Termination of clock output(for LMK04816)

Hello Jacky,

jacky lu said:

(1)The termination for dc coupled differential operation and ac coupled differential operation is different. while the dc coupled operation terminate to Vcc-2V, the ac coupled operation terminate to 2V( as the two pictures below show). Why these two voltages are different?

In the top DC coupled figure, the 82 ohms is connected to ground, which results in a thevenin equivilent voltage of Vcc - 2 V, and 50 ohms resistance.  When in operation, the LVPECL output is emitter follower and the voltage will not go below this Vcc - 2 V.  The common mode is approximately 2 V.

LVPECL is designed to operate with 50 ohm termination to Vcc - 2 V.

When AC coupled, the resistors are now setting the common mode and RF termination.  An LVPECL receiver expects a common mode of approximately 2 V.  The output DC requirement is achieved by using the emitter resistors (120 ohm).  This very similar to the next case.  Needing to terminate into an LVPECL input as shown is not common because many receivers support a wide range of common mode voltages or are self-biased.

Self-bias receivers (LMK04816 bipolar inputs are self-biased), so all that is required is to AC couple signal and provide good RF termination.

jacky lu said:

(2) For AC coupling, LVPECL drivers require a emitter resisteors to configure a DC path to ground. In some case, I find the emitter resistors is 120ohm(like the picture above), in some other cases the emitter resistors is 240ohm(like the picture below).  What is the difference?

As you show in the two AC coupled LVPECL images above... in one case the DC requirements of the LVPECL outputs are met with 120 ohm emitter resistors and in the second 240 ohm emitter resistors.

  - The traditional value for these to emulate the 50 ohm termination to Vcc - 2 V is 120 ohms.  However it is allowable to chose higher values to reduce current consumption of the device.  The penalty is reduced output amplitude.  With the reduced output amplitude, phase noise floor performance is typically still the same, only at lower output frequencies, approximately sub 100 MHz, is there a difference in phase noise floor performance.

    - Please note in the current consumption tables at end of datasheet, increased current for 120 ohm LVPECL vs 240 ohm LVPECL.

    - Please note in the typical performance charts, the 120 ohm has higher output amplitudes than 240 ohm.

73,

Timothy

Timothy:

(1) LVPECL is designed to operate with 50 ohm termination to Vcc - 2 V. An LVPECL receiver expects a common mode of approximately 2 V.

    In the first picture, the DC coupling make a termination to Vcc-2V , is it OK for the receiver?

(2)Self-bias receivers (LMK04816 bipolar inputs are self-biased), so all that is required is to AC couple signal and provide good RF termination.

    That means when the receivers is self-bias, I can use ac coupling and just bridge a 100ohm resistor between the positive and negative line, like the pic3(the third picture). Besides,pic3 use ac coupling both near the drivers and near the receivers,  must I use the two stage AC coupling ? Can it be more simple?

Thank you！

Regards

Jacky

2013-06-05