Frequency modulation? Amplitude modulation?

[bananasiong]

Hi,
I'm doing simulation on frequency modulation. The modulating signal is 5 kHz, 500 mV and the carrier is around 30 MHz, 2 V.
As shown in the schematic, the blue waveform is the modulating signal. The red waveforms are the carrier signal and the modulated signal. With the switch opened, there is only carrier signal at the output, which is around 30 MHz. When the switch is closed, I can see the modulated waveform. I found that when the amplitude of the modulating signal is peak, the frequency of the modulated is higher than the carrier frequency. When the amplitude of the modulating signal is low, the frequency of the modulated signal is lower than the carrier frequency. These are expected except the amplitude of the modulated signal keeps changing but is not as the same as the carrier signal.
I'm not sure whether the schematic is correct or not because I took the modulation part of a FM transmitter and modified it, just want to see the modulated waveform as compared to the reference book.

EDIT: The bottom waveforms are the carrier signal, I zoom in and display on the right hand side.

Thanks

 

[Roff]

The modulating signal changes the transistor current (because the voltage across the 100 ohm resistor changes), so you will get amplitude modulation along with your FM.

 

[Nigel Goodwin]

As I'm sure you've been told already, in all your posts about FM transmitters, these are only really toy devices - they give both AM and FM modulation due to the crude way they work.

 

[ljcox]

It is not really AM. It looks to me like the sum of the carrier (c) and the mod signal (v), ie. c + v.

True AM is the product, ie. c * v and is the shape of a person's eye.

 

[Hero999]

Yes but the lower carrier frequency won't be transmitted because the aerial will be far too short.

 

[Roff]

It is not really AM. It looks to me like the sum of the carrier (c) and the mod signal (v), ie. c + v.

True AM is the product, ie. c * v and is the shape of a person's eye.

Yeah, you're right, Len. I didn't even think about it. I have seen AM on these toy FM transmitters before, so I just started spouting off. I'll still bet there is some, though. You need to look on the collector to really see it.

 

[JimB]

To see what an AM signal really looks like, have a look at the pictures which I posted in this thread:

https://www.electro-tech-online.com/threads/need-advice.27014/

JimB

 

[audioguru]

That is why a super-regen "radio" picks up these toy transmitters, it detects the AM.
The "mixed" waveform bounces up and down normally with the modulation and the vertical amplitude of the carrier also changes a little with the AM but can't be seen.

 

[ljcox]

To see what an AM signal really looks like, have a look at the pictures which I posted in this thread:

https://www.electro-tech-online.com/threads/need-advice.27014/

JimB

Yes, that is a good picture. It looks like it is less than 100% modulation.

My recollection is that when the modulation reaches 100%, the minima are zero which is what I meant by it "looking like a person's eye".

 

[JimB]

Yes, that is a good picture. It looks like it is less than 100% modulation.

Yes, the sinewave is about 50% modulation.

My recollection is that when the modulation reaches 100%, the minima are zero which is what I meant by it "looking like a person's eye".

Correct, for symmetrical 100% AM, the maximum of the modulation envelope should be twice the amplitude of the unmodulated carrier, and the minimum of the envelope should be zero.

JimB

 

[bananasiong]

Yes, I've got those waveform too with the experiment kit in my college. I expected that I can get the waveform that I see from the reference book, i.e. the carrier frequency is proportional to the amplitude of the modulating signal, with the constant amplitude. But what I got in this simulation is, FM with inconstant amplitude. Both frequency and amplitude of the modulated signal are proportional to the modulating signal. I can see this type of design in a few simple FM transmitter..
And yes, I've been told about this, by audioguru. His mod4 is FM with a little AM.

Thanks

 

[audioguru]

Modulate the capacitance of a varactor diode that tunes with the coil then you will have perfect FM without any AM.

 

[bananasiong]

Do you mean use a varactor diode and the coil as the carrier? I didn't learn what a varactor diode is, just went through it. The equivalent circuit consists of resistors, capacitor and inductor. Its capacitance is varied by the voltage in reverse bias.