[time-nuts] 10 MHz notch filter

[Magnus Danielson]

Hi,

Be careful to trim this to be as balanced as possible, since inbalance
between side-bands allow for linear AM-to-PM conversion.

You could even use a AM-modulated signal and then detect the strength of
the PM/FM signal and tune for minumum leakage. You can also swap the
modulation modes between source and detector, as the leakage from PM to
AM is the same as from AM to PM.

This will not be as important in the deep notches, as to the sides of them.

Cheers,
Magnus

On 2019-10-15 16:12, Gerhard Hoffmann wrote:
>
> I have just made an experimental 10 MHz notch filter for phase noise
> improvement.
>
>
> input - 50 Ohm to 400 Ohm transformer----- 400 Ohm line  ----
> 400-to50-transformer - output.
>
> From the 400 Ohm line to GND are 2 cheap 10 MHz crystals, one with a
> 35pF series trimmer,
>
> the other with series trimmer and 4.7uH. The xtal resonances thus are
> 10 MHz +1.6 KHz and -1.6 KHz.
>
>
> Transformers are 1:8CT  Pulse electronics CX2049LNLT   digikey
> 553-2287-1-nd.
>
> That's all.
>
>
> The pics are the frequency response of the filter and the effect on
> the phase noise of an R&S SMIQ.
>
> SMIQ was chosen as an easy victim.
>
>
> The filter costs next to nothing. It won't turn my MV89A references
> into BVAs, but gives at least some
>
> keyhole insights . There could be easily 5 crystal pairs for
> strategically chosen frequencies.
>
>
> For notches closer to the carrier the impedance level must be probably
> lower (1:4 or 1:2 xformers)
>
> That makes the notches less deep, but also less wide, so they cannot
> interact at the carrier frequency itself.
>
> The filter withstands quite a lot of power because the crystals do not
> resonate on the carrier frequency.
>
> Just make sure you do not sweep them with +30 dBm accidentally.
>
>
> regards, Gerhard
>
>
>
>
>
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