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2017

The vast majority of electronic tests involve using a digital multimeter (DMM) at one time or another. There are a variety of ways to reduce DMM measurement times to improve overall test throughput. Of course, test time improvements sometimes require compromises in other areas, but knowing the tradeoffs involved in throughput improvements and identifying what is important in your specific test situation will help you determine which trade-offs make the most sense.

Auto zero: Accuracy versus test time Auto zero is a DMM feature that helps you improve accuracy. When you use the auto zero feature, the DMM makes an additional zeroing measurement with each measurement you make, thereby eliminating the offsets of the amplifier and integration stages inside the DMM. However, turning this feature off cuts the measurement time in half. These offsets are initially calibrated out, but the offsets can drift slightly with a change in temperature. Therefore, if your measurements are taken in an environment with a stable temperature, or if there are several measurements taken in a short period of time (temperature changes occur over longer periods of time), the improvements in throughput by turning auto zero off will far outweigh any slight compromise in accuracy. For example, with auto zero off in a stable environment, the Keysight 34460A/61A/65A/70A DMMs typically adds only an additional 0.0002% of range +5 μV for DCV or +5 mΩ for resistance accuracy specification. Note that with auto zero off, any range, function, or integration time setting change can cause a single auto zero cycle to be performed on the first reading using the new setting. Consequently, turning auto zero off and constantly changing settings defeats the time savings advantage. Check your DMM auto zero operation to be sure of the circumstances leading to an advantage from this change.

DMM pic

Reduce the number of changes Changing functions or measurement ranges also requires extra time in most DMMs. Try to group your measurements to minimize function changes and range changes. For example, if you make some voltage measurements and some resistance measurements, try to do all of the voltage measurements together and all the resistance measurements together instead of changing back and forth from one function to the other. Also, try to group your low-voltage measurements together and your high-voltage measurements together to minimize range changing. Voltage ranges above 10 V use a mechanical attenuator that takes time to switch in and out. Grouping your measurements by function and range will reduce your measurement times considerably.

Auto range variations Auto range time can sometimes contribute to longer test times, but not always. The time to auto range varies with the DMM design. DMMs using flash A/D converters and parallel gain amplifiers can actually reduce test times by using auto ranging, since the time to change ranges is zero. In these cases, the time to issue a range change command from a host computer and parse the command in the instrument will be slower. Manual ranging of integrating DMMs is still the fastest way to take a measurement. Manual ranging also allows you to keep the DMM on a fixed range, which eliminates unwanted zero measurements and prevents the mechanical attenuator from needlessly actuating. Note that the I/O speed and range command parse time for the Keysight 34460A/61A/65A/70A DMM is significantly faster than the auto range algorithm.

 

Integration time versus noise Integration time is another parameter over which you have direct control, but there is a clear tradeoff. DMMs integrate their measurements over a set period of time: the integration time. The biggest benefit to choosing a longer integration time is it eliminates unwanted noise from contributing to your measurement, especially AC mains line voltage noise. However, longer integration times obviously increase your measurement times. For example, if the integration time is set to an integral number of power line cycles (NPLCs) such as 1, 2, 10, or 100, the power line noise contribution will be minimized due to averaging over a longer period of time and due to increasing the normal mode rejection (NMR). With an NPLC setting of 10 in a 60-Hz environment, the integration time is 166 ms (200 ms for a 50-Hz line). The larger the integral NPLC value, the larger the NMR (for example, 60 Hz rejection), but the longer the measurement time.

 

DMMs are used in virtually all electronic test systems; therefore, making conscious choices about how to make DMM measurements can save large amounts of test time, thereby increasing throughput. Here is a helpful checklist for better throughput:

  • If appropriate, turn auto zero off
  • Minimize function and range changes
    • Group similar measurement functions together (DCV, DC ohms, ACV, etc.)
    • Use fixed ranges instead of auto range, if appropriate
    • Shorten integration time with consideration for noise rejection, resolution, and accuracy

 

For more info on Keysight DMMs click here

Power line communication or power line carrier (PLC) is getting a lot more attention these days since it is used in many of the new green energy electronics such as smart grid devices, solar inverters, and home automation. PLC is communication technique that uses the power wiring in buildings or grid power transmission lines as its communication channel. In this post, we will look how you can easily generate complex PLC signals for test purposes with a low-cost function / arbitrary waveform generator (FG/AWG). For a more general overview on PLC click here. Generating communication signals for testing typically requires costly test equipment like a signal generator for the carrier and a FG/AWG for the baseband. For PLC signals, we can skip the costly signal generator and just use a modern FG/AWG. There are two main reasons we can skip the signal generator for simulating PLC signals. The first one is the carrier signal for PLC is typically less than 1 MHz so it falls well within the bandwidth capabilities of a FG/AWG. The second is modern FG/AWGs have advanced features for creating complex signals. These features include:

  • Large waveform memory for storing not only arbitrary waveforms, but also arbitrary signals.
  • Arbitrary waveform sequencing, which is analogues to a playlist on an MP3 player. It allows you to seamlessly combine multiple waveforms from memory to create a complex signal.
  • Optional second independent channel for creating an I and Q signal.
  • Advanced modulation capabilities such as waveform summing, modulating an arb with an arb, and for two channel FG/AWGs the ability to modulate the signal from one channel with the other channel.

Let's look at a couple of examples using Keysight's 33522B / 33622A FG/AWG. Here is a simple example just using built-in waveforms. In the below screen shot from the 33522B / 33622A, a BPSK signal with a 135 KHz carrier was created. For the baseband, a built-in waveform known as a pseudo random bit stream (PRBS) was used. The PRBS waveform just delivers a close to random stream of 1s and 0s at a chosen bit rate, for this example 5.5 kbps was used. Of course, an arbitrary waveform made up of real data could have been used for the baseband as well.

JPF

For the second example let’s look at something a little more complicated. In this example a QPSK signal with a frequency hopped spread spectrum carrier was created using Matlab. The bit rate of the digital data was 10 Kbits/s. The signal lasts for 15 ms and consists of >500,000 data points. The waveform was transferred to the 33522B / 33622A via a USB stick and a CSV file. Below is a screen shot of a portion of the signal.

QPSK screen shot

In this example, we used the FG/AWG's large waveform memory to output a large arb file (greater than 500K points) to create a 15 ms signal segment with the baseband modulation and the frequency hopping already in signal. This frees up the FG/AWG's modulation capabilities for other purposes such as simulating communication channel noise. Below is an example of using the modulation function to add some channel noise. The noise signal was a sharp pulse signal representing a large load transient on the power line. This was done on the 33522B / 33622A by using the "sum" modulation feature. The source of the pulse noise signal was channel two on the 33522B / 33622A.

QPSK Pulse

In this post we talked about using a low cost function / arbitrary waveform generator for creating PLC signals. There are two main reasons why a FG/AWG makes a great solution for simulating PLC signals compared to a signal generator. The first is the carrier frequency of PLC signals is well within the capabilities of a FG/AWG. Second modern FG/AWGs have features like a large waveform memory, waveform sequencing, and advanced modulation capabilities. If you have any questions related to this post please email me and if you have anything to add use the comments section below.

 

Click here for more info on the 2-channel FG/AWG 33522B / 33622A