A Tuning Circuit

[GWS]

Hi folks,

This is an interesting tuning circuit which I've come across, that gives a wide tuning range.

To check it out, I knocked up a simple console program to do the sums.

It shows how useful Creative can be just to do a simple bit of calculation to check something out.

Two lines are commented  - these are the values when the tuning capacitance is fully open (highest frequency).

As the program stands at the moment, the values of C1 and C2 are for the fully meshed condition, and so give the lowest frequency.  C1 and C2 are a 500pF per section dual-gang capacitor - L is a 180microH inductor.

Just swop the commenting to see the two results.

I get 464kc/s for the lowest tuned frequency, and 1726kc/s for the highest frequency.  This should tune the Medium wave stations very nicely.

Here's the quickie program:

Code Select Expand


' Creative Console skeleton ..
openconsole
cls

def f,pi,l,c1,c2,cs:float
setprecision 2

pi = 4 * atan(1)

l = 180.0e-6 :' inductance value in microHenries

c1 = 500.0e-12 :' capacitance of one section of a dual-gang capacitor in microFarada - fully closed
'c1 = 25.0e-12 :' estimated capacitance when fully open


c2 = 500.0e-12 :' capacitance of the second section of the dual-gang capacitor
'c2 = 25.0e-12 :' estimated capacitance of the second section when fully open


cs = 220.0e-12 :' capacitance of the series capacitor connecting the two sections in microFarads


ctemp = (c2*cs)/(c2+cs) :' the effective capacitance of the series capacitor cs and c2 in series

print "Combined capacitance of c2 and cs: ",ctemp*1.0e12," pF",:print

c = (c1 + ctemp)

f = 1.0e-6 / (2 * pi * sqrt(L * C))*1000

print "Frequency: ",f," kc/s"

do:until inkey$<>""
closeconsole
end



The circuit is as follows ..

best wishes,

Graham

[aurelCB]

Hi sir Graham 
In my case how properly calculate coil and capacitor that cover standard FM
range 88-108 Mhz with a simple tuning resonant part like is in my case
tuning part of small superregen FM radio...

[GWS]

Hello Aurel,

That sounds a fascinating project - I might try one of those myself .. 

Where did you find the circuit from ?

The problem is, that as you move to higher frequencies, it becomes more difficult to design components.

Coils can become more like 'U' shaped loops of wire - capacitors become small, and for the very low values required, can often be made from reversed biassed diodes.

Here's an interesting note from QSL ..

http://www.qsl.net/y/yu1aw/Misc/HelixpreV.pdf

My current project is only at very low frequencies below 2 Mc/s ..  - much different from 200 Mc/s.

There are so many other factors to consider than just the L/C values. Signals can jump from one part of the circuit to another unless carefully screened. Loss reduction is a problem, although I've used thick copper wire or thin copper tube in the past.  Silver coated wire is good if you can find some.

For capacitors, varicap diodes are probably best for tuning from a control knob.

I don't know of any easy way to calculate circuit values at those frequencies.  Probably best to just copy the mechanical layout and component values from some existing published circuit for the frequencies you require 80 - 120 Mc/s.  The values will probably have been developed by trial and error to give acceptable results.

If you find a good circuit, I'd be interested .. 

best wishes,

Graham

[aurelCB]

Graham
There are lot of similar circuits on net..
Just type in google .. superregen fm radio or
super reaction fm radio..
Main problem in this circuit is operating point because this circuit is
oscilator -so called self-quenching oscilator and use so called 'slope detection' similar
like in AM radios...

[aurelCB]

Just to add ...
I build circuit from first image but without BF324 , i use BF506 but without result
Second circuit is my own design based on old superregen called Fremodyne Radio
which is originaly created with HF tube,in this circuit i use C3355 hf transistor with
very good results.
I also try other transistors like BF199,BF494 and N9018,N9011...

[GWS]

Thanks Aurel .. you seem to be very busy with electrics and programming .. 

I'll give it a try when I get my current projects finished.

As I sort of expected, the theoretical values from the above program gave nothing like the tuning coverage in practice for my little receiver   I'll play about with it for a while, as it does work in a limited way, but I'm not hopeful.

At the moment, it's too darn cold in my shed to stay there very long .. 

Best wishes,

Graham

[aurelCB]

you seem to be very busy with electrics and programming

Heh...
Not to much but because is still very cold outside for some work i must do something
in free time.I like this retro electronic..

[GWS]

Yes, it is fun .. 

Here's a Cossor 500 radio I recently rebuilt.

It was originally released in May 1950, priced at £15 2s 9d plus purchase tax .. 

I had to remake the case (the original had woodworm) - and with an 8 inch speaker, it sounds really nice.
I have it in the lounge playing in the background.  It will probably last another 60 years now .. 

best wishes,

Graham

[Doc]

That's some Bea-U-Ti-Ful work, Graham!

[GWS]

Thanks Doc .. 

That's used up the last of my 'decent' wood.  It's getting quite hard to find good wood these days.
Our local wood yard closed several years ago.

My daughter says the used-furniture dealers I go to, would be aghast if they knew the bookshelves I buy, are just to be broken up for the wood .. 

For this project, I was going to buy some oak beading to go round the dial and speaker, until I found they were asking nearly $20 for a 6 foot length .. 

I find modern products pretty cheap and nasty - I like the good old stuff .. 

best wishes,

Graham