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I have an electromagnet (coil with about 500 uH inductance and 0.8 Ohm resistance) that I want to drive at 4 kHz with about 3 Amps of current. I want to achieve this with minimum noise and power at any frequency other than 4 kHz.

I am using a waveform generator (Stanford Research Systems DS345) to generate a 4 kHz sine wave (unbalanced signal), which is then sent to an audio amplifier (Crown xls 402). The output of the amplifier is a balanced signal connected to the coil. In parallel with the coil I have two zener diodes for flyback to prevent damage to the amplifier in the event of a power failure.

In order to minimize noise coming from the waveform generator or the amplifier, which of the following is optimal? The most critical noise frequencies that I must minimize are those at low frequencies (< 4kHz and in particular those less than 200 Hz) and those over 4 MHz.

(i) Maximize the voltage coming from the waveform generator (10 V peak to peak). Then set the gain on the amplifier to get desired output current

(ii) Set gain on amplifier to maximum, and then set the waveform amplitude to get desired output current

(iii) Something else entirely

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  • \$\begingroup\$ Where is the noise coming from? \$\endgroup\$ – Dave Tweed May 22 '14 at 1:09
  • \$\begingroup\$ @DaveTweed Noise levels are already really low, but our application requires absolutely minimizing the noise as much as possible. There is some line noise at 60 Hz as well as what I believe are harmonics of the line noise at 120 Hz, 180 Hz, 300 Hz, etc.. There is some 1/f noise also. The cable from the waveform generator to the amplifier could be picking up stray RF. The waveform generator itself could have some 'noise' or rather imperfections since it is basically a digital to analog converter. \$\endgroup\$ – Joe May 22 '14 at 1:17
  • \$\begingroup\$ Is there a reason that you can't put a passive narrow band pass filter in the circuit just before the coil? \$\endgroup\$ – markt May 22 '14 at 1:46
  • \$\begingroup\$ @markt We do have a narrow band pass filter. This does help. Given that our input signal is as good as possible, I would still like to know if it is better to maximize the input amplitude or the amplifier gain? Which produces the truest output signal? \$\endgroup\$ – Joe May 22 '14 at 2:01
  • \$\begingroup\$ There are no general rules. If you're really concerned about noise at that level (and you still haven't given us any numbers), you're going to have to analyze the entire signal chain, including the internals of the signal generator and the amplifier, to determine whether they're going to meet your needs. Do you also have similarly stringent requirements with regard to waveform distortion, etc.? Finally, have you considered using a class-D driver, which would make the entire signal chain up to the final power stage purely digital? \$\endgroup\$ – Dave Tweed May 22 '14 at 2:15
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You might be missing a trick here - try series resonating the inductor with a capacitor of precisely 3.18 uF. With the 500uH coil (and series resistance of 0.8 ohms), you'll get a voltage magnification across the coil of about 15:1.

In other words, you apply a 1V RMS sine wave at 4kHz and you get 15V RMS sinewave across the coil. There's another benefit too - it's a fairly tight filter and rejects distortion/noise from the amplifier that's driving it. Your amplifier needs to be able to drive a small load resistance because now all the load the amp will see is the losses in the inductor i.e. the 0.8 ohms but, if the amp's minimum load is 2 ohms then insert an extra 1.2 ohms in series - the magnification will only be about 6.3 now but still pretty reasonable.

schematic

simulate this circuit – Schematic created using CircuitLab

Resonant frequency is \$\dfrac{1}{2\pi\sqrt{LC}} = \dfrac{1}{2\pi\sqrt{500\mu\cdot 3.18\mu}} = 3991.4 Hz. \$

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  • \$\begingroup\$ You don't really mean kHz, right? :) \$\endgroup\$ – WhatRoughBeast May 22 '14 at 12:27
  • \$\begingroup\$ @WhatRoughBeast oooooooooooops LOL.... fixed!!! \$\endgroup\$ – Andy aka May 22 '14 at 12:35
  • \$\begingroup\$ Additionally the bandwidth of the series circuit is BW = R/L \$\endgroup\$ – Marla May 22 '14 at 15:25
  • \$\begingroup\$ As yet another refinement, if you add an external resistor, say 10 milliohms, from the coil to ground, you can tune the generator to maximize efficiency. Just monitor the voltage across the external resistor and tune the generator for 0 degrees phase shift between the two. Since capacitors are famously low-tolerance, this would allow you not to worry too much about the actual capacitor and inductance values. \$\endgroup\$ – WhatRoughBeast May 23 '14 at 0:00
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In general, all things being equal the first amplification stage dominates the noise figure, so I would first try maximizing the gain of the signal generator and lowering the gain of the power amp and see if that improves things.

If you're going to use an analog signal path, instead of the digital one mentioned above, I would experiment with using the power amp as a frequency selective current source and driving the coil with a current rather than a voltage. Try putting a current shunt in series with the coil, and running the developed voltage through a graphic EQ with a deep notch at 4khz. Then send this into the sidechain of an audio compressor connected in series with the signal generator and experiment with the attack and release settings, to see if this helps null out the power line harmonics.

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