Op-Amps at high frequencies

[Roff]

Sorry for not replying any sooner. Well That's basically what I'm doing but the thing is the least prescaler value for the 5MHz input for this PLL IC is 8 so I am basically doing what you're suggesting but my prescaler values are different.

One more thing..You mentioned (and it says in the datasheet) that the prescaler has an ECL output. I checked it out and it turns out it represents zeros and ones by negative voltages? So if I pass that output through a capacitor, I'll just get the normal sine wave (without the negative DC shift) and that would be suitable as an input for the PLL IC, right?

The PLL has an internal feedback resistors around the input buffers. This automatically biases them at the middle of their active range. All you need is a capacitor (100pF or more?). I couldn't find any info about capacitor size.

 

[Roff]

I was thinking of a part like the ADF4360-8. I'm not suggesting this is the best choice, and there may be reasons that it won't work, but it appears to have what you need:
1. A VCO that works up to 400MHz
2. A feedback scaler that will divide by 63
3. A feedforward scaler that will divide by one, i.e., no division of the 5MHz reference frequency.
I have not worked with PLLs in years, and I suspect there are quite a few parts on the market that will work, unless you have some requirements that I don't know about, such as some unique property of the VCO you have chosen (e.g., low phase noise).

 

[YAN-1]

Well thanks a lot! I will definitely look into it.

Nichola

 

[Roff]

Well thanks a lot! I will definitely look into it.

Nichola

If it suits your needs, I would continue searching for better (smaller, lower power, lower cost) parts if I were you. I found that part with very little effort - I think my original search (can't remember what it was for) led me to Analog devices. From there I just went to their product tree and followed the links.

 

[YAN-1]

Well I sure will. Thanks a lot. It's just that I was running out of time and needed to get this thing working with the ICs that I already found. I will improve on it.

 

[YAN-1]

Just checking, from the datasheet of the MC145151-2 (page 17), the PDout signal is a pulsating signal between VL and VH. I just wanna make sure that VL is 0V and VH is 5V (assuming a 5V VCC) and that it stays at around 2.5V when frequency locking occurs. I wanna make sure since the VCO takes a positive input tuning voltage and it would be perfect if the Pdout levels are as I think they are since the VCO gives around 315MHz at a tuning voltage of 2.5V. I'm asking because it says in the datasheet of the MC145151 that the PDout gives either positive or negative pulses depending on the correction that needs to be made (page 4). I just hope that 'negative' is not literally negative. Please take a look at the datasheet if you can and confirm my interpretation.

Thanks a lot.
Nichola

 

[Roff]

Just checking, from the datasheet of the MC145151-2 (page 17), the PDout signal is a pulsating signal between VL and VH. I just wanna make sure that VL is 0V and VH is 5V (assuming a 5V VCC) and that it stays at around 2.5V when frequency locking occurs. I wanna make sure since the VCO takes a positive input tuning voltage and it would be perfect if the Pdout levels are as I think they are since the VCO gives around 315MHz at a tuning voltage of 2.5V. I'm asking because it says in the datasheet of the MC145151 that the PDout gives either positive or negative pulses depending on the correction that needs to be made (page 4). I just hope that 'negative' is not literally negative. Please take a look at the datasheet if you can and confirm my interpretation.

Thanks a lot.
Nichola

The pulses are positive and negative relative to the average voltage on the phase detector output (the voltage on the loop filter capacitor). The voltage will always be between zero and vcc. The actual voltage when locked will be whatever voltage is required to pull the vco to exactly 315.000MHz (assuming your reference is exactly 5.000...MHz).