Chers all !
How many of us dreamed about having some kinda extreme narrow preselector for CW contests when lower bands is full of bigguns and transceiver front-end is getting overloaded?
Well, I did it ! for 40 meter band and only some cw channels,otherwise it would be extremely expensive to be honest.
Basically it is just a crystal 3 poles ladder filter, channel selection relays, T/R relays, an Arduino controller and few t37-6 toroids for L-match circuitry for each channel.
Because the original design using a simple switch to change channel relays and wanted an Arduino afterwards, I needed an extra transistors board for Arduino interfacing.Also, a special care was to design T/R logic to avoid damaging X-tals if PTT cable is disconnected and transmitting 100 W straight into filter.To compensate this, I found that Icom has a T/R pin on DIN connector (apart of SEND RCA) ,pin that presents 6V dc when RX and zero when TX.
So was as simple as pie to use 6 V into NPN transistor base to create a TCVR detector.If cable not connected then filter goes into ByPass mode to send all the power to antenna.In the next future will implement Icom Ci-V protocol to change TCVR freq using filter rotary encoder when filter is enabled.
For each channel there are 3 crystals in series (ladder) and capacitors from each crystal terminat towards the ground. Capacitors value eas determined not by computing but using some variables capacitors at the beginning while testing bandwidth and insertion losses.After properly setting this values, carefully desolder variable ones and measure capacitance.Once measured, I replace it with fixed value by combining multiple SMD capacitors.
In this way , the more crystals we got , the more channels we can create, in my case 7000 ; 7008; 7013; 7018;7023;7028 and 7038 Khz.
7 crystal pairs (21 cristals ) not sorted were purchased from Ebay ,a bag of SMD , relays and other components from Mouser dot com and lost few hours to measure all and also developing Arduino software.
The video bellow is self explanatory, I let Icom to SSB filter while enabling / disabling my new gadget to observe real bandwidth, so all of you can conclude if is worth building it or not just for CW heavy contests.