Re: Clocks and baluns

Prune wrote:
Hi, thanks for the fast reply.
I was concerned about input capacitance of the IC making a lowpass with added resistance and limiting slew rate, though it looks like it's only 2 pF... The clock is specified for HCMOS output, 15 pF and <5 ns risetime. Not sure how performance of the clock's output stage would be affected by a non-standard load.
If I do use a resistive divider, would it be best to put the clock right next to the IC with minimum PCB track in between, just the divider?


Please post underneath the previous text, makes it easier for others to follow (and respond).

It doesn't really matter but if you place it far away you must calculate the resulting input impedance and size the traces accordingly. IOW so they have the correct characteristic impedance. It is generally best to place RF stuff close together in order to minimize EMI.



Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in
news:u9uoj.6719$Rg1.6021@xxxxxxxxxxxxxxxxxxxx:

Prune wrote:
Question for the RF-knowledgeable:
I intend to use AD9510 for clock distribution, as it has fery low
additive phase noise. My problem is, according to the datasheet
performance is degraded if input exceeds 2 V peak-to-peak, and the
low jitter TCXO I am considering has the typical either 3.3 V or 5 V
output. Someone suggested I use a 2:1 or 3:1 balun, but I'm
concerned about bandwidth, since the clock is square wave output
(because the AD9510 datasheet says it needs high slew rate on the
inputs for optimal performance). I'm also not sure how to handle
transmission line effects. With a PCB track impedance is easy to
control, and I normally use series termination resistors at the source on high speed lines to get a decent waveform, but I'm not sure
what inserting a balun will do, how to choose the balun, and where to
locate it for best performance (at the clock or at the distribution
IC's input). The TCXO I'm using has about -110 dB phase noise at 10
Hz offset from the fundamental of 24 MHz, and I'd like to not get
more than 5 dB increase from the overall distribution
circuit--transformer+AD9510. What's the best approach?

Why not just use a (symmetrical) resistive divider?




--
Regards, Joerg

http://www.analogconsultants.com/
.

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