Audioguru's FM Transmitters

[audioguru]

Hi Monkeytree,
You can troubleshoot the circuit by measuring DC voltages at its important places like the collectors and emitters. Since it operates at VHF, measure the voltages with a 100k resistor connected in series with the red lead of a working high input resistance (10M) multimeter.

 

[Antey]

Q3 is an RF amplifier, it produces much greater range, and makes it more stable by giving a degree of isolation between the oscillator and aerial.

But in order to work (amplify) the transistor must be properly biased, that is it must work in "normal, active" area (if I translate this correctly), and even without simulation, I don't expect Q3 to work like that... unless of course there's something I can't see or understand. What could it be ?

I consider simulators a waste of time, particularly for this type of circuit, where it's not likely to even get close.

Well, I already know that (and start to agree with You...)
For my bad luck, that's exactly my task - to provide working simulation, but the circuit must also "look like" real.
Not to build an actuall device, or just simulate the spectrum.
Just to make such strange so-called-simulation

 

[Nigel Goodwin]

But in order to work (amplify) the transistor must be properly biased, that is it must work in "normal, active" area (if I translate this correctly), and even without simulation, I don't expect Q3 to work like that... unless of course there's something I can't see or understand. What could it be ?

Q3 is biased by R8 - I don't see your problem?.

An RF amplifer (at least for FM) doesn't need to be linear, and there's no advantage to it being so - in fact there are advantages in it NOT being so!.

This sort of RF amplifier would commonly be used in class C, with no bias at all - I don't know if Audioguru has tried that? (connect R8 to 0V rather than 9V) - but I suspect the 47K is intended to bias the transistor towards class B, as there probably isn't enough drive signal to work in class C?.

 

[audioguru]

Hi Nigel,
I tried Q3 without bias as a class-C amplifier but it didn't have enough input level to produce much output power. With it biased on, the input signal causes it to amplify very well, and I think it operates in class-B as you say with a very high output voltage swing.
Someone 'scoped one to show its output voltage (35Vp-p) far exceeding the supply voltage due to the high Q of its tuned circuit ringing.

I don't think the sim program knows anything about Q or RF. It just thinks the transistor's collector is shorted to the supply, and displaying the battery's voltage being modulated. :lol:

 

[Antey]

I don't think the sim program knows anything about Q or RF. It just thinks the transistor's collector is shorted to the supply, and displaying the battery's voltage being modulated. :lol:

Indeed. Oscillation around average level of 9 Volts (perpetum mobile ? DC is 9 V, and I get larger voltages in the circuit... ).
... life :evil: Anyway : does anyone have an idea how to prove sim's inability to accurately perform the calculations WITHOUT assebling a circuit ? The more maths, then better.

By the way : please forgive me my strange questions. On holiday I'll take up real electronics.

 

[Antey]

Q3 is biased by R8 - I don't see your problem?.

I'll take it for granted, but still i don't see it

 

[Nigel Goodwin]

Q3 is biased by R8 - I don't see your problem?.

I'll take it for granted, but still i don't see it

Try studying the different bias classes, for a simple overview:

Class A (the only one you appear to be considering).

Class B (biased to 'not quite on', how I think Q3 probably is?).

Class C (not biased at all, switched by the signal).

Class D (used purely as a switch, either hard ON or hard OFF).

Class A is usually used in preamps.

Class B is usually used in push/pull ampifiers, but usually modified to class A/B to improve distortion figues.

Class C is usually used in RF power amplifiers, only conducts on positive half cycles, relying on the tuned circuit to 'ring'.

Class D is usually used in 'digital' amplifiers, as it's always either hard ON or hard OFF, it's dissipation is accordingly low.

 

[Antey]

Try studying the different bias classes, for a simple overview :

:shock: (sound of hand hitting the forehead ).
Well...newbie is newbie
Thank You very much Nigel, for your patience and advice.

Question for everybody : is there by any chance ready PCB for this circuit ? (mod 3, without IC ). And what the antenna should look like ?

Best Regards,
Peter

 

[audioguru]

Hi Antey,
I made mine with Veroboard (with strips of printed copper) and its antenna is just a piece of wire 30 inches long hanging down. I will probably use a telescopic whip antenna from an old RC car transmitter. My Veroboard layout for mod-4 looks like this:

 

[Antey]

My Veroboard layout for mod-4 looks like this:

THANK YOU
But... I'm afraid to ask - do You happen to have a schema for board mod.3 ? Or maybe just suggestions about putting all pieces together - especially about making connections and wiring ? I am not expirenced enough, but I know that there can be some issues regarding additional capacities/inductances...

I'd like to thank everybody once again.

 

[audioguru]

But... I'm afraid to ask

Hi Antey,
We are friends so ask what you want. :lol:

I am not expirenced enough, but I know that there can be some issues regarding additional capacities/inductances...

It is difficult making a very high frequency radio transmitter without having a printed circuit board and detailed instructions. Perhaps it is best for you to learn by making a kit that comes with all special parts, a printed circuit board and detailed instructions. :lol:

 

[zachtheterrible]

Or maybe just suggestions about putting all pieces together - especially about making connections and wiring ? I am not expirenced enough, but I know that there can be some issues regarding additional capacities/inductances...

Layout at 100MHZ isnt extremely critical, like in the GHZ region. Just make sure that you keep your leads real nice and short and you should be fine. And try to place the components as close together as possible on the veroboard.

 

[Antey]

[long, and may be considered OT]

Layout at 100MHZ isnt extremely critical, like in the GHZ region

That's what I expected, but it's always nice to get a confirmation

We are friends so ask what you want. :lol:

Well, thanks again

Perhaps it is best for you to learn by making a kit that comes with all special parts, a printed circuit board and detailed instructions. :lol:

Heh... well... the problem is, that in this particular it's not about learning but earning a passing grade, and worse, one must distinguish between those two. Quite silly, isn't it ? For my twice-bad-luck I've come across a "teacher" who doesnt' seem to know much more than anyone in the class.
(other teachers, at lab, or lecture, are very knowledgeable, but too busy to help ). My task is to provide 3 different schematics of FM modulator (100 MHz carrier and 50 kHz deviation ), which both look realistic, and work perfectly in simulator, then preapare detailed calculations and working (under Spice A/D 6.3 ) simulation of one of them.

I read some books, looked at few sites and groups, and run about 50 different circuits in simulator. Still nothing and the clock is ticking. So I decided that, because of obvious falult in simulator (which doesn't work at all under WinXP, so i had to switch to SIMetrix ) I'll try to read more about those sim-glitches(inconsistencies?) in book about SPICE itself then deliver results, assebled & working circuit (with credits included - honesty 1st! ), and explanation of difference between sim and reality, including the conclusion that the task itself was faulty and then wait for reaction. I don't expect to pass, but at least to get some arguments for the Dean to get permission to take this course once more, under sombody else's guidance.

I think that I took up real electronics far too late :cry: . So as one Russsian Prime Minister once said " I did my best... yet performed as usually".

 

[Nigel Goodwin]

Re: [long, and may be considered OT]

My task is to provide 3 different schematics of FM modulator (100 MHz carrier and 50 kHz deviation ), which both look realistic, and work perfectly in simulator, then preapare detailed calculations and working (under Spice A/D 6.3 ) simulation of one of them.

The usual way of providing FM modulation is to use a varactor diode as part of the oscillators tuned circuit, applying the audio to the varactor changes it's capacitance and provides frequency modulation.

Doing it that way will allow you to do detailed calculations, and should simulate without any problems.

The crude FM 'bugs' are far more difficult to calculate, not to mention describe - the modulation method is crude, and probably gives both FM and AM at the same time.

 

[Antey]

Re: [long, and may be considered OT]

Doing it that way will allow you to do detailed calculations, and should simulate without any problems.

Anyway, I'll have three long months to make it work - I tried to run several circuits with diodes... and nothing. One of many problems is that generation dies after disconnecting the modulating tone. I know that generators sometimes require additional "kick" to start working, but it's not also the case...

 

[TheNewGuy]

Could Q1, Q2, and Q3 be replaced with three PN3563's?

 

[audioguru]

Could Q1, Q2, and Q3 be replaced with three fairchild PN3563's?

Why?
The DC current gain of the PN3563 is such a wide range that goes as low as 1/5th the current gain of the 2N3904 and its capacitance is much lower then the entire circuit will need to be re-designed.

 

[Dragon Tamer]

The 2N3904 is such a popular general purpose transistor that you can find them just about anywhere. Just go out to your local radio snack or some other place that sells electronic components (even if they are only basic), I can garuntee that you can find a 2N3904 in stock.

If they don't, I will personaly smack myself in the face in dispileif (not really, that hurts).

 

[k7420]

Thank you Audioguru! I made 2 quantity of your MOD4. One with regulator, which cost a lot here in Pakistan about 75 rupees, or equal to 90US Pennies.... it works perfect with mic.

Another one I made without regulator, because it's too expensive, but I can't get audio from mic, only if i talk really close to mic like i'm going to kiss the mic lol... but the sound quality is poor. Can you please recommend any suggestion about my second unregulated transmitter! I'm operating it on 3 volt DC, 2 AA Cells......

 

[k7420]

I also would like to know can I power my Regulated Transmitter to a 12 volts car battery? will it cause damage to Q3?