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TLC2932A: IC with a standalone PFD like the TLC2932A

Part Number: TLC2932A
Other Parts Discussed in Thread: CD74HC297, CDCM7005

For my applications, I want to lock a vcxo to an external reference clock (10MHz). For this, I need a PFD to close the loop. The TLC2932A seems suitable; it has both a PLL and a VCO, but the two are not internally connected so I can just use the PFD. But the TLC2932A status is NRND. Are there any other parts which have a PFD I can with an external vcxo?

  • Hi Mike,

    If all you need is a phase detector that will work at 10MHz, there's a few PLLs tucked away under the Specialty Logic subheader. A promising candidate might be the CD74HC297, which includes both XOR and edge-controlled phase detectors (along with some other digital PLL stuff, which can probably be ignored for your application). Admittedly the datasheet is dodgy about the frequency range at typical modern LVCMOS voltages, but I expect that 10MHz should be achievable at 3.3V. The phase detector specifications on the logic PLLs are propagation delays and rise/fall times, so I wouldn't expect spectacular performance, but if all you need is a frequency lock they should suffice.

    For higher performance, you will likely end up paying more, or needing to program the PLL (as most of TI's PLLs are now programmable), or both. The CDCM7005 is an (expensive!) option for external VCXO-based PLLs that requires a SPI interface to configure.

    There may be a better option depending on your application. Can you tell me more about the clock tree for your system? (EDIT: I think I saw some discussion of what you're trying to do in the E2E thread below... still curious if you have more in your clock tree that might be supported by an alternative.)



  • Hi Derek, I've attached a basic block diagram of what I'd like to implement. The only outputs I need are at 10MHz. Among the slave devices is an FPGA, which can can derive any other clock frequencies I need from the 10MHz input.

    Thanks for pointing out the specialty logic page. I was aware of the CD4046, but didn't know there were updated high speed versions in HC/HCT/LV logic families. Those might be perfect for this, if they're fast enough.

    The CD74HC297 is new to me. Looks like the ECPD is a JK-FF based phase dectector. That might be sufficient for use with a vcxo though.

    Do any of these parts have a current source charge pump output? Otherwise I'll probably have to use an active filter between the PFD and the VCXO. 

  • Hi Mike,

    Thanks for the update. Most of our integrated oscillator solutions were designed for video or PCI/DDR so I'm pretty sure anything else I could recommend you from the clocks and timing portfolio would be the wrong frequency range, or just way more work to implement and higher cost than the specialty logic PLLs.

    The specialty logic PLLs predate all of TI's current source charge pump output devices by several years, so you will probably need that active filter. Fortunately there are quite a few practical implementation options for a VCXO-based PLL follower, since you probably want the loop bandwidth to be small (~100Hz) so the VCXO noise dominates across most of the carrier offset range. And an active filter would still be less expensive than a modern device from TI.

    I looked around at other vendors as well, but most vendors (TI included) trend heavily toward integrated oscillators. There are a few options for external VCXO, but which require programming, or their age and low usage drives their price up considerably. I think high-speed CD4046B- or CD74HC297-based approach is your best bet.


  • Agreed Derek, I will buy a couple HCT4046 and HC297 to test. Thanks for your help.