Fig. 2. The idea of including a switch (DPDT relay) to select band-pass or band-reject
Band-Pass or Band-Reject function
Continuous 190 KHZ - >30 MHZ tuning
RF relay switched, without diodes, varactors, small inductors
1:1, 4:1 and 16:1 transformers at the input-output to select different bandwidth
Fig. 3. A kind of variable inductor (stepped).
T1-T4 Preparation: Each transformer is wound with 0.5mm diameter enamelled copper wire with 10 bifilar turns. Cut two lengths of 1.20 cm each, twist together to 5-6 turns per inch and count 10 complete U-turns throught the core's two holes.
Separate the twisted wire and identify which winding is the A and which is B using a Ohm-meter. Twist the end of A with the start of B together to make the center.
Two of those impedance transformers connected in series have been checked for their frequency response. The -3 dB points are on 33 KHZ and 39 MHZ. The photos show the response of 2 transformers connected together.
Fig. 4. A kind of variable capacitor (stepped).
Photos of a completed PCB
Fig. 5. The complete funtional diagram. It doesn't show the preselector in/out switching which is included on the pcb.
Results so far: It is more expensive than a simpler design, because of some serious reasons, while not an overkill, but is it worth it?
Fig. 1. Out of the 4 choises, I have chosen those that do not require ground connection, because the PCB design is hard already and more special attention to the ground loops should be required.
Both inductors are wound with 0.5 mm diameter, approx 3 meters in length, enamelled copper wire. The taps position should be verified with an inductance meter because there is some variation on core permeability. Generally, precice inductance is not so important since the inductance step is so small.There is a thought that the firmware of the controller will measure the exact inductance and the true tuned frequency calculated will be shown on the display. What is important is their mechanical stability that will protect the tuned frequency drift from temperature, vibrations, even from sound waves. I have used a thick double sided adhesive tape between the inductor and the pcb and I will place a foam pad as a hat over each inductor, secured with a nylon screw at the middle. The same precautions should be taken as of a VFO coil for example.
Copyright © Makis Katsouris, SV1AFN. All Rights Reserved.