27 MHz Amplification Query

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RMMK said:
it showed some 19.76Volts!
What does it mean?? why this large voltage spike??
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It is possible for your LC tank oscillations to build to such a volt level. (As shown by simulations earlier in this thread.)

Your antenna will need a certain ACV level, in order to broadcast. I don't know how high the voltage must be.

I saw an old-timer state that the crucial thing is to make the antenna resonate to your transmitting frequency. This means tuning the antenna.

You want to get standing waves within the antenna. That is when photons are emitted most efficiently from the wire.
 


Sir what is this ACV??
how am i gonna calculate the output power from this potential difference??


and what should be the next step now???

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Borber said:
19V is OK for inductive load. Transistor 2N3866 can survive up tp 30V Vce0.
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Umm i measure it after the collector capacitor........ sir what will you say about the potential difference at the output of the pi-network?? Also when i remove the collector capacitor from the last stage (i.e., of 2N3866) and place the diode probe on the output of pi-network it still gives me some voltage........
 

Your diode detector already has coupling capacitor and you can measure RF voltages at any point without adding capacitor in series with detector. About potential difference on output we can talk only when we know the output power of amplifier and for that amplifier must be properly tuned. Assuming your amplifier will be able to deliver 250mW power to 50 Ohm load then it is easy to calculate output RF voltage.
Talking of some voltage means that output power is very low. At 50mW power your reading should be over 1V.
First try to tune amplifier. You can use detector and tune stages for max reading.
 
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how will i know its tuned or not?? You yourself said freq counter is useless for amplifiers......
should i look for maximum voltage reading on each of the collectors of each amplification stage??

By the way i only have a tuned LC tank on the 4th stage that is of 2N2219 just before the 2N3866 one......... All other stages have inductors on their collectors because when i was working with freq. counter i placed and removed components solely basing my results on the frequency counter......

Should i replace this 2N2219 collector LC tank 15pF capacitor with a trimmer one and try to vary it for obtaining a peak value on the diode probe??
or should i also place trimmer capacitors on each of the stage on collectors forming LC tanks??
 
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It looks like you did not understood.
 

okay but not all the stages in my circuit had LC tanks :???: only one had...... from the above quote all i understand is have a trimmer capacitor on the LC tanks to tune them........ my questions still remain unanswered sir!
 

RMMK said:
Sir what is this ACV??
how am i gonna calculate the output power from this potential difference??
Click to expand...

AC voltage. There's a question whether the broadcasting antenna should get true AC, or whether it should have a DC component. (That's a gap in my radio knowledge. If this has been answered already, I apologize for missing it.)

Therefore your output stage must be designed accordingly.

The nature of AM transmitting is that the photons alternate between being sparse and plentiful (I think).

You'll need more info from someone with more knowledge than me.
 

Borber said:
It looks like you did not understood.
Click to expand...

okay i understood the tuning part. I think you wanted to say that by poking the RF diode probe into the RF circuitry somewhere on the output side of the transmitter stage (or anywhere on the amplifier stages) you need to tune and tune for the maximum meter reading.....

On the second part plz confirm do you want me to place LC tanks on every stage of the amplifiers as opposed to currently one stage??? Plz sir consider the below circuit which also possess an RF amplifier without any LC tank.......


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Now i am even questioning myself whether i have designed the pi-network correctly or not??
This (below) is the current actual circuit i have built (except for the first two stages). On the advice of Borber i am considering adding trimmer capacitors thus forming LC tanks on each stage...... Plz verify!



I have uploaded previously the diode probe measured values without 100ohm resistor across the input of the rf diode probe...... with a 100ohm (i couldt find a 50ohm resistor in my inventory and had only one 100ohm resistor) resistor across the input of the diode probe i get these readings:-

1. Voltage at pi-network output = 0.96Volts (with all preceding stages connected)

2. Voltage at the last amplifier stage collector = 1.5Volts (with pi network disconnected but all previous stages connected)

3. Voltage at the last amplifier (2N3866) stage collector = 1.72Volts (with pi-network and preceding stages connected)

4. Voltage at the second last amplifier stage (2N2219) collector = 3.7Volts (with all succeeding and preceding stages connected)

5. Voltage at the third last amplifier stage (2N2222) collector = 3.4Volts (with all succeeding and preceding stages connected)
 
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Nothing wrong - it means there is RF present at that point. If you were measuring the output voltage AT THE ANTENNA CONNECTION you should add the extra load resistor across the input to simulate the real antenna being there. Without it you may get a false high or low reading because the output network can work like a transformer, stepping the voltage down or up. For other places in the circuit you don't want the resistor so it presents a higher impedance and resistance.

As pointed out by others, the output stages have no bias except that provided by the signal itself. If you make the collector tuned circuits tunable with a variable capacitor you can adjust their resonance to 'peak' the signal level to the next stage. As more bias is produced the current through the transistor will increase so you can monitor it to find the best tuning setting. I will urge caution though, when you tune several circuits in an amplifier to the same frequency, the chances of instability are greatly increased. This is why you often see each stage of an amplifier in it's own screened box, it helps to stop them interacting with each other.

Brian.
 


sir with the 50ohm resistor across the input of diode probe; when i measure output voltage AT THE ANTENNA CONNECTION its reading at some 0.5Volts!
 


Is it good or bad: that is the question.
 

i followed the instruction from borber regarding the trimmer capacitors! I placed a 6.8-45pF trimmer cap across the 1uH inductor (at the third last stage), then 4.2-20pF (Second last stage) on the 2.2uH and 5.6uH (last stage of 2N3866) ones.....

I also tuned them to maximum peak values (with RF diode probe) which are as follows:-
1) Third Last stage => 2N2222 => 1.28Volts
2) Second Last stage => 2N2219 => 1.36Volts
3) Last stage => 2N3866 => 1.86Volts
(all the above readings were taken with a 100ohm resistor placed across the input of the diode probe- b/c i lost my one and only 50ohm resistor)

I out of curiosity also checked on the freq count on each of the above stages with the succeeding stage connection removed (preceding stages connection intact) and found these readings:-
1) Third Last stage => 2N2222 => 27.130MHz
2) Second Last stage => 2N2219 => 27.130MHz
3) Last stage => 2N3866 => (drifting values) 13.56-15.7MHz
 
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is it that the last stage is interacting with some other stage or is it all okay and it will work?? Should i proceed with building the receiver?? if not, then what should be the next step b/c i am pretty confused now!
 

Since the receiver is half of your R/C system, you might as well divide your time to work on that too. The receiver needs a couple of LC tank circuits too, and amplification.

Once you build that, you'll adjust everything to the same frequency.
Then your job will be to obtain most reliable performance. Fortunately you now have the instruments you need.

I am still wondering if you need that many stages of amplification between oscillator and antenna. I can be wrong of course.
 

Intuitively, I would have one oscillator stage, one buffer and one output stage. That doesn't mean I'm right but from experience I would think it sufficient. For R/C work you only need a few mW but with three transistors as I suggest you should be able to produce at least 1 Watt. Note that if you manage 1W output the last transistor will draw ~400mA collector current so it will need a heat sink.

Brian.
 

So sir are you saying that the transmitter is working as it should have been considering the above readings i posted??
I am still getting a count of some 8-9MHz at the output of pi-network!
uptill now i was giving this transmitter power from a 6Volts/ 1Amp battery to inhibit any heating of the last stages because i havent installed any heat sink as of yet!

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Sir this is the design for the receiver i am thinking to use! Its actually preety same to waht i shared before as well but this time i tried to interface the super regenerative receiver output eith RX-2 IC.



The pinout for RX-2 IC i found from the datasheet in the below link:-
**broken link removed**

You may see that the components at the IC were not exact (especially the capacitors) because i couldnt find find them and tried to form substitutes..... could you please verify it??

Also sir i am as before i am thinking of placing the below pre-amplifier in place of the antenna shown in the above circuit. The C' is going to be a trimmer capacitor with L' probably a 1uH inductor.....
 

It is very crude but it should work. Check the value of the capacitor at the collector of Q3, you show 100pF, should it be 100nF?
Also double check the value of the capacitor in the base of Q1, it sets the 'squegging' rate of the detector, 4u7 seems too high to me, check it shouldn't be 4n7.

Incidentally, radiation resistance in a receiving antenna has no meaning!

Brian.
 

then plz help me make modifications in it! And yeah capacitor at the collector value was 100nF..... my mistake! I will check on the Q1 base capacitor!
thanks.....
 


Sir i checked some designs on the net and it seems like all those had Q1 base capacitor in micros.....
**broken link removed**

http://jap.hu/electronic/sregrcvr.html

but then i found a design which had the capacitor values in nano:-
http://www.talkingelectronics.com/projects/200TrCcts/200TrCcts.html#55

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and all the design were identical in the sense that they all were regenerative receivers!
 

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