[time-nuts] 5>10 doubler
csteinmetz at yandex.com
Wed Jan 28 14:28:27 EST 2015
>It is a different game when you want to notch away sub/harmonics.
One problem with using crystals as traps (notch filters) is that the
series resistance of a crystal is several orders of magnitude higher
than that of a good series-resonant LC -- generally in the 50-100 ohm
range. So, although the notch is very narrow, it will not be very
deep unless it is in a high-impedance circuit. For example, in a 50
ohm circuit (50 ohms looking each way, so 25 ohms at the node) you
will be lucky to get 3dB of attenuation. To get 40dB of suppression,
the nodal impedance would need to be at least 5k ohms, perhaps
even >10k ohms -- and the high impedance adds noise, which means
there is a phase noise penalty. Another problem is that the narrow
notches are prone to sliding off frequency with small temperature changes.
Also, while a 5MHz trap crystal will almost certainly be a
fundamental-mode resonator, that will probably not be true at, say,
30MHz -- so a 30MHz trap would most likely have a notch at or near
the desired output frequency.
Made with good, high-Q RF inductors (forget SM parts), an LC trap is
generally preferable to a crystal trap. There is still some
temperature sensitivity, but the greater width is much more
forgiving. At the same time, the Q is high enough that you don't
have to worry about effects 5MHz away when you are trapping
frequencies of 30MHz and below.
More information about the time-nuts