CDC7005 EVM - VCXO output

As you correctly pointed out, you can get the VCXO-OUTB signal via connector J21 (closing 1-3) and drive the CDC7005 VCXO input with the remaining VCXO_OUT signal. Notice, that this might have some performance impact, as the differenctial  VCXO input of the CDC7005 gets only half of the signal.

Also, keep in mind to correctly terminate the redirected VCXO_OUTB signal at the end of the line (50 ohm into Vcc-2V).

Regards, Georg.

Thank you, Georg.

I will introduce the redirected VCXO_OUTB into a circuit with a 50 Ohm input impedance.

Regarding the remaining VCXO input, I have seen in the Figure 9 of "Application Report SCAA063A – March 2003 – Revised January 2004" an schematic about how single ended VCXO can be interfaced with CDC7005. Is it necessary? Is the PCB adapted for such implementation?

Good point! The "open" VCXO input of CDC7005 need to be bias to 2V. The PCB has a resistor devider implemented (130/82ohm) which need to be turned (82/130ohm) to generate the righ bias voltage.

Regards,

Georg. 

Hi Georg,

I'm a bit confused about the required bias at the VCXO input of CDC7005. You say that a 2V bias is needed. However, in the user guide it is said the proper termination is 50 Ohm to (Vcc-2V), which is achieved with the 130/82 Ohm divider. Then, do I really have to swap R62-R64, and R58-R59?

As a first step, I have closed 1-3 of J21 and I have connected J24 (through an SMA connector) to an ocsilloscope (50 Ohm load). The VCXO led is off, and I don't see the VCXO output in the oscilloscope.

Thanks.

 Hi,

you are right, there is no need to swap the 130/82 ohm divider (R58/R59, R62/R64), I mixed it up with AC coupling but it's DC coupled (C59/C61 are not implemented).

As for no VCXO signal, the reason there is due the AC coupling (C60=10nF in J21-J24 path). In case you keep the AC coupling, a 150 ohm resistor (next to VCXO output) is needed to bias the LVPECL output of the VCXO as well as provide a dc current path for the source current. Alternatively, you can change AC coupling to DC coupling by replacing C60 with 0 ohm resistor.

Let me know if this helps.

Regards,

Georg. 

Hi,

I think it is better to keep AC coupling, replacing C60 with 0Ohm could be dangerous when connecting to other circuits, as there is no DC block. However, there is no space in the PCB for sodering a 150 Ohm. What can I do?

I understood from the user guide that the VCXO output could be extracted without PCB modifications,  as in section 3.3.6 it is said that "The circuitry lets the user: Measure the on-board VCXO output from outside on J23 and J24 (close 1−3 of J21 and 1−3 of J22)."

Maybe I can use VCXO_OUT instead of VCXO_OUTB, as there is a resistor R63 connected to Vcc that can be used for proper biasing. Is this possible?

Thanks

Hello Georg,

I would like to know if TI has another board with the capability I asked you before. I mean, a clock sintetizer with the possibility of extracting the VCXO signal.

Thank you very much.

Best regards