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I'm wondering why digital oscilloscopes are using switching power supplies instead of linear power supplies.

The SMPS has a higher efficiency, but it can produce some noise during high frequency switching (PWM) and also EMI that can affect the signal shown on the display.

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    \$\begingroup\$ Do you think there is no transformer in an SMPS? There is a transformer in an SMPS too. But it is smaller than in a linear supply because the switching freq. is much higher than 50/60Hz. Back to your question the main reason is money and weight. Linear supply is bulkier and more expensive. \$\endgroup\$ – user16307 Aug 10 at 15:05
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    \$\begingroup\$ @user16307 Don't answer questions in the comment section. \$\endgroup\$ – pipe Aug 10 at 17:19
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    \$\begingroup\$ The digital circuitry in the DSO is far noisier than the SMPS, larger dI/dt & dV/dt. If you look at high-end scopes the entire analog front-end is in a shield-can, in an attempt to filter all the switching noise on-board, not just the SMPS. \$\endgroup\$ – sstobbe Aug 10 at 18:29
  • \$\begingroup\$ One word: price. \$\endgroup\$ – winny Aug 10 at 18:39
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    \$\begingroup\$ Heat generation and weight are important issues in portable equipment. The switching power supply design in oscilloscopes do use techniques such as resonant mode operation with zero-volt switching to minimize noise generation while still having the advantages of high-efficiency and low weight. \$\endgroup\$ – Kevin White Aug 10 at 20:45
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The SMPS advantages over linear supplies, of compactness, high efficiency so low heating, light weight and wide input voltage range, are very valuable in portable equipment like an oscilloscope.

The principle disadvantages of SMPS over linear supplies are switching noise on the output and radiated EMI.

(Circuit complexity used to be considered another but not nowadays.)

Well-designed output filtering can reliably reduce ('remove') this noise and an oscilloscope carries a high enough cost to afford it.

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If proper engineering is followed, the switching noise shouldn't impact the readings and the oscilloscope weighs less, takes up less space, and draws less power. Thus, many benefits to a single increase in engineering difficulty.

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    \$\begingroup\$ Also note that oscilloscopes aren't usually precision measurement instruments either. Most are only 8-bits. \$\endgroup\$ – DKNguyen Aug 10 at 13:16
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    \$\begingroup\$ The SMPS output can also be post-regulated using linear regulators giving the best of both worlds - good efficiency and very clean output. \$\endgroup\$ – filo Aug 10 at 14:36
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    \$\begingroup\$ @filo, high frequency noise will pass straight through linear regulators - they won't magically slice the top of the voltage graph off :-) That can only be done by filtering. You could feed that into linear regulators but that wasn't your point. \$\endgroup\$ – TonyM Aug 10 at 14:58
  • \$\begingroup\$ @TonyM that's true, but if you've got a good linear regulator "dampening" the majority of the energy in voltage fluctuations, then much smaller (or fewer different, see the good ole "need a ceramic in parallel to an electrolytic cap, one has small capacitancy the other small ESR") filtering components can be used. \$\endgroup\$ – Marcus Müller Aug 10 at 15:16
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    \$\begingroup\$ At high frequencies, the Linear Regulator needs help: a low-pass-filter composed of series inductor (perhaps with a parallel Rdampen 1-10 ohms) and large shunting capacitor that connects to a wide GND bus or to a GND plane. \$\endgroup\$ – analogsystemsrf Aug 10 at 17:03

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