[time-nuts] Re: RCB-F9T Adapter PCB with USB and 50 Ohm Timepulse SMA Connectors

[Carsten Andrich]

On 24.08.22 10:05, Erik Kaashoek wrote:
> Carsten,
> Forgive me asking an uninformed question
> The LM27761 switches at 2MHz and is claimed to be low noise as the 
> switching is so fast but I expect it radiates at 2MHz and all harmonics.
> You are looking for a 1ns rise time, probably to avoid diluting the 
> 1PPS accuracy, so the moment the 1PPS signal causes the line driver to 
> switch must be very well determined and stable
> How do you avoid these harmonics to interfere with this switching?
> [...]
>
> On 24-8-2022 9:19, Carsten Andrich via time-nuts wrote:
>> I'm using the LM27761 on my RCB-F9T adapter board. See my initial 
>> post for the layout. 
>
Hi Erik,

don't hesitate to ask these kinds of questions, as explaining makes me 
rethink my design*. Look at my schematic/layout while reading my 
explanation.

As a charge pump, i.e., no inductor, radiated electro-magnetic 
interferce (EMI) is presumably not an issue with the LM27761. That 
leaves conducted EMI, for which it has an integrated linear 
post-regulator to minimize output ripple, see Figs. 1 and 8 in the 
datasheet [1]. In combination with the BUF602's >30 dB power supply 
rejection ratio (PSRR) up to 10 MHz, the amount of switching noise on 
the outputs is probably negligible.
*) I could probably add another ferrite bead (or one per BUF602) between 
the LM27761 and the BUF602s to filter fast transients. The LM's 2 MHz 
switching frequency is relatively low compared to, e.g., a 10 MHz 
rectangular signal or transients of a 1 PPS output signal. So the charge 
for switching low/high and potentially driving a 50 Ohm load during high 
states must anyway come from the BUF602s' local decoupling caps.

To minimize switching noise from leaking back into the +5V net, I've 
placed a ferrite bead at the LM27761's input. With decoupling caps 
between the ferrite and any IC's supply pins, the decoupling capacitors 
are the lowest impedance path for current transients. The ferrite bead 
significantly increases the impedance of the board's power supply 
compared to the local caps, so the ferrite kinda forces the transients 
into the decoupling caps. I generally place ferrites for power supply 
filtering and for anything analog or digital that may cause switching 
transients. Unlike LC filters, which can oscillate, ferrite-based RC 
filters are fairly safe. You "only" have to consider their added DC 
resistance, power rating, and saturation effects with increasing current.


> I'm asking because I'm experiencing some problems with leakage of a 
> 10MHz clock into the input of a fast comparator used for edge 
> detection and input to a counter, both on the same PCB, so I hope to 
> learn from you.
That could be a sign of insufficient or improper decoupling. Your 10 MHz 
clock may somehow leak back into your power supply and the comparator 
has insufficient PSRR to suppress the clocks transients on the power 
supply. More decoupling and ferrite beads (one per digital IC) should 
help in that case.

It may also be indicative of a general issue in your PCB design. Did you 
ensure proper grounding, considering all signal return paths? On that 
topic, I cannot recommend this video enough: 
https://www.youtube.com/watch?v=ySuUZEjARPY
It is truly the single most important video on PCB design I've ever watched.

Best regards,
Carsten

[1] https://www.ti.com/lit/ds/symlink/lm27761.pdf