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.
Wednesday, July 28, 2021
Q - What is the "Q" of a coil?
Thursday, July 22, 2021
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
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.
Monday, June 28, 2021
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
|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.|
|Lower coupling caps cause the low end response to drop off. This will make a 'tinny' sound.|
Thursday, April 15, 2021
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.