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CHALLENGE - SS rig for the AM PW (QRP) net.




 
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Author Topic: CHALLENGE - SS rig for the AM PW (QRP) net.  (Read 51006 times)
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ssbothwell KJ6RSG
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« Reply #100 on: January 26, 2012, 11:42:43 PM »

i just tried that test using an 8.3W bulb for a car brakelight. i hooked up the lineout from my computer to the audio in of the modulator and played some music. the bulb glows but it is pretty dull (especially compared to if i sort a 12V/3A power supply with the bulb) and it doesnt change brightness much at all. i guess i should try in the other room using my stereo receiver and see what happens.

same situation with my home stereo. on my scope i can see the audio modulating the carrier signal and it is 6VRMS but it is only 600mV Pk-Pk.

(in the attached image 0V is at the bottom of the window.)


* image (39).jpg (195.46 KB, 1280x960 - viewed 441 times.)
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W1FVB
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« Reply #101 on: January 27, 2012, 10:50:49 AM »

i just tried that test using an 8.3W bulb for a car brakelight. i hooked up the lineout from my computer to the audio in of the modulator and played some music. the bulb glows but it is pretty dull (especially compared to if i sort a 12V/3A power supply with the bulb) and it doesnt change brightness much at all. i guess i should try in the other room using my stereo receiver and see what happens.

same situation with my home stereo. on my scope i can see the audio modulating the carrier signal and it is 6VRMS but it is only 600mV Pk-Pk.

(in the attached image 0V is at the bottom of the window.)


Sol,
It looks like there's a problem with the circuit. It shouldn'T have any problems "swinging" the voltage to a double.
I hooked up my scope and made a little movie this morning:
http://youtu.be/uttBWvyDK1w

(You can see that my DC at carrier level is not nice and flat under load of the RF deck! However this isn't noticeable
on the audio from the receiver)

I expect that something is not working right with your circuit. I'll attach the latest schematic of the circuit and that is basically
the one I'm using in the video. (I just noticed that the 10uF cap symbol after the 5K6 should be an electrolytic)
Maybe check pin 2 & 3 to the op-amp?


* modulator.jpg (59.28 KB, 960x548 - viewed 526 times.)
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ssbothwell KJ6RSG
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« Reply #102 on: January 30, 2012, 11:22:09 PM »

hi frits. sorry i dissipeared for a couple days. i got busy with work and school.

i am gonna go over the my modulation circuit tomorrow and try to figure out what is going wrong.

just to be clear, if the modulator is working correctly and i am inserting an audio sinewave into the carrier i should see a 12V Peak to Peak signal?
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« Reply #103 on: January 31, 2012, 08:02:26 AM »

hi frits. sorry i dissipeared for a couple days. i got busy with work and school.

i am gonna go over the my modulation circuit tomorrow and try to figure out what is going wrong.

just to be clear, if the modulator is working correctly and i am inserting an audio sinewave into the carrier i should see a 12V Peak to Peak signal?


No problem,
The audio signal "rides" on the carrier. If it peaks reaches 2 times the carrier voltage you've achieved 100% positive modulation
And with the circuit , yeah set the carrier voltage half of the power supply voltage and the audio peak should be able to reach the 12 volt again. However there is a little loss in the 2N3055 ,to compensate , just turn the carrier level a little bit lower.
Keep testing it on the light bulb

Hope you find the problem,

Frits
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« Reply #104 on: January 31, 2012, 02:28:41 PM »

SOL,

Let's say you expect a 5 Watt output from the circuit. (Assuming Class C or E final)

This is about 450 mA of current at 12 V peak.

Rload = V^2/(2XP) = 144/10 = 14.4  or 15 ohms.

Recommend you place a 15 ohm resistor between emitter of 2N3055 and ground rated at 5 Watts or more, and measure emitter voltage.

Also check your voltages at pin 7 and 4 of the IC. 7 = 12V (or whatever supply voltage) and  4 = zero.

As the 100k potentiometer is moved up toward 12v, you should see the emitter voltage of the final pass transistor 2N3055 rise as the potentiometer wiper goes toward 12V, since the wiper supplies voltage to the non-inverting input of the opamp. Stop at say 6 V.

Apply audio. Voltage at emitter should vary between I would think about 2V and 10 Volts in step with audio.

Phil
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« Reply #105 on: April 04, 2012, 11:53:51 AM »

It came to my attention that the feedback resistor in the modulator schematic was incorrectly listed as 50K instead of 150K.
The 50K value is most likely to much feedback to the TL071 opamp.

Here's the updated schematic:
http://files.myopera.com/Frister/projects/modulator.jpg

Hope you didn't toss it in the garbage yet Sol  Wink
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ssbothwell KJ6RSG
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« Reply #106 on: April 05, 2012, 01:14:32 PM »

very interesting. my board has been sitting on my desk forever.i assumed i design it incorrectly and havent gotten around to checking it against the schematic. i'll try switching out the feedback resistor and see what happens.
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ssbothwell KJ6RSG
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« Reply #107 on: April 16, 2012, 05:38:22 PM »

hi frits. i tried assembling the modulator in LTSpice  but it doesnt seem to be operating correctly.

the audio signal gets biased to ~30V at the op-amp input and the op-amp output is a flat ~29V dc signal.

does this circuit look right?


* modulator-ltspice.jpg (578.83 KB, 1440x874 - viewed 419 times.)
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« Reply #108 on: April 18, 2012, 08:00:30 PM »

simulation needs a load resistor between L1 and ground
Bias the op amp positive input so the output is just under 50% VCC of your final
Add a resistor between the positive input of the op amp and ground to pull the voltage down. this can be a trim pot to make it adjustable.
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