Designing Resonant Circuits

Inductance-Condenser-Design

This is a link to the 1930s  correspondance course that trained a lot of radio men on into the 1970s. How to design resonant circuits with performance in mind.

Radio designers handbook

This is a very large download but is worth the effort if you have a decent internet connection. Radio design from A to Z in this 1500 page book.

Combined antenna coil and V-cap to make tank

Put an antenna coil on the variable cap. The tuning knob (red screw) adjust from about 7 - 10 Mhz. When I put the amp and antenna on I got radio China and a couple of the usual stations in that range. I will make another amp for radio listening. The hearing aid amp works but I think it was desinged more for fidellity than gain.  I could put a preamp on it but the little box is crowded with three transformers in it and it works well for what it is.

double balanced ring detector

This is the circuit I was playing with using the dipmeter as the LO.

homemade selector switch.

 Building a tapped coil selector switch. I cut a thin slice from the end of a 3" x 3" block and laid out the taps every 30 degrees. I pushed a sequel pin through at each point. drilled the center tap to hold the armature and coil. Rolled a hook in the pin to solder the wires to.

This is the back side with the coil mounted. I ran a wire from the bottom of the armature for one side of the circuit. It is basically the same as the tapped toroidal coil I posted earlier.

note: The flat metal piece is a spring. Attaching a flattened #16 copper wire under it provides the contact. The spring will work but adding the copper makes a better job of it.

another switch here:

Playing with the dip meter

Using a grid dip meter as the LO and the hearing aid amp I'm driving a ring modulator circuit as a direct conversion receiver. Picked up Cuba, Fla, and Tenn stations. (some Spanish sounding station)
EDIT:
4 diodes
3 transformers
3 transistor amp
dip meter
I did a followup on this with Gilbert cell instead of ring mixer. 6 transistors and two resistors = Gilbert cell.

hearing aid amp using a single cell power supply.

I wanted a general purpose amp and built this one. The only thing I might would change would be to use a 100k pot for the input. With a single dry cell supply it will hurt my ear if I drive it to hard. I probably should put a pair of diodes limiters across the output.

How to make a frequency doubler

The question was asked. How to make a frequency doubler. Several answers were offered. The advice of one member was one test is better than the advice of one thousand experts. I kind of like the empirical method myself so here we go. What does it take to make a doubler? A signal source. Check i have a dip meter so far so good. A tank circuit. I took an inductor and a capacitor from the parts bin. Check have a tank. A way to clip the input to less than 90 degrees pulses. I have a diode, resistor and a battery. Check can build a clipper. Bread boarded the circuit like this.

This simple circuit puts a 1.5 volt battery potential across a diode in series with a 10 K ohm resistor reverse biasing the diode. As I drive the circuit my input will have to exceed 1.5 volts and forward bias the diode. If I can the wave forms will look something like this.

 This is actually from a transistorized doubler but the wave forms will be the same. As the diode switches on it will pass the peaks. I hope to get less than 90 degrees. My signal is adjusted by adding or removing loops on the dip meter coil. My test setup looks like this.

My son gave me the scope and I'm still learning how to use it. It does have a frequency reading at the bottom of the screen. I'll zoom in on it.

As you can see the frequency is just over doubled. If it were a serious circuit I would tweak it a little. I'll call it a proof of concept and set it aside for now.
EDIT:
The green waveform is not distorted as it seems. Every other peak 'shrinks' as power is drawn by the tank.