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Understanding RF

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I am using Tracy Shields' collection of Multisim experiments entitled "Understanding RF". I'm having major issues with Experiment 14 FM Modulation and Demodulation. Essentially, the setup for the voltage-controlled sine-wave oscillator (VCSWO) does NOT yield the desired results. Are the instructions incorrect, or is the VCSWO not up to the task? bottom line is we are NOT getting what should be a constant amplitude waveform whose frequency varies with an applied control voltage, specified, per this experiment, to be a 50% duty cycle triangular wave of 500 Hz, 6 Vp and 7 volts DC offset. The scope display looks horrid. something is not right. any suggestions?

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@Nowradioguy wrote:

I am using Tracy Shields' collection of Multisim experiments entitled "Understanding RF". I'm having major issues with Experiment 14 FM Modulation and Demodulation. Essentially, the setup for the voltage-controlled sine-wave oscillator (VCSWO) does NOT yield the desired results. Are the instructions incorrect, or is the VCSWO not up to the task? bottom line is we are NOT getting what should be a constant amplitude waveform whose frequency varies with an applied control voltage, specified, per this experiment, to be a 50% duty cycle triangular wave of 500 Hz, 6 Vp and 7 volts DC offset. The scope display looks horrid. something is not right. any suggestions?


Alright, so let me ask the question another way: are there restrictions on the VCSWO re both the amplitude and the frequency of the control signal? I'm thinking that 500 Hz is a bit of a stretch for that VCSWO to sweep its frequency up and down each second. And the control voltage swing between +1 and +13 volts may not be what the doctor ordered. I did try connecting a small capacitor to ground at the VCSWO output, and, while it cleaned up the horrid-looking output considerably, it would eventually cause the simulation to overload memory and stop FM'ing. Dare I say this is one abysmally poorly designed experiment? In its present form it is unusable and clearly fails to demonstrate the generation of a swept FM signal to electronics communications students.

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Hi,

 

You are right, it looks like the values described in the PDF are not the best for this experiment. The componets are working correctly, if you test a DC signal on the VCO you will see the corresponding frecquency.

 

Look at this configuration, this should be better to explain the experiment:

 

Capture.PNG               Capture2.PNG

 

I used a lower frecquency and less voltage to have something more visible. The frecquency is changing very fast with the values of the experiment, so we are not even able to see a cycle or something understandable. 

 

I hope this helps!

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Christopher,

 

I REALLY appreciate your replying!

 

In addition to the lower modulating frequency and amplitude, I was able to get some decent results by shunting a small (2 nF.) capacitor across the VCSWO output, AND using the Tektronix scope instead of the Multisim default scope. The capacitor cleaned up the sharp edges and the scope had better capability for viewing the changing waveform. We we able to see an FM wave compressing and expanding as the frequency changed. 

 

THANKS.

 

 

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You're welcome! That sounds good, thanks for sharing that as well.

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@ChristopherBrenes wrote:

Hi,

 

You are right, it looks like the values described in the PDF are not the best for this experiment. The componets are working correctly, if you test a DC signal on the VCO you will see the corresponding frecquency.

 

Look at this configuration, this should be better to explain the experiment:

 

Capture.PNG               Capture2.PNG

 

I used a lower frecquency and less voltage to have something more visible. The frecquency is changing very fast with the values of the experiment, so we are not even able to see a cycle or something understandable. 

 

I hope this helps!


Christopher,

 

Might I impose upon your good graces once again? In part 2 of this same experiment, we construct an FM demodulator. Some concerns:

 

(a) what is the purpose of the VIRTUAL (or IDEAL) CRYSTAL in the circuit? A crystal is a resonator, so, to what purpose?

(b) exactly what type of FM demodulator is this? It doesn't look like - at least to me - a Foster-Seely discriminator or a ratio detector, and it is certainly not a PLL; my (SWAG) sophisticated wild-a$$ guess would be a quadrature detector, but even that appears "out there";

(c) after the simulation runs for a while, the 10-kHz modulating signal decreases in amplitude, almost down to nothing; pause and restart, and the amplitude comes back up, but one can clearly see it decaying during each sweep of the scope; any idea?

 

Again, this is one of the most half-baked circuits in the collection. I've assigned it to more than one class, hoping either a newer version of Multisim resolves the issues, and/or we ourselves can solve them. Only partial success so far.   

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