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### Topic: how to calculate the exact frequency of this FM transmitter? (Read 17701 times)previous topic - next topic

#### firashelou

##### Jan 16, 2014, 10:14 amLast Edit: Jan 16, 2014, 10:38 am by firashelou Reason: 1
hello guys,
i have a schematic of FM transmitter so i build it on breadboard but i got some components that are different from the schematic, so i guess this changes the frequency which should be 102.3 MHZ, in fact this frequency in my country is used (can i still be using it and interfere to the station that is using it ?)
so i need to calculate the new frequency which i should check to hear the sound from my transmitter, how can i do that ?

the original schematic in attachment file

my changes are :
instead of R5 47K i used 50K
instead of 4.7uF i used 2.2uF
instead of of 220K i used 200K
instead of variable capacitor C5 i used a ceramic 0.1uF
instead of the inductor used i put a piece of wire and made it into a coil
as an antenna i used a piece of wire about 17cm attached to the + of the inductor

i assume there is connection between R2, R3 and the base of transmitter is that right ?
and a connection between the LED, 330ohm and the base ot the second transmitter ?

Aiming for the unreachable !

#### mauried

#1
##### Jan 16, 2014, 01:00 pm
What are you trying to do with this transmitter?
The frequency is given by the formula 1 / 2 X pi X sqroot (L X C), but as
the circuit has the oscillator as the output stage , its frequency stability will be terrible.
You need some kind of crystal oscillator followed by a multiplier to get a stable frequency.

#### MarkT

#2
##### Jan 16, 2014, 01:21 pm
Nice thought, but you really don't want a stable frequency for wideband FM, you
have to shift the frequency +/-200kHz or so according to the modulation input,
0.1% or so, far more than a crystal can be "bent".   _Narrowband_ FM can be done
with crystals (deviation is more like 2.5kHz), wideband you need either PLL or DDS
techniques or an modulated LC oscillator like this.

The frequency is set by the tank circuit, C6/L1, since C6 is a trimmer anyway that
should give a wide range of adjustment.

Without a spectrum analyser you'll have to use a radio receiver to tune this
transmitter.

Note that this circuit is primitive and the antenna will affect the tuning so you must
tune with the antenna you will be using.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### michael_x

#3
##### Jan 16, 2014, 01:57 pm
Quote
instead of the inductor used i put a piece of wire and made it into a coil

Hopefully you manage to hit a frequency in the range of your radio receiver (88 .. 108 MHz is not a too broad range).

Quote
Note that this circuit is primitive ...
... so chances are good the mic audio signal affects frequency somehow and produces an FM-like signal. I guess if it works at all, it will affect all FM radio frequencies. No need to worry about 101.3 in special.

#### polymorph

#4
##### Jan 16, 2014, 04:19 pm
So you used a 100,000pF ceramic capacitor in place of a 6 to 60pF variable? So I'd say the frequency is 0Hz, because it won't oscillate.

As for the coil, there is no way anyone can tell you the inductance with the information you gave.

This is a rather unstable oscillator, for the reasons given. Pretty much anything you change that is connected to Q2 (including the battery condition) will change the frequency. You can get close by calculating the resonance of C5 and L1, although the frequency will be something lower than that due to the feedback capacitor C6 and the transistor's operating point characteristics.

So the only way to know for sure is either tune a radio into its signal, or use a frequency counter. If you connect an oscilloscope probe or anything else directly to it, you'll lower the frequency.

What a horrid schematic. Terrible place to draw a (wasteful) power LED. And the LED is pointing the wrong way.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### MarkT

#5
##### Jan 16, 2014, 04:32 pm

What are you trying to do with this transmitter?
The frequency is given by the formula 1 / 2 X pi X sqroot (L X C), but as
the circuit has the oscillator as the output stage , its frequency stability will be terrible.
You need some kind of crystal oscillator followed by a multiplier to get a stable frequency.

Actually it may more complex than you think, since its a modified Hartley
oscillator several capacitors are contributing to the overall capacitance.
(Including those of the device itself which are modulated by the input)

Tuning a radio into the output is a fairly sane way to tune this (although
it will also detect harmonics, so a rough calculation based on the component
values is a good safety check.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### polymorph

#6
##### Jan 16, 2014, 05:24 pm
Like 100,000pf for the resonating capacitor? ;')
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### firashelou

#7
##### Jan 16, 2014, 05:33 pm

Like 100,000pf for the resonating capacitor? ;')

well yea i thought this will affect it big time, i just used because i didn't have the 60pF trimmer but seems it's very wrong !

about the tuning using a radio, what do you mean by that ?

about the inductor, it's 6 rounds, 0.5 cm diameter
Aiming for the unreachable !

#### polymorph

#8
##### Jan 16, 2014, 06:07 pm
Well, it is only about 2000x larger. :'/ What happens if I put mining truck tires on my car? It isn't meaningful any more to ask how fast it can go, because it won't work.

Tuning the radio? I meant tuning your receiver around until you hear the signal from the transmitter.

We need to know a lot more about the inductor to calculate the inductance. Is that 5mm inner or outer diameter? How long is it? Wire gauge?
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### Grumpy_Mike

#9
##### Jan 16, 2014, 06:31 pm
Quote
in fact this frequency in my country is used (can i still be using it and interfere to the station that is using it ?)

Most countries will not allow you to transmit in this band anyway.
If you fill in the location information in your profile everyone can see it and give you better advice.

In general this is a broadcast band and using any frequency is illegal especially using such an unstable circuit.

#### firashelou

#10
##### Jan 16, 2014, 08:18 pm

Well, it is only about 2000x larger. :'/ What happens if I put mining truck tires on my car? It isn't meaningful any more to ask how fast it can go, because it won't work.

Tuning the radio? I meant tuning your receiver around until you hear the signal from the transmitter.

We need to know a lot more about the inductor to calculate the inductance. Is that 5mm inner or outer diameter? How long is it? Wire gauge?

i guess it's better to buy an inductor than build one like that
Aiming for the unreachable !

#### firashelou

#11
##### Jan 16, 2014, 08:19 pm

Quote
in fact this frequency in my country is used (can i still be using it and interfere to the station that is using it ?)

Most countries will not allow you to transmit in this band anyway.
If you fill in the location information in your profile everyone can see it and give you better advice.

In general this is a broadcast band and using any frequency is illegal especially using such an unstable circuit.

than what is the solution ? how can i choose a specific frequency and make a circuit for that frequency ?
Aiming for the unreachable !

#### polymorph

#12
##### Jan 16, 2014, 09:50 pm
First, tell us what you are trying to accomplish.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### firashelou

#13
##### Jan 16, 2014, 10:06 pmLast Edit: Jan 16, 2014, 10:08 pm by firashelou Reason: 1

First, tell us what you are trying to accomplish.

ok my goal is to go from here to build a system like the walkie talkie or something small similar to the communication system used in racing when the team speak to driver and the driver speak back

i can say after i finished the book i was reading i managed to build a small speaker system, and i can say i have an idea (not very advanced but i am not much confused like when i first started) of what is going on in sound stuff
Aiming for the unreachable !

#### Grumpy_Mike

#14
##### Jan 16, 2014, 10:10 pm
From the document attached:-
Quote
Article 14: Requirement of a Frequency License
1. Subject to Article 14 (3), no Person may use Radio Frequencies or operate a radio apparatus in Lebanon, or on board of any ship, aircraft or space object that is registered in Lebanon, unless authorized by a Frequency License of the Authority.

See the last section of the document for:-