Caps and Resistors for RF applications

[O. P. E.]

I am trying to make the ns pulse generator in Linear's App Note 98.
The basic idea of the circuit is to use comparators to produce a
delayed pulse and an inverted pulse, and then use an AND gate to
produce a pulse. It uses a resistor and capacitor to form the delay
lines. It specs 8pF caps and the circuit also has a variable resistor
in it.

From the looks of it, the caps and variable resistor do not appear to
be in the RF "side" of the circuit. Still, I am planning to use

surface mount components to reduce RF problems, and I was thinking of
purchasing some "High frequency" ceramic caps, as Mouser has some from
AVX and Murata. I am wondering whether these parts RF properties will
be so different from that of "standard" ceramic caps as to cause
problems? (The app note and associated articles do not specify the
caps beyond 8pF).

Also, I was planning to use a surface mount pot to reduce RF problems
with the leads. Since pots are relatively large compared to other
components, I was wondering if it is likely to cause problems. Would
wrapping it in AL foil help?

Thanks

 

[Lostgallifreyan]

"O. P. E." <mleigh@gmail.com> wrote in
news:1176319237.355422.172020@d57g2000hsg.googlegroups.com:

Also, I was planning to use a surface mount pot to reduce RF problems
with the leads. Since pots are relatively large compared to other
components, I was wondering if it is likely to cause problems. Would
wrapping it in AL foil help?

I think that would just be a new source of stray capacitance. Some SMT pots
are very small, so it's worth looking for one, if it's a preset you want.

 

[LVMarc]

O. P. E. wrote:

I am trying to make the ns pulse generator in Linear's App Note 98.
The basic idea of the circuit is to use comparators to produce a
delayed pulse and an inverted pulse, and then use an AND gate to
produce a pulse. It uses a resistor and capacitor to form the delay
lines. It specs 8pF caps and the circuit also has a variable resistor
in it.

From the looks of it, the caps and variable resistor do not appear to
be in the RF "side" of the circuit. Still, I am planning to use
surface mount components to reduce RF problems, and I was thinking of
purchasing some "High frequency" ceramic caps, as Mouser has some from
AVX and Murata. I am wondering whether these parts RF properties will
be so different from that of "standard" ceramic caps as to cause
problems? (The app note and associated articles do not specify the
caps beyond 8pF).

Also, I was planning to use a surface mount pot to reduce RF problems
with the leads. Since pots are relatively large compared to other
components, I was wondering if it is likely to cause problems. Would
wrapping it in AL foil help?

Thanks

Depending on the RF freq uencies you use, the smt pots may not be a good

idea! The size and convulted leads add inductance, at some point the jwL
impedance becomes significant..thats your personal cutooff frequency. if
you below that go for it, if not use RF resitors and caps.

what makes the rf labeled r and c different? these parts all have
parasitic elements. caps have hidden resitors and inductnce,, inductors
have hidden lead to lead capcitors, so these also have a

self resonant frequency. thats the indivduals part's limit! so your "nS"
pulse sytem needs a clear DC - > 1GC frequency......

Pulsed applications also need "low esr" to be able to respond to the
transtional state where the impulse from the leading edge occrus.

Best Regards and good luck

Marc