Franklin VFO FM TX built entirely on LtSpice

Wouldn't it be a good idea to start your own website with your transmitter designs?

Del

There's one problem with the last circuit. LtSpice simulation showing excessive current to the final transistor. 132ma something. Is it being overdriven?

Del

Del

Hi Ivan,
Yes, that sounds even better. It would be even easier than one would think to make a variable, temperture compensated cap using a bi-material principle.nThat does open up the mind for creative ideas.
BR Harry

Hi,
I imagine an air capacitor with two plates, one of them made from bimetal sheet. It changes its capacity strongly when the temperature changes and the bimetal bends. It would be possible to make a capacitor exactly compensating the coil drift.

BR from Ivan

Correct!

One other idea is to use a coil from another metal that has a different expansion coefficient.

X 0.000001 mm / degree C:

Brass = 19
Aluminium = 12.8
Copper = 10
steel = 7
Iron (grey) = 5.8
Platinum = 5
Silicon = 2.8
Stainless steel = about 5 (varies with quality/type)
Titanium = 4.8

Another method is using conductive paint on a stable surface, like glass.

Alternatives you can try are use a tuning capacitor with a negative temperature coefficient. Remember the old "compression trimmers"? Some had a brass screw down the center, which moved the plates (reduced the tension) when heated due to expansion of the screw.

I don't know if you have access to a vacuum jar, but I once experimented with vaporising copper (high current through thin wire) to deposit copper onto non-metal surfaces. I did that to make strain gauges. I put a schablon/mask on the material so the copper was deposited, much like a PCB. I used two elements, one to bind with any last traces of oxygen in the jar.

The jar was borrowed from a school laboratory where they hung an electric bell inside, pumped out the air and the ringing disappeared when the air was sucked out.

Just a couple of bits of info to get you thinking. Maybe you can get some ideas?

BR Harry

Yes Harry, it makes sense. A small coil has less surface area too so better the temperature isolation. The Franklin oscillator has no DC Current flowing through the coil so it should definitely improve stability right?

dare4444 wrote:Harry, do you think the stability would be excellent like 5KHz drift in 24 hours? I used a small 30nH coil and large tuning capacitance. Does this improve drift?

Hi Joy,
In general, coils used copper, which has a temperature expansion coefficient. So when the coils are heated they expand. Coils in air are also subjected to vibrations.

Capacitors, on the other hand, use different materials and can have a very low capacitance change with temperature.

This means that larger capacitors and smaller coils can give you a better long-term stability.

For my V5 FM transmitter I etched the coils on the PCB and that gave a very good stability.

Does this answer your questions?

BR Harry

Del

Harry, do you think the stability would be excellent like 5KHz drift in 24 hours? I used a small 30nH coil and large tuning capacitance. Does this improve drift?

Del