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Crystal oscillator connection with frequency synthesizer

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jocarreira

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Hi,

I have a very basic question, I think... I have a Synthesizer (Silabs si4136) and I want to connect this crystal oscillator (CX49GFWB) to provide the reference frequency.
How should I connect the crystal to the synthesizer? One pin from the crystal connects to XIN pin of the synthesizer and the other pin of the crystal connects to ground?
Is it as simple as that? Will it work?

Thanks
 

Si4136 doesn't have an internal oscillator, where you can just attach a crystal to oscillate.
The chip requires an external clock oscillator, and XIN pin is the clock input (AC coupled).
 

Data sheet shows a "RF BUFFER AMP" in the block diagram, which would imply they can handle a sine wave reference and the amp will square it up for the dividers, or a digital reference. Later they show an external capacitor required on the Xin pin, so there is probably a DC resistive divider network inside of the chip to provide the correct DC operating level.

I do not see anywhere where they specify how big the signal on the reference can be. I would assume it might run fine with Xin from 0.3 to 3 v peak to peak.
 

So, are you saying I should use a clock generator to feed the XIN input? I will choose one that works with +3V (or +3.3V) since that is the voltage I will use for the synthesizer.
 

jocarreira said:
So, are you saying I should use a clock generator to feed the XIN input? I will choose one that works with +3V (or +3.3V) since that is the voltage I will use for the synthesizer.
Click to expand...

Yes, either a sine wave with 3Vpp, or a square wave with 0/+3V. You would not want to drive it with a voltage higher than your VCC rail, so if VCC=3.3, you want something a little less than or equal to. If using an external source, like a lab oscillator, you want a little margin in case you bump the knob, so 3V.

But since this chip has an amplifier on the Xin port, it could be less. Just don't know how much less.
 

The external signal level shall not overpass the Absolute Maximum Ratings for XIN (spec on Table 2, page 4)
 

I will use a 555 timer with a Vcc=+3.3V (the same Vcc for the synthesizer). It has CMOS levels so I will be able to connect directly the output of the 555 to XIN pin.
The only thing I see that I have to be carefull is the oscillation frequency, because the synthesizer has a minimum of 2 MHz.

Thanks for your help :)
 

Yes you can start that way. But realize that the frequency out of the 555 timer chip will not be very stable. The PLL will effectively multiply up the frequency jitter of the reference by a big number, so the resultant microwave or high-RF frequency may look pretty bad.

There is a chance that if you drive Xin with a square wave, that the PLL chip will work below 2 MHz in.
 

Do you think the LTC6908-1 from Linear Technology is better than the 555 timer?
Is more expensive, but the 555 timer work to 3 MHz (maybe 5MHz) the most, and this LTC6908-1 can go to 10 MHz, so I would have better working margin, if the synthesizer doesn't respond well to 2 MHz input reference frequency...
 

Assuming you are talking about bench testing, and did not have a high quality low frequency lab synthesizer to take the place of a fixed crystal reference...I would go with a tunable crystal oscillator. Something like this from digikey:

CTX721CT-ND

You can drive the Xin port directly. But you can slightly tune the XIN frequency with a control voltage. Being able to tune the XIN frequency is a useful thing--you can vary the frequency ΔF slightly, and see if the VCO frequency N*ΔF follows indicating lock. You can modulate the XIN frequency from 10 Hz to 1 MHz, and see where your control loop bandwidth ends by watching the modulation sidebands on a spectrum analyzer.
 

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