a very simple FM Tx

[audioguru2]

Hi Walid,
A parallel resonant LC has a very high impedance, nearly infinite ohms. A series resonant LC has a very low impedance, the resistance of the coil plus the resistance of the wires on the capacitor. Look at LC Resonance in Google.

 

[walid1]

Hi guru

I go to google and search for LC Resonance and one of them was:
http://www.pronine.ca/lcf.htm
It is a calculator
substitute L=330n and C=6.7p
u will have f=106 MHz and Zo=220 ohm as i calculate it, it is not nearly infinite ohms
It is very important to me to correctly calc the o/p Z of the oscillator because i want to discuss with u the pi C-L-C filter between this osc and the 50 ohm antenna from this site:
http://hem.passagen.se/communication/antenimp.html
and to ask u why u don't use it in your FM Tx
don't worry i understand 95% of what he said but still to know how to cal the o/p Z of the oscillator to match it with the antenna
I make a page in MATHCAD for quickly calculations
thank u guru

 

[audioguru2]

Hi Walid,
The calculator you found is WRONG!
It has the wrong formula for the impedance at resonance of a parallel LC.

You shouldn't calculate things at such a high frequency because you don't know how much stray capacitance is in parallel with your C, and you don't know how much inductance is in the wiring of the L.

I tested my FM transmitter only one day and didn't improve it by adding a matching network at its antenna. Its range and sound quality are much better than I expected and since my FM band is full, it causes interference. It is illegal (and not nice) to cause interference here.

Here is another formula for the impedance at resonance of a parallel LC:

View attachment 40180

 

[walid1]

Hi guru

The calculator you found is WRONG!
It has the wrong formula for the impedance at resonance of a parallel LC.

yes I agree, it It has the wrong formula and they calc it as 220 and i calc it the first time as 110 and mistake when tell you it 220 ( I was hurry)

You shouldn't calculate things at such a high frequency because you don't know how much stray capacitance is in parallel with your C, and you don't know how much inductance is in the wiring of the L.

yes, I know, but still must calc it to know the o/p impedance to correctly fit the matching network, then may modify it lettle to compensare for the stray capacitance and inductance

Here is another formula for the impedance at resonance of a parallel LC:

I calc it, using your formula, look below:


View attachment 40182

 

[audioguru2]

No Walid.
Your calculation doesn't include the +90 degrees and -90 degrees phase shifts of the capacitor and inductor and they cancel. Therefore at resonance a parallel LC has an infinitely high impedance.

 

[walid1]

hi guru

If they cancel then the Z is zero not infinity
I know it is not zero
the ohmic resistance of the coil wire and cap materials .. etec


please look, the man who do this: http://hem.passagen.se/communication/antenimp.html

say:

the matching between Zout and the 50 ohm antenna using Pi C-L-C filter
if Zout is infinity, how can he put the matching network
to design the Pi C-L-C filter you must know Zout??

Your last answers on this topic were not very good
it takes me up then down, I'm confused now

thank u

 

[audioguru2]

Hi Walid,
I was talking about the parallel resonant LC at the collector of the RF amplifier transistor on my FM transmitter. Its impedance is infinity at its resonant frequency without a load. But it is a terrible match to a 50 ohm antenna.

The article you are looking at shows a pi (CLC) lowpass filter/matching circuit that has a high input impedance from a transmitter and a low output impedance to an antenna. It is a good matching circuit because it has low loss. It also reduces harmonics that cause interference.

 

[walid1]

Hi guru

I was talking about the parallel resonant LC at the collector of the RF amplifier transistor on my FM transmitter. Its impedance is infinity at its resonant frequency without a load. But it is a terrible match to a 50 ohm antenna.

I can't understand or accept that the LC impedance is infinity at its resonant frequency. Here is your formula:

At resonance XL = Xc in magnitude and if they cancle each other because of phase.................... Yes Yes its impedance is infinity at its resonant frequency.

But still some point obscure:
If U GURU, for example, want to replace your C14 (=22p) in your FM Tx

with a Pi C-L-C network as in that site: http://hem.passagen.se/communication/antenimp.html
you have to know the Zout of the RF amplifier transistor (Q3) to match it with the 50 ohm antenna using the calculated values of caps and coil.

May I worgly understood what the above link want to say, may be he want to tell me that the Pi C-L-C network match any Tx at 50MHz.

so tell me what you have to do if u decide to use that network.

thank u guru

 

[audioguru2]

Hi Walid,
I never have used an RF pi network.

The output impedance of the Tuned RF Amplifier depends on its DC current and on its drive level if it operates in class-B or class-C. With a high DC current and high drive level then the resonance of the tuned circuit causes the output voltage to exceed the power supply voltage. Then its output has high power into a fairly high impedance.

I think that a load line must be used to determine the optimum matching impedance of the pi network.

 

[walid1]

Hi guru

two weeks ago, i discussed and write an article about a very simple FM transmitter, the one whose circuit is appear at the top of this topic- in an arab community.
my discussion was depending

 

[audioguru2]

A few things affect  why his transmitters have poor range:
1) His battery is not new?
2) His radio has poor sensitivity?
3) His circuit layout is not compact?
4) His transistors are cheap Chinese copies?
5) His transmitting and receiving antennas are pointing in different directions?