I'm trying to think of the best way to convert a PWM signal into an analog signal. I could use an RC filter, but that requires a very high PWM frequency to create a faithful reproduction of the signal. In my case I'm dealing with audio frequencies for the analog signal - up to 20 kHz. So, what other options are there?
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2\$\begingroup\$ How did you get a PWM signal that represents up to 20kHz without a very high frequency PWM in the first place? \$\endgroup\$– MartinCommented Mar 31, 2011 at 9:06
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3\$\begingroup\$ @Thomas, Fine, so you need an active low pass filter with a fairly steep roll off. \$\endgroup\$– MartinCommented Mar 31, 2011 at 10:32
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1\$\begingroup\$ @Thomas - Have you actually looked at your audio frequencies? 20 kHz is about the theoretical limit of audible signals, which probably isn't the standard you need to meet. Voice, for instance, can be well represented entirely under 3 kHz. \$\endgroup\$– Kevin VermeerCommented Mar 31, 2011 at 15:41
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1\$\begingroup\$ @reemrevnivek Depending on your application, < 3khz could be entirely unacceptable. You can get the gist of a voice in this range, but to say it can be really encoded in this range is not correct. \$\endgroup\$– Joe MacCommented Mar 31, 2011 at 20:55
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1\$\begingroup\$ @JoeMac A standard phone call is encoded with 8-bit audio at a sample rate of 8 KHz. But the antialising filter normally is set for a cutoff of 3 KHz (and a high pass filter cuts off everything below about 300 Hz). So, for many speech applications, 3 KHz is just fine. Higher does sound better, however, but doesn't add to the intelligibility. \$\endgroup\$– user3624Commented Apr 1, 2011 at 0:49
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4 Answers
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The simple answer is that you can't. There are several things going against you:
would be that for good audio quality you'd need to be very precise in your PWM generation. This is fairly difficult, as your master clock frequency (the freq that the logic used to generate the PWM from) would have to be around 100 MHz just to get the equivalent of an 11-Bit DAC. There are ways to generate PWM that good without that, but then you'd be starting with an analog signal and thus wouldn't have this problem.
as noted you would need a super steep rolloff of a filter. Just to match your 11-bit DAC equivalent you'd need something like 60 dB/octave-- which is unreasonable even for the pro's. There is a lot of difficultly in doing this kind of filter which is why everyone went to Delta-Sigma DAC's for their audio, which require about a 6 db/octave filter.
If your filter is not 60 db/octave then you'll need the PWM frequency to be super high. If you have 60 db/octave then you could have a PWM freq of 40KHz. At 54 db/octave then maybe 80 KHz will work. 48 db/octave = 160 KHz. Etc. Very quickly you get into the high MHz range-- and then your master clock frequency would have to be into the GHz.
All is not lost, however. Do you really need to filter out the high frequency stuff? In many applications (not all) you can either not filter it or use a simple RC filter at 1-5 MHz. There will still be high frequency stuff getting though but either the speaker or your ear will filter it all out. But the audio will still sound bad. AM Radio on a bad day quality. That's just the way it is.
Update: My numbers for #3, above, are somewhat wrong. But before I get to that, let me explain where I got all the numbers from.
Let's say that you are generating the PWM using digital logic (FPGA, Microcontroller, etc.). And then let's say that your audio sample rate and thus your PWM frequency is 48 KHz. And you want 8-bit resolution. That means that your master clock frequency should be 12.288 MHz. I calculated that this way: Master_Clk_Freq = Sample_Rate * 2^n_bits. Doing that again for an 11 bit resolution is 98.304 MHz.
The theoretical best noise level of an ideal DAC is about -6dB/Bit. So a 24-bit DAC has no better than -144 db noise. (Note: I'm playing a little loose with the terms here, lumping SNR, THD+N, and dynamic range all together.) Of course, no real 24-bit DAC can do this, but we're talking theoretical here. What this means is that an 11-bit DAC has about a -66 db noise level, so there is little point in making a filter that works better than this. AM Radio has approximately 60-ish dB signal to noise ratio, for comparison.
Ok, here's where I royally messed up the numbers. Assume that the PWM Rate = 48 KHz, and filter Cutoff=24 KHz (to make the math easy). With a -60dB/Oct filter we'll be -63dB @ 48KHz. If our filter were changed to -54dB/Oct then we'd be -57dB @ 48 KHz and -62.4dB @ 50.4KHz. What this means is that by changing our filter from -60db/oct -54db/oct we would have to change the PWM rate from 48 KHz to 50.4KHz to achieve the same filter blocking performance. Not a doubling of PWM frequency as I mentioned before. In the same way, if the filter were changed to -18dB/Oct (which is a manageable design) then the PWM frequency would have to be 104 KHz to still have -63dB filter attenuation at the PWM frequency. I calculated this by making a small spreadsheet and playing with the numbers.
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1\$\begingroup\$ This answer could be improved a lot if you explained how you derive your numbers... (11-bit DAC, 60 db/octave \$\rightarrow\$ 40 KHz, etc.) \$\endgroup\$ Commented Mar 31, 2011 at 13:24
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\$\begingroup\$ @reemrevnivek -- You're right. Maybe that's why I shouldn't be writing answers just after waking up in the morning... I have to go into a meeting right now, but when I'm done I'll go back over the numbers and make sure they are correct. \$\endgroup\$– user3624Commented Mar 31, 2011 at 14:16
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\$\begingroup\$ This is precisely why I said an RC filter probably wouldn't be an option. I was considering something else, like integrating the PWM ramp up, capturing the voltage and then resetting the ramp. I'll see if I can get an example working... \$\endgroup\$– Thomas OCommented Mar 31, 2011 at 14:38
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\$\begingroup\$ @reemrevnivek - I updated my answer with how I got the numbers. @ThomasO - While integrating the PWM as you described might make the resulting waveform look all clean, it would still have a huge frequency spike at the PWM frequency. So you'd still need the same nasty low pass filter. \$\endgroup\$– user3624Commented Mar 31, 2011 at 15:34
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The RC Low-pass filter would be the natural solution, but you can also use a microcontroller to measure the duty cycle and use that value to control a DAC. To measure duty cycle you need a timer with input-capture capability. The higher your timer clock, the more accurate your measurement. If the microcontroller doesn't have on on-chip DAC you can use an external one.
answered Mar 31, 2011 at 14:34
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I'm integrating the PWM ramp up, capturing the voltage with a sample and hold circuit and then resetting the ramp. This seems to get a relatively faithful representation of the signal, but it still has some issues, like PWM noise on the peaks, and it's slightly nonlinear. But, it does work fairly well, and it allows me to use a 100kHz PWM signal.
Simulate it here.
answered Mar 31, 2011 at 15:56
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1\$\begingroup\$ That's a cool website simulation thing! Anyway, when you clean up the "PWM Noise on the peaks" you will find that you have lots of quantization noise (a.k.a. aliasing) on the output waveform. I think that the noise that you have now, and the 1K/10nF filter on the output is hiding (but not removing) the quantization noise. \$\endgroup\$– user3624Commented Mar 31, 2011 at 16:13
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One issue not yet mentioned is knowing how the PWM signal was generated, in particular whether the leading edge, center, or trailing edge occurs at a uniform rate, and whether the PWM signal represents uniformly-spaced samples, or whether the PWM signal represents the samples taken at uniform intervals, or whether they represent the value of the input signal at the time of the rising edge, falling edge, or both. Knowing how a PWM is generated, it should be possible to do a good job of recovering the original signal (as well as it could be recovered from a signal sampled at the PWM rate). Attempting to reconstruct a signal which was sampled via some means other than what was expected is apt to cause phase distortion.
