another FM transmitter project

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I am thinking about stereo encoder and Pre-emphasis to connect to oscillator. Which series is correct?
-Stereo encoder- Pre-emphasis and Oscillator
-Pre-emphasis- Stereo encoder and Oscillator

I think connecting pre-emphasis first is not more practical bacause there are two channel in stereo audio.
 
A stereo encoder needs a transmitter circuit that has a flat frequency and phase response to 53kHz. Most simple FM transmitter circuits do not go anywhere near that high.

Each channel needs its own pre-emphasis before the stereo encoder. A compressor/limiter might be needed in between.
 

looks like little complicated.
Um....can I connect your pre-emphasis at the beginning and then stereo encoder and then your Tx?
What kind of Tx has flat friquency and phase response of 53KHz? Mean?
 
The oscillator in my FM transmitter has a 470pF capacitor to ground at its base so that it is common base at 100MHz. Then if the stereo encoder has an output impedance of 10k ohms, the frequency response will be -3dB at 34kHz which is no good for FM stereo.

You have another thread about a stereo encoder but I can't remember its output impedance. Its output impedance must be less than about 600 ohms for the response and phase to be fairly flat up to 53kHz.
 
Willen said:
looks like little complicated.
Um....can I connect your pre-emphasis at the beginning and then stereo encoder and then your Tx?
What kind of Tx has flat friquency and phase response of 53KHz? Mean?
Click to expand...
Stereo is a little complicated.
With mono you need pre-emphasis before the transmitter. Bandwidth 20 to 19khz.
For stereo you need two pre-emphasis, one for each channel (R and L). After the encoder the bandwidth is 20 to 53khz.
 
Are there any simple oscillator shematic where I can inject (generally) composite stereo AF from stereo encoder?

This oscillator has no Base-Ground capacitor, so will it work with **broken link removed**?
 
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Of course the base of the oscillator transistor has a capacitor to ground, it has a 1nF capacitor to the positive supply that is grounded by C1 and the output of the 5V regulator. The oscillator transistor will not oscillate without a capacitor causing it to be a "common base amplifier".
Your new circuit has the opposite of pre-emphasis since it cuts high audio frequencies (and cuts all stereo signals).

If you look at Harry's stereo encoder, it has a high output impedance so the oscillator capacitor will cut even more high frequencies.

EDIT: On Harry's website, does he say which of his transmitters will work with his stereo encoder?
 
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OH MY GOD! I am going to be mad! Then what to do by making this encoder!!

I think he didn't say where to inject its composit stereo but he said his small bug Tx cannot be driven without modify.
 
There are manufacturers of "Pirate" FM stereo transmitters (Ramsey is one) who sell them to boats in international waters near England.
The web has their complicated schematics.

Here is a site with a lot of information about FM stereo transmitters including "Stereo for Dummies":
https://transmitters.tripod.com/
 
Willen said:
Wow! Here is a simpler onother without crystal.
www.hqew.net/circuit-diagram/Stereo-encoder-circuit_8197.html
How it can be stable without crystal? Is it better or average?
Click to expand...

I have worked on a number of FM transmitters. The 19khz (38khz) should be crystal. An RC oscillator can not be stable. You should at least use a really good capacitor. R13 is not temperature stable. Several of these circuits say adjust the frequency until your radio works. It will work, as a hobby thing, but this is not how it is done in real transmitters. You might have to readjust R13 on hot days or after months. This circuit only has to work with one radio. A commercial transmitter has to work with all radios.
 
Willen said:
Wow! Here is a simpler onother without crystal.
www.hqew.net/circuit-diagram/Stereo-encoder-circuit_8197.html
How it can be stable without crystal? Is it better or average?
Click to expand...
The designer used a very unstable ceramic capacitor in the oscillator so its frequency will wander when the temperature changes.
A poly capacitor is much more stable but maybe the fixed resistor and the variable resistor values also might change with temperature change or with age.
This simple circuit also does not have important filters.

The author knows nothing about transistors when he says that a BC549C will provide higher amplification.
The transistor is an emitter-follower with a voltage gain of 1 so there is no amplification anyway.
A BC549C has a little more current gain than a BC549B but that does not affect the voltage gain.
 
The 47k resistor R1 and parallel 1nF capacitor C2 is for Europe and Australia pre-emphasis like mine. But the 22k resistor R2 destroys most of the pre-emphasis.

Where do you find those horrible schematics?

EDIT:
Isn't Electronics-DIY.com in India? On their first page they show the obsolete and poor performance BA1404 IC that has not been made for at least 8 years.
 
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When I was teenager, I made a basic small Tx(attached) as a first Tx of mine. But I got very bad hum (hiss) noise at background of transmitted audio. Then I removed BF494 and placed BF495 transistor, and got little less hiss/hum.
But I got crystal clear audio when I made Mod-4 (post #49).
So I think there is a problem of biasing in my 1st Tx. I cannot understand what was the wrong!
 
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Hum is a low frequency, usually from the 50Hz or 60Hz of mains electricity. It could be 100Hz or 120Hz from a full-wave rectifier in an AC-DC power supply.
Hiss is higher frequency random noise above 2kHz. It sounds like "sssssssss".

Your first transmitter has a 1k resistor powering the electret mic directly from the +3V supply without any filtering so any noise on the +3V is fed directly to the mic.
The series 18k resistor between the mic and transistor simply attenuates the mic signal.
It has no voltage regulator so its frequency changes when its battery voltage runs down.
It has no pre-emphasis so it will sound like it has no high audio frequencies.
It has its antenna connected to its tuned circuit so the changing capacitance of something coming towards or away from its antenna changes its frequency.
It is low power so maybe a radio that is not sensitive will produce hiss when receiving the weak signal.
 
You found another very simple FM transmitter that is almost the same as hundreds of others.
It has most of the problems listed in my last post.

Harry's stereo encoder has a simple additive mixer with a very high output impedance and a capacitor that filters away most of the very important frequencies of composite stereo. It needs an opamp as a mixer because the output impedance of an opamp is very low.
 

Wow! I got solution!!!
OK I will add another TL072.
Small help please, May I connect this 'bias' pin to same bias pin of previous opamp?
 
Willen said:
Wow! I got solution!!!
OK I will add another TL072.
Small help please, May I connect this 'bias' pin to same bias pin of previous opamp?
Click to expand...
Yes, the (+) inputs of the previous dual opamp in Harry's stereo encoder.
 
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