frequencyfunction generatorsquare
I connected the output (50Ω) of a function generator to a 35 watt 5Ω speaker.
When I sent square wave signals of 1 Hz-20 kHz to the speaker, my friends and I were able to hear them. But when I sent sine waves across the same range, no one was able to hear anything below 100 Hz.
Although I still wasn't able to find a manual online, in an ad for one of the products for the similar SG100x series (same as mine w/o an ADC or second TTL port), it mentions the sweep function going from frequency f1 to f2. As mentioned in my question, I couldn't find any direct way to set the starting and ending frequencies, but seeing them labelled f1 and f2 got me thinking.
There is a Save function which saves the current parameter setup to one of 10 "slots", numbered 0 through 9. Sure enough, setting up the starting frequency and saving the current setup as slot 1 (skipping slot 0), and then setting up the ending frequency and saving the setup as slot 2, and then setting the Sweep function to Run causes it to use these two frequencies as the start and end point. All the the other parameters such as the signal type (square/triangle/square, sweep duration, etc. are taken from the current setup when Run is selected.
Rather obscure user interface.
Update
I was finally able to find the manual for my specific model online here.
Any waveform other than a sine at 12.5 MHz will be composed of the fundamental frequency 12.5 MHz, plus harmonics above that. For ideal waveforms with sharp edges, the harmonics extend to infinity.
It sounds like you have added a filter with a cutoff frequency around 12.5 MHz, so this is why your non-sinusoidal outputs don't look like they should -- your filter is stripping off the harmonics which give those waves their non-sinusoidal identity.
To illustrate, here are a few animations from Wikipedia which show various non-sinusoidal waveforms as harmonics are added/removed:
What your filter should be doing is stripping off the harmonics generated by the impulses at each sample output by your DAC. This is called an anti-aliasing filter, and ideally it's a brick-wall low-pass filter with a cutoff equal to half your sampling frequency. Given that true brick-wall filters aren't realizable, the cutoff is usually a bit below half the sampling frequency, and the roll-off is made as steep as feasible. If you can sample significantly more than twice the highest frequency of interest, then the filter design isn't as critical.
It also follows that if you want to synthesize a 12.5 MHz square (or triangle, or sawtooth...) wave, you need a DAC sampling fast enough to also synthesize the higher harmonics in that wave. How much higher depends on how closely you want to approach an ideal square/triangle/sawtooth wave.
Best Answer
Most people can't hear sinewaves below about 20 Hz. The fact that you couldn't hear them until they got to 100 Hz is probably due to poor low-frequency response in your amplifier/speaker.
When you use a square waveform, you are adding a rich series of harmonics to the fundamental frequency — these are what you're hearing, and your brain extrapolates the fundamental frequency for you. Down to a point, that is — at some point you just start hearing individual clicks.