bernard ang

How to Create Differential Signals with a Function Generator

Blog Post created by bernard ang Employee on Apr 23, 2018

Aside from wireless and fiber optic transmissions, in this modern digital and mixed-signal age, many of our data transmissions go through good old-fashioned conducting cables. The two most common types of cables are differential and single-ended. There are obviously pros and cons in choosing either type of cable, but differentials have many advantages over single-ended cables. Differential signaling is usually used in conjunction with tightly twisted pair wires to reduce or cancel out the generation of electromagnetic noise. Hence, it has superior signal-to-noise ratio and fewer timing errors.

 

There is a Low-Level Differential Signaling (LVDS) standard for electrical transmission and communication protocols that are used in very low voltage and sometimes high-speed data transmissions such as video, graphics, and digital data bus transfers. Other applications include transmitting sensitive analog signals with audio microphones and medical heart monitoring devices.

 

What is a Differential Signal, and How Do I create a Differential Signal?

 

Figure 1 below shows two complementary signals (one signal inverted from the other) being used to create a differential output transmission signal. Here’s how to create differential signals for testing.

 

Differential output derived from two complementary signals (out+/out-).

Figure 1. Differential output derived from two complementary signals (out+/out-).

 

Step 1: Use a function generator to create the first of your two complementary signals (out+/out-). For example, create the (out+) signal on Channel 1.

 

Step 2: Select Dual Channel Inverted Tracking mode (see Figure 2).

 

As a result, Channel 2 will output an identical, but inverted, signal of Channel 1 (out-). Both signals will be amplitude and phase synchronized.

 

Differential channel output function generator setup on a Keysight Trueform function generator.

Figure 2. Differential channel output function generator setup on a Keysight Trueform function generator.

 

Step 3: Combine the differential outputs. To physically combine the two channels into a single differential output, connect the two common connections (the connector shells) of both channels together. Use the middle signal pin of Channel 1 as the high signal path of the differential signal, and use the signal pin of Channel 2 as the inverse return path.

 

Then connect a twisted pair cable to signal pins of Channel 1 and 2, as shown in Figures 3 and 4.

 

Differential signal block diagram.

Figure 3. Differential signal block diagram.

 

Function generator setup based on block diagram.

Figure 4. Function generator setup based on block diagram.

 

On the other end of the twisted pair cable, connect the signal pin of Channel 1 to the signal pin of the receiver’s BNC connector (Differential Signal Input) and the signal pin of Channel 2 to the ground connector shell of the receiver’s BNC (Figure 4).

 

Why and When Do You Use Differential Cables?

 

Basically, a product or systems designer will choose differential over single-ended signaling if:

  • The designer needs to reduce EMI or electromagnetic interference
  • The designer needs to reduce crosstalk or interference from nearby cables
  • The designer needs to transfer very low voltage signals, especially in millivolt range, to simulate bio-signals for medical applications. Low voltage signals are susceptible to noise interference
  • The designer needs to transfer low digital voltage signals to save power
  • The designer needs precise timings of digital signal crossover or digital switching

 

What Type of Function Generator Should You Use to Simulate Differential Signals?

 

Create a Balanced Pair of Signals

Use a function generator with “inverted tracking mode” to create an inverted mirror image of a signal. The signals on both channels will then be perfectly balanced pairs with synchronized amplitude, offset, and phase. Make sure the lengths of your differential twisted pair wires are matched too.

 

Reproduce Actual Signals from Your Design

Consider using an arbitrary function generator and its ability to reproduce actual signals from your design. You can quickly recreate your design signal by capturing it with a modern oscilloscope, then saving the captured trace to a .csv file.

At this point, you can use a USB thumb drive to import the data in the Waveforms > Arbs menu of your function generator to recreate and playback the signal from your design. You can even change your signal’s frequency, amplitude, or offset.

After recreating your signal, you can use the Dual Channel Inverted Tracking mode to recreate the output as a differential signal.

 

Create Arbitrary Waveform Signals

Differential signals are not limited to basic function generator signals such as periodic pulse, sine, square, and ramp signals. A lot of applications need complex waveforms, such as ECG bio-signals for medical applications, automotive CAN bus test signal simulations, telecommunication network test signals, and more.

Keysight’s Trueform function generators provide software tools such as BenchVue Waveform Builder (see Figure 5) and a built-in waveform editor to create your arbitrary waveforms. You can also use tools such as Excel or Matlab to create your arbitrary waveforms and transfer them into your function generator via .csv file format.

 

BenchVue function generator app to create arbitrary waveforms.

Figure 5. BenchVue function generator app to create arbitrary waveforms.

 

Use a Function Generator with the Lowest Jitter

If timing of your signal is very critical, differential signaling is a better option than single-ended signaling since it eliminates the uncertainties of transition crossover points. This takes care of jitter noise from cable transmission.

How about jitter noise from the source of the signal? Not all function generators are built the same! Keysight’s Trueform function generators have superior jitter noise compared to the conventional DDS technology used by most function generators in the market. (See Figure 6.)

 

Trueform technology, shown on the left, has significantly better jitter performance compared to conventional DDS technology.

Figure 6. Trueform technology, shown on the left, has significantly better jitter performance compared to conventional DDS technology.

 

Add Noise to Your Signals

Sometimes, you need an ideal signal with low noise, low distortion, and high signal integrity for your tests. However, at the same time, you want to introduce a controlled imperfect signal with noise for your tests.

Make sure to use a function generator that allows you to add noise to your signals. Press the Modulate button, select Source as Internal/Shape as Noise, and turn on Modulation. (See Figure 7.)

 

Adding variable bandwidth noise to signal for testing.

Figure 7. Adding variable bandwidth noise to signal for testing.

 

Conclusion

 

You can see that it’s easy to create differential signals if you have the right function generator. Choosing a function generator that has built-in differential capabilities will streamline your testing and ensure your signal’s phase and amplitude stay balanced.

 

To get better at using your function generator in the lab, download the Creating Differential Signals with a Waveform Generator application brief.

 

Visit Keysight.com for more information on Keysight’s Trueform function generators.

 

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