C Turner
Joined: 10 Nov 2003
Posts: 40
Location: Utah
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 Posted: Wed Mar 17, 2004 1:52 pm |
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I take it that you are referring to FMing it? It's been a while since I looked at the 9830 data sheet, but I don't recall that there are provisions to affect output amplitude via the programming.
Do you care about lots of sidebands, aliasing effect, etc, or not?
To simply put some 1 KHz (FM) energy on it, one need only load the two frequency registers with accumulator values correlating with a frequency on either side of center, and then switch between the two at 2 KHz: It will be (essentially) and FMed square wave, complete with lots of sidebands.
Similarly, you could simply switch the phase 180 degrees (or less) at the same rate and effect a modulation - also a rather sideband-rich signal. That method would be more akin to PM, but if only a single modulation frequency is involved, who can tell?
If you want more of a "sine-ish" modulation, then you'll have to do the whole deal with sine lookup tables, sample rates, etc. You'll still end up with a rather interesting spectrum, however, as you'll have aliasing effects on your modulated signal.
If you *do* need a "clean" sine wave on the modulated signal, you could do two things:
- Bandpass filter the resulting signal. This might be tricky, as the aliased components may be too close to remove cheaply/easily.
- Regenerate the signal using a tracking PLL with a bandwidth-limited loop. This scheme is commonly used where DDS techniques are employed in signal generation for transmit and (especially) receive applications where they myriad low-level spurs intrinsic to DDS techniques could cause a problem. |
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