As I stumble my way through an "RF" project I keep coming up with more questions than answers.
"Simple Project", a simplistic, (i.e. Hobbist grade), signal generator using the Analog Devices AD9833 chip, (0-12.5MHz, Sin, Triangle, Square Waves).
I'm running my ATMega168 at 5V, LCD at 5V, and hence SPI bus is also at 5V.
I'm running the AD9833 chip at 5V to match the above.
I ordered some 25 MHz external Osc from Jameco, 1/2 can, TTL, 5V units. Turns out, reviewing their data sheet, they are 3.3 V units. :(
I called Jameco, they acknowledge that both their web page, and the catalog are in error.
So, I check their full can oscillators, 5V units, and it turns out they are also 3.3 V units, with the web and catalog again being in error.
So, I go to Mouser, find several 25 MHz, 5 V Oscillators. This one, for example: #520-TCH2500-X, by ECS Inc, is listed as 5V, and the data sheet says 5 V. So far so good.
The output waveform, however, lists a minimum High of 2.4 V, (on a 5V supply). The AD9833 lists, under logic I/O, (There is no separate listing for the clock signal), that the Minimum, High, Input is 2.8 V, when running at 5V.
Even if I use a "true" 5V ext osc, running In Spec, it could have insufficient drive for the sig gen chip.
So.... Any words of wisdom or experience and insight to be offered?
I'd like to just tie the ext osc to the chip and hope there is enough leeway that it works, BUT I hate designing out of spec, especially when I don't have enough experience in this area to know what latitude is reasonable.
I'd hate to put a 1 transistor amlifier/driver on the board, and I really don't want to switch to a 3V design.
Edit type, and: So do the Logic Input levels apply to the External Clock Input pin?