[time-nuts] 10 MHz notch filter

[Gerhard Hoffmann]

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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