[time-nuts] FE 5680A "new version" - Filtering the 10 MHz
John Ackermann N8UR
jra at febo.com
Tue Nov 15 16:18:15 EST 2011
If the problem is only harmonics (cleaning up a square wave), a simple
LC low pass filter with a cutoff midway between the fundamental and
second harmonic might be a better choice because it minimizes the
tempco-related phase shift that either a high Q filter, or one with a
cutoff near the desired frequency, can exhibit.
On 11/15/2011 11:21 AM, ed breya wrote:
> You can add a crystal filter using a common 10 MHz crystal in series
> with a low noise, high impedance amplifier. Terminate the input side
> with about 50 ohms, and set the Q of the filter with the load impedance.
> Since you're only worried about harmonics, the Q can be fairly low,
> making the exact crystal characteristics (frequency precision
> especially) less important. Extra components can be added for more
> sophistication and compensation for parasitics (especially parallel Co),
> if needed. More stages will make it even cleaner, but the amplifier
> noises will add up. If there is enough signal power available in the
> original output, you can leave out the amplifier and use an impedance
> matching transformer instead, avoiding the amplifier noise.
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