[HomeBrew] notch filter

[Richard Blackman]

Hello again to group.
I have decided to go ahead with a very simple filter, a .244uH coil and a 
20pF cap to ground in series. I have built the prototype and it gives a very 
deep null at 72 MHz by trimming either the cap or the coil. Trimming the 
coil gives a smoother steadier null as the changes are more gradual than 
when trimming the cap. I think that the  notch is good enough for my needs, 
and I will build a partitioned 3 pole filter if I find I need a lot more -dB 
gain.

BTW-  This is going to be part of a homebrew all band txr that I have been 
building for 10 years or so.
It is from the  book "How to build an intelligent amateur radio transceiver" 
by Randy Henderson, WI5W. It has been a very involved project. I found that 
the VFO, a motorized capacitor with an analogue to digital PLL feedback 
worked poorly and the motor did a lot of hunting  until the PLL  locked and 
the motor/ capacitor  stopped.  It produced 72 -102 MHz.  I decided to rip 
out the A-D converter / motor/combo  and I have built a PIC controlled VFO 
using a 9851 DDS chip from Analoge Devices. This  DDS VFO  MHz signal needs 
to be mixed with a 72 MHz carrier to bring the output from 0-30 MHz up to 
the required (and hi pass filtered)  72-102 MHz. This signal now goes out to 
the rest of the radio and further mixers and filters until  0-30 MHz is 
transmitted.
Therefore the need for a notch filter to remove the 72 from the output, even 
with the hi pass filter..

More to come.

Richard VA3NDO