[time-nuts] Low-pass Filter for 5 and 10 MHz

[Rick Karlquist]

Luciano Paramithiotti wrote:
> A simple low pass filter to cut second and third harmonics from a 5 or 10
> MHZ signal.
> See the paper:
> http://www.timeok.it/files/5_and_10mhz_low_pass_notch_filter.pdf
>
> Luciano Timeok

It is easy to use filter software to design a filter like this,
but how well it actually works is highly dependent on the
quality of implementation.  Suppression is a function of the
Q of the resonators, the tuning tolerance, and possibly the
tempco.  Tuning tolerance gets into the issue of tuning granularity.

In the 5071A, what I elected to do is design the filters to use
off the shelf Coil Craft air core inductors and pairs of standard
value capacitors in parallel (coarse and fine).  The capacitor values
were chosen to account for PC board parasitics.  I lived with whatever
suppression I could get under these constraints.  I then designed
a filter with multiple traps per frequency such that I could achieve
100+ dB suppression.  This is not a minimum parts count design.
These boards were automatically loaded with components off a tape
and they simply worked when we turned them on.  Every time.
When it came to getting complaints from the production engineer,
I was as lonely as the Maytag repairman.  Nothing ever went wrong.
Compare this to the finicky 5061 frequency multiplier.
Filter software (at least what I have seen) doesn't do this
type of design.

Rick Karlquist N6RK
(5071A RF designer)