Why You Can't Connect Two Signal Generators in a Series: Causes and Solutions

Connecting two signal generators in series is a common requirement in many engineering applications. However, it is not always recommended due to various technical challenges. This article explains the reasons why you cannot connect two signal generators in series and provides practical solutions.

The Reasons Against Series Connection

1. Impedance Mismatch

Signal generators are designed to drive specific load impedances, typically 50 ohms or 75 ohms. When connected in series, the load impedance seen by each generator changes. This can lead to reflections, signal loss, or distortion as the generators struggle to maintain the intended frequency and amplitude.

2. Output Voltage Levels

Connecting the output of one signal generator to the input of another can cause the first generator's output voltage to exceed the second generator's input voltage range. This can result in damage to the second generator or signal distortion.

3. Isolation Issues

Signal generators typically have output stages that are not designed to handle being driven by another generator. This can lead to issues with signal integrity, where the output of one generator may attempt to drive the input of the other, leading to unpredictable results.

4. Signal Interference

Each signal generator may introduce noise or artifacts into the output signal. When connected in series, these interferences can compound, significantly degrading the overall signal quality.

5. Control and Synchronization

Even if the generators are synchronized in frequency and phase, the signals can interfere destructively. This can lead to a complex and potentially unusable output waveform.

Potential Solutions for Connection

Although connecting two signal generators in series isn't advisable, it is possible under certain conditions. Here are some practical solutions to consider:

Using a Proper Combining Circuit

One effective solution is to use a dedicated combining circuit such as an RF combiner or mixer. These devices are specifically designed to sum signals while managing impedance and isolation to prevent the issues highlighted above.

Example Circuit

The following is an example circuit simulated in LTspice. R1 and R2 represent the output impedance of each signal generator, while R3 is the load. The simulation shows that the 50Hz and 60Hz signals have added together to produce a complex waveform:

Simulation Output:

Output of the LTspice simulation where 50Hz and 60Hz signals are added in series

Note that in this case, the second signal generator is floating, i.e., not connected to ground. This means the ground side of the generator will have the first generator's output impressed on it, which can create EMI and affect nearby circuits.

It is also important to consider the internal coupling of the second generator. If the output is already connected to ground internally, attempting to connect the two generators in series can create a direct short to ground, damaging both generators.

In the scenario where both generators are internally connected to ground, they can still be wired in series, but only by their ground sides, and only if the load is floating. However, the output impedances of the generators are also added, which could be a problem if the load requires a matched impedance to prevent reflections.

Conclusion

While connecting two signal generators in series poses several technical challenges, it is possible to achieve the desired results with appropriate circuit design and the use of specialized combining circuits. Understanding the risks and carefully planning the connection can ensure that your signal quality remains optimal and your equipment is protected from potential damage.