Wednesday, July 28, 2021

A little mind experiment about the coil - core - inductance relationship.

Q - What is the "Q" of a coil?
A - The quality of the coil.
Q - OK? Plain English please.
A - The reactance and the resistance divide the energy. A pure reactance has no loses. A pure resistance has a power factor of 1.
Q - OK? So what is the power factor?
A - Pure reactance PF = 0. Pure resistance PF = 1.
Q - How do we determine the PF?
A - We would use vectors or phasers or trig factors.
    The simple math for Q is X / R. So if R = 0 Q is infinity. This is why the wire size, wire type and core material matter.
    Now on to the mind experiment part 1:
    Assume we wind an air core coil for my oscillator. It would have a Q determined as any other air core coil. Assume we set the circuit to oscillate at 4 MHz. All is well but it is fixed frequency. Now I want to lower the frequency by inserting a ferrite core. As I insert the core the frequency lowers. Assume the frequency changes to 3 MHz. This would be a small change from 30uh to 15uh with a 100pfd cap. What happens to the Q? Remember Q = X / R. The wire has not changed so R remains the same. The core causes the X to increase so Q increases.
    Now on to mind experiment part 2:
    Here is the real question to consider.
    The air core coil is 15uh.
    The core raises it to 30uh.
    I place a magnet against the core and it saturates, what happens?
    The relative speed at which the field expands or contracts is the variable.
    Consider the speed of sound in air and in water.
    Consider the speed of light in air and in water.
    When we place a straw in a glass of water it appears to bend, why?
    The basic laws for motors and generators tell us the field strength and relative motion determine the motor input requirements or generator output.
    Mind experiment final:
    The air core core has a Q of some value.
    The metal core has a higher Q.
    Assuming the air core Q is high enough for our circuit inserting the core should be ok.
    Saturating the core makes the core become "invisable" or in other words it will make the coil to act the same as the air core version.
    So in theory can I adjust the oscillator frequency by adjusting the bias on the core.

Thursday, July 22, 2021

Variable inductor controlled by a pot.

 A weird idea bouncing around in my head. It goes like this: I can use a pot and diode to acts as a variable cap. Why not use a pot and coil to act as a variable inductance. We have magnetic amplifiers which are basically variable inductors controlled by a signal applied to a control winding. On to the sims.

A simple 2N2222 oscillator. This one does not seem "strong enough". I think we need more output to drive our DBM.

The lambda diode oscillator is much stronger. Could be a good choice.

Ok , now we need a control winding and an out put winding. 

The sim supports the theory.

The sim supported the theory using Andy's oscillator. I wound a core with 3 coils, 1 turn, 3 turn and 16 turn and got the values I was looking for.

Time for a build when I get some bench time?


Thursday, July 1, 2021

Converting the Andy Osc to a GE transistor

 My young advisor looked the previous post over and gave me two points to consider. The transistor I used was an audio transistor which is not capable of operating at RF frequencies and we should consider using GE diodes with the GE transistor.

Both points are well taken. I do have some transistors to try but am woo-ful-ly lacking in data and the SPICE models. I did find I have a model for the P416 RF amp.

The original circuit.

The AC128 appears to work well. Sadly it has a very low Ft.

Still functions with one 1N34.

Second 1N34 'breaks' the circuit. Why?
Consider the voltage drop across the 220k resistor.

The current is to high and the 220k resistor is dropping to much. Let us try a smaller resistor.

OK. the 10k resistor restores our circuit operation. 
Next step is to find a suitable core.
I have a green core (43) that should work in the short wave range. The coupling being tight we will have to observe the loading effect. That will be the next post.

Monday, June 28, 2021

The Andy oscillator low voltage version

 My friend gave me two versions of this oscillator to use in the 40/80. They both gave a flat output over their operating range. I have a 6 volt lantern battery which was damaged (dropped I suppose). I took it apart and found one cell was shorted. The connecting strap was against the zinc shell. So I have 3 1.5 volt cells about half again as large as a D cell. I'm thing a 1.5 volt radio. A direct conversion receiver with an oscillator feeding a DBM and a a hearing aid amp audio section. 

This is the original circuit.

This is what I'm starting the build with. I am using a germanium transistor for 1.5 volt version. 

The number 1 issue with the direct conversion set is the oscillator will feed the antenna so I am thinking a ferrite core to help reduce undesired coupling.

Time to try a build.

Sunday, May 9, 2021

Using a piezo element

The High Z element will require a high voltage low current drive. To control the frequency response we must adjust the coupling caps and the load resistance. The high R high C circuit gives good low frequency response but drops off on the high end.

Lowering the load resistance flattens the response.
Lower coupling caps cause the low end response to drop off. This will make a 'tinny' sound.      

Thursday, April 15, 2021

Turret board amp upgrade.


I will use the double row layout. First step is to layout the components.

I laid it out in spice so i could test the circuit. The sim works so I didn't miss any jumpers or cross fed any points.

My first build worked very well but left room for improving the layout.

I may want to put it in a box. The components on both side and the transformer mounted on the turret pins will not fit a box well.

This will install in a box much easier. The transformer on a separate board will allow it to be placed so the jack is accessible.

The little amp works nicely with a DBM.
I wound 9 turns of 3 strand wire. One coil is the primary. The other 2 form a tapped secondary. The taps are the AF output. RF and LO feed the primaries.
The platform is a pill bottle cap. The base is a piece of press board. I used turret pins to mount the diodes. I had the terminal strips on hand so I used them for the I/O connections. Turret pins would work just as well for the I/O.
This setup received a station in Texas and one in Tenn.
The 80 meter ham band was active.
WWV on 5Mhz.
Several shortwave stations around 6Mhz.
This was using my bench signal generator as the LO. Next step would be to build a generator.