Technical Insights > Benchtop

How to couple the frequency and amplitude of two function generator channels

2018-10-25  |  7 min read 

In electronics design and testing, you sometimes want to have two synchronized clock signals that are related by a frequency ratio; one clock needs to maintain a certain frequency ratio with the other clock.

Or perhaps, you want to simulate an amplifier with an offset; the amplifier output needs to maintain a defined offset from the input amplitude.

These requirements may sound basic, but building you own clock reference device or frequency/amplitude coupling circuit takes time and resources. It’s much easier to use a dual channel function generator that has built-in signal coupling.

Let’s look at how to generate frequency-coupled and amplitude-coupled signals using a dual channel function generator.

How to generate two frequency-coupled signals with a function generator?

Frequency coupling allows you to specify the frequency relationship of two signals using either a ratio (multiplying) or an offset (adding). Figure 1 shows an example of a frequency coupling ratio of “3”. When the frequency of Channel 1 is set to 3 kHz, the frequency of Channel 2 automatically sets to 9 kHz (Figure 2).

Dual channel frequency coupling setting on ratio on a Keysight 33612A Trueform function generator

Figure 1. Dual channel frequency coupling setting on ratio on a Keysight 33612A Trueform function generator.

Frequency ratio of 3 as observed on an oscilloscope

Figure 2. Frequency ratio of 3 as observed on an oscilloscope (see box marked in red).

Here is an example of frequency coupling by offset where the offset difference between Channel 2 and Channel 1 is 2 kHz (Figure 3). When the frequency of Channel 1 is set to 3 kHz, Channel 2 automatically tracks Channel 1 and outputs 5 kHz signal (see Figure 4).

Dual channel frequency coupling setting on offset

Figure 3. Dual channel frequency coupling setting on offset.

Frequency offset of 2 kHz as observed on an oscilloscope
Figure 4. Frequency offset of 2 kHz as observed on an oscilloscope (see box marked in red).

How to generate two amplitude-coupled signals with a function generator

You can also couple the amplitude and offset of two signals (Figures 5 and 6). Enabling this dual channel feature and setting the amplitude and offset saves you configuration time. Rather than setting the amplitude and offset of both channels independently, the function generator will keep track of your settings for you. You can even couple signals of different waveform shapes! Figure 6 shows how you can control the amplitude of a square wave and a sine wave using only one amplitude setting.

Dual channel amplitude and offset coupling setting on a Keysight 33612A Trueform function generator
Figure 5. Dual channel amplitude and offset coupling setting on a Keysight 33612A Trueform function generator.

Amplitude and offset coupling of 2 signals as observed an oscilloscope
Figure 6. Amplitude and offset coupling of 2 signals as observed an oscilloscope.

Benefits of Dual Channel Frequency Coupling and Amplitude Coupling

Here are some situations where coupling the amplitude and/or frequency of your test signals is useful.

Creating multiple reference pulse clocks to test circuitry

Some electronic devices operate with multiple frequency reference clocks. Dual channel frequency coupling comes in handy for quick design verifications.

Creating two very different arbitrary waveform signals that track each other in frequency

Consider this example of simulating a pacemaker pulse and an ECG arbitrary pulse wave that has the same frequency. When combining these two signals together, pacemaker manufacturers can test a pacemaker’s pulse rejection capability using the resulting signal, shown in Figure 7.

Two frequency-coupled signals combined into a single signal output
Figure 7. Two frequency-coupled signals combined into a single signal output.

Testing the differential gain of an amplifier

Testing the differential gain of an amplifier requires synchronized input signals. A function generator and oscilloscope are the ideal test setup for this (Figure 8). If you have perfectly identical input signals for the amplifier, you should see a zero-difference output (straight line), as shown by the oscilloscope’s output in Figure 9. If the output is not flat, you know there’s something wrong with your amplifier.

Function generator and oscilloscope setup to test an instrumentation amplifier
Figure 8. Function generator and oscilloscope setup to test an instrumentation amplifier.

Green and blue lines are identical input signals into the Op Amp, and the yellow line is the resultant output from the Op Amp

Figure 9. Green and blue lines are identical input signals into the Op Amp, and the yellow line is the resultant output from the Op Amp.

Using the dual frequency coupling with ratio of 2, the output of the amplifier will show the differential gain output (yellow line), as shown in Figure 10.

Green and blue lines are frequency coupled signals (with a ratio of 2) generated by a function generator for the input of Op Amp, and the yellow line is the resultant output from the Op Amp
Figure 10. Green and blue lines are frequency coupled signals (with a ratio of 2) generated by a function generator for the input of Op Amp, and the yellow line is the resultant output from the Op Amp.

Benefits of Frequency Coupling and Amplitude Coupling

Clearly, dual-channel tracking simplifies your configuration settings. You don’t have to configure channels separately, which is often tedious and repetitive. As a result, your testing will be less error prone and more efficient. If you are using a programming interface, automatic frequency coupling and amplitude coupling will simplify your code.

Now that you know how to couple two signals together, you should be able to perform fast simulation on your products or processes. Thanks to this coupling capability, the signal generation process has never been simpler, quicker, or less frustrating.

To learn more about frequency coupling and amplitude coupling your signals using Keysight’s function/waveform generators, read this eBook "5 Tips for Getting the Most Out of Your Function Generator".