Hi guys, this is probably a very stupid question but I have a circuit that uses a crystal oscillator for a microcontroller clock signal. The output of the XO goes directly to the micro but there is a capacitor and resistor connected at the output of the XO. They are in parallel and both connected to ground. What are these components doing in this case? filtering?

 

Are you talking about this configuration:


The capacitors are required to load the crystal and are described in every application note.

 

There is often,but not always, a high value resistor across the the connections to the IC. The reason is that most CMOS devices need a return connection for all of the input connections. If those high impedance inputs do not have a current return path their voltage will wander to a value that will cause undesired operation, which is often drifting into a linear operation point, with unwanted results.
The capacitors to ground are there to adjust the circuit impedance at the operating frequency to the values that provide the best waveform and stability. The application literature may discuss that, or it may just recommend specific values. My experience has been that using the wrong values causes the frequency to be incorrect and the waveform to be very distorted.

 

Resistor improves phase relations for oscillation. Also, the inverting gate at the inlet increases the amplitude and the edges are shorter. This reduces the chip consumption current.

 

There is often,but not always, a high value resistor across the the connections to the IC. The reason is that most CMOS devices need a return connection for all of the input connections. If those high impedance inputs do not have a current return path their voltage will wander to a value that will cause undesired operation, which is often drifting into a linear operation point, with unwanted results.
The capacitors to ground are there to adjust the circuit impedance at the operating frequency to the values that provide the best waveform and stability. The application literature may discuss that, or it may just recommend specific values. My experience has been that using the wrong values causes the frequency to be incorrect and the waveform to be very distorted.

This makes sense. The resistor value is 10Mohm. I guess it doesn't look like it is for the IC because on the schematic it is located directly at the output of the XO rather than the input of the micro, which is in a different section of the page.

 

To maybe illustrate what I'm talking about here is a snipping of the circuit. The 15pF cap is described in the XO datasheet as the output load capacitance for 3V3 supply.

10Mohm seems very high for a pull down resistor on the clock input of the CPLD IC the oscillator connects to. So what am I missing? I can't find any documentation that supports this setup.

 

To maybe illustrate what I'm talking about here is a snipping of the circuit. The 15pF cap is described in the XO datasheet as the output load capacitance for 3V3 supply.

10Mohm seems very high for a pull down resistor on the clock input of the CPLD IC the oscillator connects to. So what am I missing? I can't find any documentation that supports this setup.

That 15 pF cap is NOT a component you need to connect to the oscillator; it represents the load conditions under which the oscillator is tested. It does not represent a real circuit component. Note on the datasheet, the diagram showing it is titled "Test Circuit." Regarding the 10 MΩ resistor, I've no idea where that came from.

Eliminate both the 15 pF capacitor and the 10 MΩ resistor. They shouldn't be there.

 

That 15 pF cap is NOT a component you need to connect to the oscillator; it represents the load conditions under which the oscillator is tested. It does not represent a real circuit component. Note on the datasheet, the diagram showing it is titled "Test Circuit." Regarding the 10 MΩ resistor, I've no idea where that came from.

Eliminate both the 15 pF capacitor and the 10 MΩ resistor. They shouldn't be there.

I'm afraid I can't do that as I didn't design the circuit and it has been qualified with these components included. I'd really just like to know why they are there. The people who did the design can't be contacted to ask.

 

I'd really just like to know why they are there. The people who did the design can't be contacted to ask.

I'm afraid the answer to that question is probably that the original designers weren't experienced enough to understand what the data sheet was telling them.

 

I'm afraid the answer to that question is probably that the original designers weren't experienced enough to understand what the data sheet was telling them.

Well, that'd be a first.

 

The circuit shown in post #6 is not even similar to the one shown in post #2. And none of my comments address that second circuit. Suddenly the comments are addressing a circuit that is quite different from the initially posted one. So while threads are allowed to wander a bit, this seems quite far from the original question.

 

Why don't you remove the capacitor and resistor and see if the circuit still works?
(That is usually not my recommendations. It is always best to know why a given circuit is designed that way in the first place.)

 

The way the original post was stated and the absence of specific information such as a schematic or part numbers, I assumed the TS was talking about a typical crystal oscillator circuit for an MCU.

Apparently, that is not the case. The TS seems to be using a specific crystal oscillator IQXO-71 made by IQD corp. Here's a typical datasheet:

https://www.iqdfrequencyproducts.com/products/details/iqxo-71-11-30.pdf

It shows the load capacitor for testing, but not the resistor.

 

This is simply an example of the designer not understanding "test circuit" as already pointed out in post #7 and post #9.

Firstly, the 10MΩ resistor in parallel is used as the feedback element in the oscillator circuit of the MCU. This is not required for the application in this thread.

Secondly, 15pF is the test load for characterizing the circuit under test conditions. Again, this is not required.

The crystal oscillator in question is an active oscillator module and requires no additional components except a 100nF ceramic capacitor across the power supply rails.

 

OK, I got it wrong this time, relative to which circuit came from the TS. The one in post #2 is the typical one that I am used to seeing in the less expensive assemblies. Using a separate oscillator module is different. The R and C shown with that circuit seem to be simulations of a scope probe, or a frequency counter probe.. Fooled me this time, my problem for not checking closely enough.