fm tranismiter

[trigger]

My veroboard sample will not as tidy as Audioguru does............

Good job Audioguru!!!

I got to learn from you... ;D

 

[audioguru2]

Thanks Trigger,
I learnt that keeping wires very short in VHF circuits is very important.  ;D

 

[Alun]

Yes connecting a varactor diode in with the capacitcor will give pure FM with little AM. You sitll probably want to have a vairable capacitor in paralel as well.

Can't you play around with the pre-amp biasing to clip or limit the outout to prevent over modulation or at least reduce it?

Have you tried taking the output to the antenna via a smaller coil wound within the output tuned circuit?

 

[audioguru2]

Hi Alun,
You have an excellent idea to solve my overmodulation problem!  ;D

Let the preamp transistor symmetrically clip at full modulation:
1) Its existing max 5Vp-p output is way too high since full modulation is only about 20mVp-p. Use a decoupled voltage divider to feed its collector. Maybe only 1V.
2) Use a voltage divider at its output to feed max 20mVp-p to the RF oscillator. The attenuation of 1V to 20mV is 50 times, that must be made-up by having 50 times more gain.

OOps! The preamp transistor is already operating at max gain with pre-emphasis, more gain would require it to operate at nA. Also, without having the 100 times of headroom and negative feedback it has now, its distortion would be terrible.

Mmmm, I have some opto-FETs that distort badly with a 20mV input, maybe they can be used as an audio limiter. Nope, they work backwards. With up to 20mV of input they are a linear resistor. Beyond that, their resistance increases. For a shunt limiter I need the resistance to decrease. I'll try them as the input resistor of a unity-gain inverting opamp. Then they should limit perfectly at 20mV output.

See, I am talking about adding 3 ICs to this simple FM transmitter toy. Forget it.  ;D 

 

[audioguru2]

Hi Sasi,
I don't know where to buy oriental transistors in Canada. Maybe in Hongcouver.

 

[trigger]

nice pictures..... Sasi.

So how are their performance then?

 

[spleblem]

that is really great
im just wondering what are u are using alot of batteryies in series(if so what batterys AND HOW MANY)
or do u have a mains power thought a transformer power pack

KIND REGARDS
SPLEBLEM

 

[Alun]

Thanks Dazza,  ;D
Yours should also work fine.

Thanks Alun,  ;D
Over here, a very popular TL071 costs less than a lousy old 741 or LF351. A TL072 dual costs even less! I always buy at least 10 and get a nice discount. Some experts like quieter audio opamps that cost an arm and a leg. This circuit is fine with just my transistor.

I'm still working on my FM transmitter, trying little ferrite chokes throughout it. I had the RF oscillator direct-coupled from the preamp but the preamp's bias went haywire. I didn't think that the oscillator's base voltage would be dragged down so low I guess by the oscillations. It works best with a little choke replacing C3, but I didn't bother changing the schematic yet.
I am also concerned with overmodulating it. A compressor/limiter would be too complicated for such a simple transmitter circuit. Because it has a lot of AM modulation, popping "P's" and other loud sounds cause its oscillator to stop for a moment. It will probably end up being modulated and tuned by a varicap for pure FM.
Then (if it is ever finished) I'll post it as a project!  ;D

I've manage to get my hands on a few TL081, are they suitable?

 

[audioguru2]

Hi Alun,
A TL081 isn't spec'd for noise. It is the same as a TL071 but the quiet ones are labelled and sold as "TL071" and the lousy ones "TL081".
Maybe a TL081 is made late on a Friday or early on a Monday, and whatever makes it noisy might affect its reliability.
Try your TL081. Maybe its noise is at the borderline for quietness and you won't notice its hiss. Maybe all TL081's made recently are low noise.  ;D

 

[Alun]

Hi audioguru,

I've just attempted to build the oscillator section of the transmitter, it worked but the frequency was 200MHz with the capacitor set to it's highest value, the sinewave output was very smooth and clean too.

I increased the diameter and number of turns on the inductor, and still no change in frequency.

I replaced the inductor with a ready built 4.7uH ferrite inductor, and it made no indifference what so ever.

The circuit is compact and less than 40mm square, and I ran it from 5V with a 100nF decoupling capacitor.

I didn't have a plastic screwdriver handy to tune it so I just used a metal one, and I only took notice of the frequency when I wasn't touching the capacitor. I reduced the capacitors value and the frequency increased to about 250MHz then stopped oscillating, it was funny because once when I touched the capacitor the frequency increased to 438MHz, this was unexpected, because I thought  more capacitance it would decrease frequency.

The capacitor is a 39pf, it's quit a bit bigger than the one you used, do you think the capacitor has a parasitic inductance causing it to resonate at this frequency?

Why else could this circuit be stubbornly oscilating at 200MHz?

 

[audioguru2]

Hi Alun,
I don't know why your oscillator's frequency is so high. My FM transmitter can be tuned below the FM band, how far below I can't measure.
You would expect your 39pF cap to work well at VHF due to its small value.
Try another transistor. Maybe the one you are using has unusually low capacitance.

 

[Alun]

It was a 2N2222A.  ???

I'm going to change the capacitor for a fixed 33pF ceramic disk and see what happens.

 

[audioguru2]

Hi Alun,
The 2N3904 transistors that I used have half the output capacitance of your 2N2222A.  ;D

 

[Alun]

I still haven't had any luck, I going to try a different type of oscilator like a colpits, I'll search the internet and read a few books and see if I can put together aworking design. I've never had any problem with this sort of thing, I've even built and experimental 1.5GHz oscilator before so 100MHz should be simple, oh well back to the drawing board.

 

[audioguru2]

Hi Alun,
You can't get 100MHz? The only other RF thingy I made was another FM transmitter in about 1968, it used a germanium transistor and worked just fine. Nope, I just remembered that around that time I was also fooling around with a tunnel diode transmitter. A wire broke off the tunnel diode before I got it to transmit.
I guess you are just unlucky with this one like me with my tunnel diode.  :'(

 

[Alun]

Oh c'mon Alun be little pro...... try this link and find the Veronica 1 Watt or 5 Watt

http://www.geocities.com/Area51/Nebula/3736/

You will get the PCB Design layout and all info.

Just see the PCB made by me. But not assembled yet....

I wasn't planning to build the whole project just yet. I intended to build just the oscilator section to conduct experiments on. I want to add a varactor diode to get prure FM. The origional circuit would give both AM and FM because the signal fed to the transistors base mostly alters the amplitude of the signal, the frequency only changes because parasitic capasitances in the transistor changes the resonant frequency of the circuit.

The capacitance of any diode changes with voltage, like the capacitance of the transistor did. You can buy special diodes called varicaps, but I intend to just use a normal 1N4001 diode in the tuned circuit to alter the frequency. Because only the capacitance in the tank circuit is changing and not the bais voltage to the transistor the frequency should change but not the amplitude.

On the left is a graph from the 1N4001 datasheet showing the capacitance change with voltage, on the left is the modified tuned circuit I plan to use with the oscilator. I wanted to get the plain oscilator working first before I added the diode and other coponents. I plan to use a PNP transistor with the power reversed in my final oscilator design to give me a more sensertive input.

But thanks, a PCB is exactly what I need for my experiment, I have 4 A4 pieces of copper clad board lying around so etching a PCB for ths experimet is definitely worth the trouble. Hopefully if my experiment is successful the transmitter will be the best you can build only using a couple of transistors and maybe an op-amp for pre-emphasis without using any complicated transmitter ICs.

 

[audioguru2]

Hi Alun,
The BB109 varicap has been used in some FM transmitter projects on the web, for FM modulation and for channel tuning. I couldn't find one anywhere near me. It was made by Philips or Vishay-Siemens.

Here is a very long essay by some University students for them to transmit data using FM. They used a BB109 varicap to modulate a 10.7MHz ceramic filter oscillator and used a CD4069 Cmos inverter as the RF amps. They claim that the CD4069 produced the 9th harmonic at 93MHz. I didn't think it would go that high. They say their transmitter is inductorless!
They tried a supreregen FM receiver but it picked up too much interference. The essay is here: http://www.ee.ucr.edu/~dgiles/sendesign03/transmission%20system.pdf

Here's an FM transmitter schematic I found that uses a varicap for modulation and tuning. It has about 250mW output for a claimed 2km range:

View attachment 36845

View attachment 36846

 

[Alun]

Hi Alun,
The BB109 varicap has been used in some FM transmitter projects on the web, for FM modulation and for channel tuning. I couldn't find one anywhere near me. It was made by Philips or Vishay-Siemens.

Yes I could use a proper tuning varicap (which would be ideal) but as you said they're not easy to get hold of. Looking at the graph I shouldn't have a problem using the 1N4001 but I will use a vairaible capacitor to tune it, the diode will just be for the frequency modulation.

Here is a very long essay by some University students for them to transmit data using FM. They used a BB109 varicap to modulate a 10.7MHz ceramic filter oscillator and used a CD4069 Cmos inverter as the RF amps. They claim that the CD4069 produced the 9th harmonic at 93MHz. I didn't think it would go that high. They say their transmitter is inductorless!
They tried a supreregen FM receiver but it picked up too much interference. The essay is here: http://www.ee.ucr.edu/~dgiles/sendesign03/transmission%20system.pdf

I found this very interesting, I've not had time to read it fully but I get the general idea.

If they can get the oscillator to work at 31MHz with a square wave then the 3rd harmonic would be 93MHz so I don't think it's that hard after all. Looking at their circuit it appears they're driving the aerial with square wave and they're relying on its resonance to filter out the harmonic. But what's the 10.7MHz ceramic filter for?

Here's an FM transmitter schematic I found that uses a varicap for modulation and tuning. It has about 250mW output for a claimed 2km range:

This is also quite interesting, what sort of oscillator is it? I doesn't look like a conventional hartly or colpitts design. Does L1, C9 & D1 form a series tuned circuit and the feedback must be via the parasitic collector-base capacitance?

 

[audioguru2]

Hi Alun,
The ceramic filter is the crystal of their 10.7MHz oscillator. The note I've attached shows that a Cmos inverter has enough gain at 10.7MHz to oscillate. N3 squares it up and the paralleled inverters even more. I can't believe they have any output of the 9th harmonic at 96.3MHz.

In the varicap tuned FM transmitter I posted, the varicap is in series with C9, making an effective 10pF in parallel with L1.  ;D

View attachment 36847

View attachment 36848

 

[audioguru2]

Hi Sasi,
That's a neat circuit.  ;D
It oscillates at 10.7MHz but you have its output tuned to the 9th harmonic at 96.3MHz.
I didn't know you could do all that with only a single transistor.  ;D

Watch Alun modulate it with a varicap for "pure" FM modulation.
Our world is going to be crowded with 96.3MHz FM transmitters!