[time-nuts] 5>10 doubler

Charles Steinmetz csteinmetz at yandex.com
Wed Jan 28 14:28:27 EST 2015

Gerhard wrote:

>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.

Best regards,


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