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How to Couple the Frequency and Amplitude of Two Function Generator Channels

Blog Post created by bernard ang Employee on Apr 17, 2018

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 a 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 the oscilloscope (see box marked in red).

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

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

 

How to Generate two Amplitude-coupled Signals from 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 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 two signals as observed on an oscilloscope.

Figure 6. Amplitude and offset coupling of two signals as observed on 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 set up to test an instrumentation amplifier.

Figure 8. Function generator and oscilloscope set up 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 simplifies 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 a Keysight function/waveform generator, read this app brief “Effortlessly Couple or Synchronize Two Signals on a Waveform Generator.”

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