So I recently purchased a ML405 development board and I need to now find an oscillator for use with the userclk. The manual specifies that I need a "half-socket LVTTL Oscillator that accepts 3.3V." However, in my search of both Digikey and Mauser I was only able to find a set of TTL oscillators, no LVTTL oscillators. Can anyone point me to a place where I can find these parts? Or do TTL oscillators work fine in these boards?
I've used the Epson Toyocom SG8002 programmable oscillators, page 1268 of the current Digikey catalog (T073). The SG531 package ("DC" suffix) is the eight-pin DIP needed for your board, and you need the output enable option (not standby) and 3.3V. The Digikey part number would be SGR8002DC-PCC (100 ppm) or SGR8002DC-PCB (50 ppm). Specify the desired frequency (1-125 MHz) when ordering.
Actually, now that I look at the specs, either the standby or output enable option would be fine, as they both give normal oscillating output when the enable pin is high.
Thanks a lot for your help. I've not had any real experience with programmable oscillators, anything I should know about them in particular? Is the difference between the 50ppm and 100ppm models just that the 50ppm has a cleaner signal? And what's the difference between standby and output enable? I'm curious. :)
A programmable oscillator has an internal PLL, so that the vendor only makes one part, and the distributor (or the end user) programs it to the specific frequency desired. In this case, Digikey will program it for you.
100 ppm or 50 ppm are the frequency stability, in parts per million. For most applications 100 ppm is good enough.
Oscillators with an output enable will put the output pin (5 for the 8-pin DIP) into a high-impedance state when the output enable (pin 1) is low. However, they continue oscillating, so they draw approximately the same power whether the output is enabled or not.
Oscillators with the disable feature will stop oscillating and draw less power when the enable (pin 1) is low.
Your board has pin 1 tied to +3.3V, so it doesn't matter which kind you use.
One more thing to check when you use a programmable or "quick-turn" oscillator. Make sure you look at the jitter specification if you intend to use the oscillator with a DCM in your design. Oscillators with internal PLL's often have enough jitter to be a problem for the DCM.
If you need a non-programmable 3.3V oscillator in an 8-pin DIP, those seem harder to find, but Mouser does have some, the Abracon ACHL series. Mouser has 17 frequency choices between 3.6864 MHz and 50 MHz.
While looking for those, I was stunned to see that Mouser sells Ecliptek programmable oscillators for ten times the price of Digikey's Epson-Toyocom programmable oscillators. Wow!
Fox offers low-jitter programmable oscillators, claimed to have less than 1/10 the jitter of normal programmable oscillators. I'm not sure whether that is good enough for use with the FPGA DLLs. Mouser doesn't have them in DIP, though.
Thanks a lot for all the feedback and info guys, 'tis nice to both get an answer and learn something in the process as well. Since we will be using DCM's in our design I'll go ahead and pick up the 50PPM parts in a few ranges of frequencies; with luck the jitter will be low enough to not cause problems.
The ppm isn't the issue, that's a measure of frequency accuracy and long term stability, and is essentially independent of jitter. For the DCM, you're better off using the true fixed-frequency oscillators like the Abracon ACHL series carried by Mouser rather than the programmble ones like the Epson-Toyocom SG8002. (Epson-Toyocom makes suitable fixed-frequency oscillators as well, but Digikey doesn't stock them.)