Keysight, Teledyne LeCroy, Tektronix, Rohde & Schwarz (R&S), and others have provided built-in digital oscilloscope Bode evaluation for a while, and this function has trickled all the way down to low-cost DSOs just like the Siglent SDS2000X Plus and the brand new SDS814X HD. These DSOs function built-in Bode evaluation when working with a companion AWG, or generally embrace the AWG inside (SDS2000X Plus), at an inexpensive value level.
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DIY common-mode choke
One of many fascinating functions of this Bode functionality is investigating the open-loop response of closed-loop techniques, similar to oscillators. This typically requires an costly isolation transformer, which might be limiting. Nevertheless, for these with a DIY spirit, a reconfigured common-mode choke serves as a pleasant isolation transformer for Bode evaluation (Determine 1) [1].
Determine 1 A reconfigured common-mode choke isolation transformer used to analyze the open-loop response of closed-loop techniques, e.g., oscillators, utilizing the Bode functionality of a benchtop oscilloscope.
Creating the isolation transformer is easy. Bodily bigger common-mode “chokes” utilized in AC mains, just like the one proven, make good candidates, particularly for decrease frequencies.
Right here, 5 mH and a couple of mH Prod Tech PDMCAT221413 sorts had been utilized after unwinding and rewinding. First, after the unwinding, the pair of wires are stretched, after which the pair of wires are twisted collectively (a hand drill helps). This leaves a protracted twisted pair which is threaded by the core as many occasions as attainable.
As proven in Determine 2, the wrapped core now has two ends with the twisted pair, and at every finish, a pair of wires. The ends of the wire on both sides are widespread with the opposite pair of wires’ ends (use an ohmmeter), changing into the first or secondary. Both approach, it doesn’t matter because the isolation transformer has a 1:1 turns ratio and is symmetrical. The first and secondary might be resistively terminated as wanted for particular functions.
Determine 2 A side-view of the DIY isolation transformer exhibiting the wrapped core and terminated with 4 2-W, 100-Ω resistors.
Determine 3 reveals the check setup using the DIY isolation transformer to measure the open-loop response of a Peltz oscillator, as described in one other Design Concept (DI): “Easy 5-component oscillator works beneath 0.8V.”
Determine 3 Take a look at setup utilizing the DIY isolation transformer to measure the open-loop response of a Peltz oscillator.
Peltz oscillator check circuit and outcomes
The isolation transformer secondary is linked 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 (Determine 4).
Determine 4 The configuration of the Peltz oscillator circuit, the place the isolation transformer is linked between Q2 base and Q1 collector to measure open-loop response.
For comparability, an LTspice circuit mannequin was created. The simulated and measured outcomes utilizing the SDS2504X Plus are proven in Determine 5.
Determine 5 Simulated (prime) and measured (backside) outcomes with the circuit beneath check in Determine 4 working with the next values: L is 470 µH, C is 0.022 µF, and R is 510 Ω.
Altering the inductor to 100 µH (measured 97.3 µH) which strikes the middle frequency to 34.4 kHz (Determine 6).
Determine 6 Simulated (prime) and measured (backside) outcomes with the circuit beneath check in Determine 4 working with the next values: L is 100 µH, C is 0.022 µF, and R is 510 Ω.
Usually, bodily bigger common-mode chokes have larger inductance, which may prolong the measurement vary to decrease frequencies. Having a bigger core additionally permits for extra turns, which additionally helps with decrease frequencies.
Nevertheless, bigger cores and extra turns restrict the higher frequency finish, and having extra cores, smaller and bigger, can cowl a wider frequency vary than a single-core transformer. I’ve had good outcomes with the cores proven from lower than 100 Hz to over 1 MHz.
That is simply one of many many makes use of for contemporary Bode-enabled DSOs with companion AWGs and some DIY isolation transformers.
Michael A Wyatt is a life member with IEEE and has continued to get pleasure from electronics ever since his childhood. Mike has a protracted profession spanning Honeywell, Northrop Grumman, Insyte/ITT/Ex-elis/Harris, ViaSat and retiring (semi) with Wyatt Labs. Throughout his profession he collected 32 US Patents and prior to now printed a couple of EDN Articles together with Greatest Concept of the 12 months in 1989.
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References
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