How does a FIR equiripple filter behave close to DC? I am multiplying two sine waves with the same frequency (f), but might have a phase difference 0 &lt;-> 90 degrees. The product is a two frequency sinusoidal wave with f1 = f-f = DC and f2 = f+f = 2f. I now want to filter out the 2f component. I am currently sampling at Fs = 32 x f. I get a 40th order equiripple FIR filter in Matlab which is ok, but if I later am to increase the sampling rate to 64xf, 128xf etc the filter order gets very high.

I assume however that f1 will not be 0 Hz exactly but somewhere close to 0 Hz. Over this region the ripple in the passband might be unwanted. Can the ripple change that much over such a narrow range? (within whatever limit one has set as the maximum ripple in the passband).

I am using the Filter Design &amp; Analysis tool in Matlab and saw that it does not seem like an equiripple filter has any ripple close to DC (0&lt;->1Hz)? Is this correct? 

I need a filter that can attenuate the 2f component with 60dB and need as little computing power as possible. (working on a MCU without FPU using fixed-point).

Is there any other filter that can do a better job than equiripple for what I described? I am open for other types of filters as IIR if someone can convince me of their advantages.
If you want to know the exact DC gain of your filter take the sum of the coefficients. This is true for an equiripple filter as well as any other FIR filter.  There is no such thing as ripple at DC, because "ripple" is a term used to describe a magnitude variation over some range of frequencies.

If what you are looking for is a flat passband then equiripple can fit the bill.  When using the fdatool in Matlab, notice that the equiripple FIR filter allows you to specify a parameter Apass.  This paramter lets you adjust the maximum allowable amplitude of the ripples in the passband as shown below.  By default it is set to 1, which means that you will have a maximum ripple of 1 dB in the passband.



If you are not constrained to FIR filters, then you might want to consider an IIR filter, depending on your needs.  You can usually achieve the desired result with a lower filter order when using IIR.  This means less computation and less delay.