Simple AM modulator

Thread Starter

tubedude17

Joined Jul 6, 2020
4
Hi,

Probably a dumb question but I'm no expert on analog design, so forgive my ignorance. The goal of this circuit is to take a carrier signal generated by my Tek cfg280 and modulate a fixed audio signal generated by the oscillator. The reason for this is to troubleshoot and align antique am radio sets. I'm not seeing anything on the output. Am I missing something? Both the oscillator and the bjt amplifier work on their own.
Any help or tips are appreciated!

IMG_20200706_184921__01.jpg
 

sparky 1

Joined Nov 3, 2018
260
Charles Wenzel describes his own mixer for cd player and other inputs that uses the LM833 op amp, he describes a little about the (modulator) section for his AM transmitter. The mixer schematic shown in the 4th picture . I believe this unit was designed for outdoors and camping, helping others to build radio as a hobby. An American classic the micropower AM radio transmitter.
http://www.techlib.com/electronics/amxmit.htm
 
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Irving

Joined Jan 30, 2016
797
The BJT emitter has no DC path to ground!

- Nandu.
I was about to say the same. I'm guessing that's meant to work as an RF amp and the audio injected into the emitter moves the bias point, but I still think there needs to be some DC component in the emitter circuit. Either another resistor to ground across the 1nF capacitor or, since the
audio is biased at 4.5v possibly across the 15nF coupling capacitor. Though that's less likely as the base is at approx 9 x 42/(89+42) = 3v. Assuming Vc should be 4.5v I'd try a 3.3k from emitter to ground and short the 4.2k resistor from the opamp.

Or ground the 4.2k resistor and move the 15nF capacitor to the emitter.

@tubedude17

What are your RF and audio levels going into the mixer and can you post the DC levels at b, c and e.
 
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Irving

Joined Jan 30, 2016
797
@tubedude17

Been playing with your circuit in the simulator and I can't make it work. The reason is simple, this is a class C amplifier which means the transistor is mainly off and only conducts for a tiny part of the cycle - normally the collector of the transistor is connected to a tank circuit at the required frequency which resonates with the kick from the transistor. The modulating signal moves the bias point to change to 'on' time of the transistor and therefore the amplitude. Without the tank circuit it just generates an output which is a highly squelched and distorted RF signal which I doubt is much use to your testing.

I appreciate a tank circuit may not be what you want, as it requires tuning and you want a wide-band mixer but this isn't going to work for you. Anyway, here's the circuit, tuned for 1MHz, 1vp-p RF in, 9v p-p audio in,

1594131171883.png
 

Thread Starter

tubedude17

Joined Jul 6, 2020
4
@Irving thanks for your answer! I took your advice and updated the circuit. I'm getting fairly correct voltages now: Vc=4.56v, Vb=2.66V, and Ve=2.13V. My carrier signal is .1Vpp and the waveform generated by the oscillator is 5Vpp.

There is an output now but it doesn't resemble a textbook modulation signal. It's also distorted- I probably need to reduce the amplitude of the oscillator to keep it from driving the bjt out of its active region. Any thoughts on how I could get this to perform better? Thanks!
 

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Irving

Joined Jan 30, 2016
797
I don't think it will, for the reasons I've cited above. All the examples I've found on various circuit compendiums show a tank circuit in the collector or hung off the output to ground.
 

jbeng

Joined Sep 10, 2006
74
I guess my question would be why aren't you using the external modulation input on the CFG 280? On page 12 of the user manual, it says to connect a voltage source (no more than 5v p-p) to the "Ext AM Input" jack on the rear panel of the unit and it will modulate the output using AM. All you really need is a sine wave generator connected to the jack and you're there.
 
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Thread Starter

tubedude17

Joined Jul 6, 2020
4
Oh crap that's embarrassing... I got the thing used and it didn't come with the manual. I guess I didn't think to download it or take a good look at the back (till now). I'm an idiot, hehe. I'll go and rtfm now. Thanks!! xD
 
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