another FM transmitter project

Status
Not open for further replies.
I do not know why you cannot buy common capacitor values. Farnell (now called Element14) has a warehouse in India:
**broken link removed**
I use metalized plastic EPCOS film capacitors for 1nF to 1uF and they have a tolerance of 5%.

The total input resistance of the oscillator of the MOD4 transmitter is about 22.7k ohms so a 330nF coupling capacitor produces a cutoff frequency of 21Hz, 220nF makes 32Hz, 150nF makes 47Hz and 100nF makes 71Hz.

What is "an output coupling capacitor" used for?
 
High quality electronic parts are available everywhere and are inexpensive on this side of the world.
 
Ok, I am searching a shop like Element14 because almost nothing I can find in my local market.
I was asking that- in pre-emphasis, if I used 330nf output coupling capacitor and 1uf input coupling capacitor anything wrong?
If 2nd output 330nf capacitor has 21Hz cut-off then can I use 1st input coupling capacitor- 1uf or 4.7uf electrolytic?
For now, I am just searching alternative way without any huge mistake.
 
Last edited:
I don't know what you are making.
Are you making a Mod4 transmitter or are you making Harry's stereo encoder and coupling it to the oscillator of the Mod4 transmitter?

In the Mod4, the 330nF capacitor C1 between the biased electret mic and the base of the preamp transistor should be a non-polarized film capacitor) because the polarity in the circuit is unknown.
The 330nF capacitor C3 between the preamp and the oscillator should also be a non-polarized film capacitor for the same reason.
 
I designed and built my MOD4 FM transmitter using 330nF metalized plastic film capacitors. They are non-polarized.
You can try 1uF electrolytic capacitors instead but their polarity might be backwards then they will mess up the circuit.

I forgot. I used EPCOS European capacitors. You are near China so maybe you can use metalized plastic film "greencaps" (some are reddish-brown) labelled 334.
220nF Chinese capacitors will also work and are marked 224.
 
Only the preamp's emitter resistor R5 and its bypass capacitor C4 make pre-emphasis in my MOD4 FM transmitter.
Pre-emphasis is high frequencies boost. The input and output capacitors affect only the low frequency cutoff.
 
* What is the purpose of low frequency cut-off? I think this cut-off reduces extremely low frequencies like some type of noises, isn't it?
* And can I understand like this way- '21Hz cut-off mean the capacitor reduces the frequencies of less than 21Hz'
 
Willen said:
* What is the purpose of low frequency cut-off? I think this cut-off reduces extremely low frequencies like some type of noises, isn't it?
Click to expand...
FM radio stations and FM radios reduce frequencies below 30Hz or 50Hz. If one RC cutoff is at 21Hz then two of them make a cutoff frequency at 42Hz. Then 50Hz is reduced a little.

And can I understand like this way- '21Hz cut-off mean the capacitor reduces the frequencies of less than 21Hz'
Click to expand...
The capacitor feeding an impedance makes the low frequency cutoff. The reactance of the capacitor makes a voltage divider with the impedance it feeds. When the amount of reactance euals the amount of impedance then the level is reduced -3dB (0.707 times), not -6dB (half) because the reactance creates a phase shift.
 
- Does this chain of RF amplification might work? (correct?)
- The last transistor needed a good heat sink but does this Driver amplifier by 2N2219 might need a heat sink? It is Class C, so it might be warm only, not hot, isn't it?

Something please!
 
The output level from the RF oscillator might not be high enough to properly turn on the class-C RF preamp. So use a linear class-A RF preamp. With an output of 50mW and a 12V supply it will be hot.

I think the 2N2219 RF driver will also be hot without a heatsink.
 
I hope, 2n2222 at 10v with class A amplifiers will produce 50 mW and might not be hot. Does it?
 
My FM transmitter has a 2N3904 class-A transistor on its output producing about 23mW into a 75 ohm antenna and with a 9V supply it gets pretty warm.
 
I recall Realistic CB radios using a 2SC1226A TO-202 transistor, able to achieve a maximum 4W signal output and provided the audio volume for PA systems. I have a Realistic TRC-10 CB radio that I haven't opened for service, and models from Cobra and Uniden yet to be tried. Assuming it would be a short range transmitter, where is the antenna connection?
 
Audioguru,
If i add a class c amplifier in Mod4 using 2n2222 with 9V supply, how much power can i get (approx)? Double...triple...or....
 
My Mod4 FM transmitter already has an RF amplifier at its output providing about 3.8V peak into a 75 ohm whip antenna (96mW) on a simulation, or about 20mW as calculated by an expert.

I added a class-C 2N2222 output stage and the output is 4.8V peak into the 75 ohm whip antenna (154mW) which is not much more.

I did not match the impedance of the antenna to the output transistor.
 
Now i made another Mod 4 on quality PCB, being very careful. There are two way input:- Audio and Mic with toggle switch. Wow! I got very very very crystal clear audio quality when i was playing song. It has higher quality in audio than few local FM station of my area. My friend also made this tx and got shocked when he listened such quality audio and no noise in background. But it finished my old 9 v battery so fast and frequency started to drift accordingly. I don't have well conditioned multimeter, so please tell me its current consumption.
 
Radio stations compress the audio so that its average level is louder. The compression boosts sound levels that are not loud and reduces sound levels that are loud. Frequently you can hear the compressor "pumping" the level of background music with the beat of the music.

I think my Mod4 FM transmitter draws 53mA from a new 9V alkaline battery. The current (and range of the transmitter) drop as the battery voltage runs down.
Here is a graph of the voltage dropping as shown on the datasheet of an Energizer 9V alkaline battery:
 

Attachments

  • 9V alkaline, 53mA load.PNG
    7.8 KB · Views: 289
Status
Not open for further replies.

Similar threads

D
  • Question
Bluetooth integrated FM transmitter project
Replies
19
Views
5K
audioguru
R
  • Question
(Another) analogue voltmeter does not go back to zero
Replies
13
Views
3K
schmitt trigger
S
  • Question
My Christmas Ornament project
Replies
1
Views
884
ronsimpson
P
  • Question
Project or Book with 2 Watt VHF transmitter for tracking
Replies
16
Views
2K
preWarp
P
J
  • Question
A circuit to block back-feeding to another line
Replies
15
Views
6K
Nigel Goodwin
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…