Keysight, Teledyne LeCroy, Tektronix, Rohde & Schwarz (R&S), and others have offered built-in digital oscilloscope Bode analysis for some time, and this feature has trickled down to low-cost DSOs like the Siglent SDS2000X Plus and the new SDS814X HD. These DSOs feature built-in Bode analysis when operating with a companion AWG, or sometimes include the AWG within (SDS2000X Plus), at an affordable price point.

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DIY common-mode choke

One of the interesting applications of this Bode capability is investigating the open-loop response of closed-loop systems, such as oscillators. This often requires an expensive isolation transformer, which can be limiting. However, for those with a DIY spirit, a reconfigured common-mode choke serves as a nice isolation transformer for Bode analysis (Figure 1) [1].

Figure 1 A reconfigured common-mode choke isolation transformer used to investigate the open-loop response of closed-loop systems, e.g., oscillators, using the Bode capability of a benchtop oscilloscope.

Creating the isolation transformer is straightforward. Physically larger common-mode “chokes” utilized in AC mains, like the one shown, make good candidates, especially for lower frequencies.

Here, 5 mH and 2 mH Prod Tech PDMCAT221413 types were utilized after unwinding and rewinding. First, after the unwinding, the pair of wires are stretched, and then the pair of wires are twisted together (a hand drill helps). This leaves a long twisted pair which is threaded through the core as many times as possible.

As shown in Figure 2, the wrapped core now has two ends with the twisted pair, and at each end, a pair of wires. The ends of the wire on each side are common with the other pair of wires’ ends (use an ohmmeter), becoming the primary or secondary. Either way, it doesn’t matter since the isolation transformer has a 1:1 turns ratio and is symmetrical. The primary and secondary can be resistively terminated as needed for specific applications.

Figure 2 A side-view of the DIY isolation transformer showing the wrapped core and terminated with four 2-W, 100-Ω resistors.

Figure 3 shows the test setup utilizing the DIY isolation transformer to measure the open-loop response of a Peltz oscillator, as described in another Design Idea (DI): “Simple 5-component oscillator works below 0.8V.”

Figure 3 Test setup using the DIY isolation transformer to measure the open-loop response of a Peltz oscillator.

Peltz oscillator test circuit and results

The isolation transformer secondary is connected between Q2 base and Q1 collector. Q1 and Q2 are 2N3904s, L is 470 µH, C is 0.022 µF, and R is 510 Ω (Figure 4).

Figure 4 The configuration of the Peltz oscillator circuit, where the isolation transformer is connected between Q2 base and Q1 collector to measure open-loop response.

For comparison, an LTspice circuit model was created. The simulated and measured results using the SDS2504X Plus are shown in Figure 5.

Figure 5 Simulated (top) and measured (bottom) results with the circuit under test in Figure 4 operating with the following values: L is 470 µH, C is 0.022 µF, and R is 510 Ω.

Changing the inductor to 100 µH (measured 97.3 µH) which moves the center frequency to 34.4 kHz (Figure 6).

Figure 6 Simulated (top) and measured (bottom) results with the circuit under test in Figure 4 operating with the following values: L is 100 µH, C is 0.022 µF, and R is 510 Ω.

Typically, physically larger common-mode chokes have higher inductance, which can extend the measurement range to lower frequencies. Having a larger core also allows for more turns, which also helps with lower frequencies.

However, larger cores and more turns limit the upper frequency end, and having more cores, smaller and larger, can cover a wider frequency range than a single-core transformer. I’ve had good results with the cores shown from less than 100 Hz to over 1 MHz.

This is just one of the many uses for modern Bode-enabled DSOs with companion AWGs and a few DIY isolation transformers.

Michael A Wyatt is a life member with IEEE and has continued to enjoy electronics ever since his childhood. Mike has a long career spanning Honeywell, Northrop Grumman, Insyte/ITT/Ex-elis/Harris, ViaSat and retiring (semi) with Wyatt Labs. During his career he accumulated 32 US Patents and in the past published a few EDN Articles including Best Idea of the Year in 1989.

Related Content

• Simple 5-component oscillator works below 0.8V
• Injection locking acts as a frequency divider and improves oscillator performance
• Investigating injection locking with DSO Bode function
• DIY custom Tektronix 576 & 577 curve tracer adapters

References

1. https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/

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