Type of Crystal as reference with ADF4157 PLL Synthesizer

[jonnasi]

Hi,

I am trying to design a PLL Synthesizer using Analog's ADF4157 PLL chip.

As a reference I am going to use a TCXO, but Im not sure about the difference between Clipped Sine Oscillator and HCMOS Oscillator.

In the datasheet they state the input can be driven from a TTL or CMOS crystal, or it can be ac-coupled.

If I use HCMOS osc. will I not use ac-coupling? Or is ac-coupling for clipped sine wave osc.

Im a little confused, can someone point out some articles/reports/tutorials/other? about this. I am supposed to write a report about this design, so it would be nice to get some information that I can refer to in my report.

 

[jonnasi]

Re: Type of Crystal as reference with ADF4157 PLL Synthesize

Nobody?

 

[G4BCH]

Re: Type of Crystal as reference with ADF4157 PLL Synthesize

If you look at figure 6 you will see that the input sensitivity on the reference input is down to below -20dBm, since no impedance is given it is up to the reader to work out actual input voltage. Since the reference input is internally biased to half rail this is the time that AC coupling is of most use i.e. at low reference input levels.
Your crystal oscillator may well have an AC coupled output so just connect it directly to the input. So long as the reference voltage is large enough to allow the chip to work it will not care, just make sure that the voltage is not too large, see absolute max ratings. I would avoid TTL levels and use a 3V oscillator or use a pad to reduce the reference to a safe level. Sine or clipped sine will not matter the IC will clip the sine wave anyway.
I have always used the AD PLL chips fed from a 50 ohm external source so terminate my unit's reference input with 50 ohms and use AC coupling.

 

[biff44]

Re: Type of Crystal as reference with ADF4157 PLL Synthesize

Yep, I agree. The key is to see if your output of the oscillator stays lower than the supply rail, which is probably 3.3 V for this chip. If you have a 5 volt output on the oscillator, you will have to resistively pad it down before applying it to the chip.