1. Home
  2. Field-Programmable Gate Arrays
  3. 5 V oscillator output to GCLK
  4. Page 2

5 V oscillator output to GCLK

Oscillators typically can't drive 50 ohms impedances either so impedance matching to the PCB would not be relevant....the optimal solution would be a series resistor of ~1.75x - 2x, parallel to ground of 3x where 'x' needs to be determined based on the spec'ed drive capability of the oscillator. An IBIS or Spice model would determine 'x'. Assuming the osc to be 50MHz or less, a PCB impedance of ~50 ohms, I'd suspect that x ~ 50-75 would give good signal quality and meet Vih requirements at the FPGA.

If power consumption was even remotely close to the case that the OP was has in mind he wouldn't bother with any of this...he would've replaced the 5V osc with a 3.3V one. Presumably there is some reason for even wanting to have a 5V osc on the board in the first place, but minimizing power consumption is not the reason.

Kevin Jennings

Reply to
KJ
Loading thread data ...

Correction to previous post. I'd suspect 'x' to be ~25-40, not 50-75.

KJ

Reply to
KJ

Just to belabor a point: When the output side of a transmission line is parallel-terminated, there is no requirement to also properly terminate the driving side. Using a "wrong" series resistor to adjust the amplitude is ok, since there is no signal coming back to the driver, and thus no need for proper termination at that end. Peter Alfke

Reply to
Peter Alfke

essentially

No. It is a transmission line with an impedance matched driver. You can get any frequency accross that setup that you like.

The setup will not change the rise time of the signal. The slew rate will be halved, because the voltage swing is halved, but that's it. The receiver will see the same waveform as the source with halve the voltage.

But other posters where correct with noting that the oscillator needs to be capable to drive a 100R transmission line, which might not be the case.

Kolja Sulimma

Reply to
Kolja Sulimma

If matched with the characteristic impedance it works like a transmission line, if not it looks like a capacitor or inductor depending on the termination.

The phone system (US, anyway) seems to use 600 ohm termination on about 100 ohm UTP cable. The result is that the cable looks like a capacitor and, for long lines, results in a high frequency drop off.

formatting link

The telephone solution is loading coils that increase the series inductance (on average) resulting in a flatter response to the required 4kHz, and a sharp drop after that.

The effect also shows up in antenna design when the frequency doesn't match the appropriate length for the antenna elements.

As previously stated, though, it is only necessary to match one end. If the sink impedance is matched the voltage will be reduced appropriately by the series source resistor and the cable impedance as a voltage divider.

-- glen

Reply to
glen herrmannsfeldt

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.