Target of "Frequency Stability" plus-minus 50ppm.

Hi GuenterL,

Thank you for your reply!
I could understand the +-50 ppm is only needed for the crystal.

Please let me ask some questions.
When customers use ADV devices with a XTAL which "Frequency Stability" is +-50 ppm,
how much "Frequency Stability" by the external related circuit (including Rdump and C etc.) is supposed to be allowed?
If you have any criteria of "Frequency Stability" of the external circuit, could you tell us it?
I read the AN-1260, but I couldn't read it.
Also, does it depend on the type of ADV devices? (For example, is it different between video decoders and HDMI receivers?)

My customer uses ADV7480 now.
He uses a XTAL IC keeping the "Frequency Stability" plus-minus 50ppm.
But he says the error/variation of the external related circuit is +25ppm(only plus 25ppm, not plus-minus) in room temperature.
So he wants to know how much criteria/range of "Frequency Stability" of the external circuit is allowed.
If it's difficult to mention about the criteria of the external circuit, could you tell us whether the "+25ppm in room temperature" of the external related circuit for ADV7480 is good or not?

Thank you!
Best regards.
Tamu

Hi Tamu,

To come up with an absolute answer to how the frequency stability is related to external crystal components would require full sensitivity analysis of the equations in AN-1260. I have not done this type of calculations since I was in colledge. Looking at the circuit however I believe the frequency sensitivity for the external R's and C's would be less than 0.1 compared to the internal crystal equivalent LRC's. So for external components with 25ppm would affective sensitivity ppm would be reduced to 2.5ppm.

If you are really worried about frequency accuracy then you have to consider the tolerance of the resistor you are using. Normally we use 1% resistors. This 1% would convert to 1000ppm when applied to the sensitivity calculations. And since I know from experience that 1% parts work just fine it indicates that the frequency sensitivity is a lot less then 0.1 against external components.

There is no criteria for external components other then to use the correctly calculated values and of the highest cost effective tolerance you can get. For resistors this would be SMD 0402 1% and capacitors would be SMD X7R 0402 10%.

On a side note if you go through the capacitor calculations, the calculation might require 45pF where the closest available part would be 47pF 10%. So now you would be using a potential cap with a value of 51.7pF (47pf * (1+10%)) or 14% over the calculated value of 45pF. This doesn't even include the possible temperature variations. And it still works fine.

In conclusion the external componenets are only needed to set the proper loading in the inverter circuit to begin oscillation. Since the sensitivity is so low the external part ppm has very little affect on the stability.

Bottom line from experience I've not seen any problems using good quality external components with the correct crystal on any of our video parts.

A comment about the ADV7480. As stated in the previous post crystal accuracy is most important when generating the color burst frequency. The ADV7480 does not generate any color burst frequency. It is a completely digital device. The MIPI output clock is derived from wither the HDMI clock or LLC pin. The audio clock is derived from the HDMI source.

25ppm external components are good for the oscillator circuit.