electronic systems today has evolved into the integration of some core integrated circuits, PCBs, connectors, power supplies, clocks and other components. Therefore, in recent years, the industry has attached great importance to the problem of PI.

Figure 10 N7020A (white)/N7024A (black) probe

Schematic diagram of the welding front end of N7021A

Figure 11 N7023A point measurement front end

Figure 12 Schematic diagram of actual operation of N7023A detection front end

Figure 13 N7032A/N7033A 4GHz/5GHz power supply noise point measurement front end

The N7032A point-and-shoot front end can easily detect 0805 and 0603 packaged devices, while the N7033A front end can easily detect 0402 and 0201 packaged devices. It should be noted that the N7032/N7033A can be used not only for the N7024A but also for the N7020A.

In addition to small voltage signals such as ripple and noise, the pursuit of battery life drives the terminal system to accurately measure the current consumption in various modes.

For example, the following figure shows the current changes of a typical smartphone in various modes. The current when sending a signal may reach 1-2A, while the current in standby mode is as low as sub-mA level. Here, there are not only sensitivity or resolution issues for small current signal testing, but also challenges for the huge dynamic range of the signal to the test equipment, which is of course a great challenge for conventional Hall effect current probes.

Figure 14 Typical smartphone operating current changes in various modes

For example, Keysight's new Hall effect-based current probe N7026A with a maximum bandwidth of 150MHz has a minimum sensitivity of 1mA/Div. Even if a sensitivity of 0.1mA can be achieved by winding 10 turns of wire for small current signals, it cannot meet the test requirements of the standby current of the smartphone in the figure above.

Figure 15 N7026A current probe with maximum sensitivity of 1mA/Div

Figure 16 N2820A (left)/N2821 (right)

High sensitivity, high dynamic range low current probe

N2820A is a dual-channel access oscilloscope, and N2821A is a single-channel access without a secondary cable or global signal channel access. Its principle and architecture are shown in the figure below. Based on Ohm's law, an RS -sensitive precision resistor is inserted into the measured signal loop , and the oscilloscope will obtain the current waveform based on the voltage signal fluctuation and resistance relationship on it.

Figure 17 N2820/N2821A architecture principle

The figure below shows the measurement of the N2821A current probe. Channel 2 is connected with a Secondary Cable passive cable, showing global signal observation, observing the current signals in normal working mode and standby state at the same time. Channel 1 is a high-sensitivity amplified observation specifically measuring the small current signal in standby state.

Figure 18 N2821A high sensitivity and high dynamic range current signal measurement

The N2820A/N2821A probes can also be used to measure small voltage signals if a 1Ω sensitive resistor is inserted.

Based on the oscilloscope, EMI/EMC diagnosis and debugging can also be performed using a near-field probe. Keysight can provide the N9311X RF/Microwave probe accessory.

Figure 19 N9311X RF/Microwave near-field probe

Figure 20 Antenna provided by N9311X

After using the above-mentioned near-field probe to pick up the signal, the EMI frequency can usually be detected by using the oscilloscope's FFT mathematical analysis. If the EMI is an occasional signal, can FFT still capture it? The answer will be given in the last article.

The above is this issue’s introduction to some typical small signal measurements, including equipment requirements, some testing techniques and precautions, and specialized test probes.

In fact, small signals are numerous and have no regularity compared to general digital signals. Therefore, in actual measurement work, it is necessary to determine according to the actual situation. However, the fundamental purpose is how to improve the signal-to-noise ratio in the measurement, that is, how to reduce the various noises introduced in the measurement, and finally obtain the real signal.