Multisim (EWB) FM problem

[fasti]

Has anyone been successful at constructing an FM transmitter in Multisim?

I HAVEN'T!

I could use an advice...

I've been entering all sorts of simple FM Tx circuits (amature crappy ones) and I've been expecting to see the carrier has been modulated.
The method of checking the modulation I used, was connecting the simulated oscilloscope and switching it to A/B mode.
If the carrier wave is modulated you're supposed to see a trapezoid (sideways), and if it's not (meaning it's a clear sine wave) you'll see a square.

Am I correct?
Well, then I've been seeing squares.
For each and every circuit.
Could it be that multisim can't handle that kind of circuits?
Or is my methode wrong, and these (many and from different hobbyist sites) circuits actually work?

BTW, in Y/T mode, the carrier wave doesn't look modulated (it doesn't expand and shrink like fm does. even if the modulating signal is in the MHz frequencies..).

Help..!

 

[audioguru]

Full modulation of an FM broadcast band transmitter is only plus and minus 75kHz. There is no way that you will see such a tiny change of the frequency of a 100MHz wave.
You must think that the deviation is from 10MHz to 1GHz. Then you would see the waves expand and shrink.

 

[JimB]

The trapedoidal display on the scope is is for AM type modes not FM.
It is possible to see the "time spead" caused by FM on an oscilloscope provided that the scope has a stable trigger and an expandable x-trace.

As for the simulator, forget it and look at a real circuit and test equipment.

JimB

 

[fasti]

and for FM

but still, is there a way to see if the modulation is okay in the A/B mode of the oscilloscope?
or a better way than looking at the carrier frequency change when the signal frequency is close (like when carrier=100MHz and signal=20MHz).
any better way?


BTW, I tried changing the modulating signal to 10MHz and 50MHz which should be visible in the carrier (spacing of the frequency), and still there seems to be no difference.. maybe Multisim doesn't modulate?
it couldn't be that all the "FM bugs" actually don't work, right?

 

[audioguru]

An FM bug can be modulated up to about 5kHz so it sounds like a telephone or an AM radio. The capacitor that connects the base to ground cuts higher modulating frequencies. The amount of frequency deviation with a loud modulating signal is 75kHz. It is way too small to be seen on a 'scope.

Simple FM transmitters cause the oscillator transistor to be amplitude modulated. The changes in its amplitude changes the transistor's capacitance which deviates its frequency making FM. Better FM transmitters use a varactor diode to directly change the frequency of the oscillator.

Some simulation programs don't work. Circuits do.

 

[JimB]

but still, is there a way to see if the modulation is okay in the A/B mode of the oscilloscope?

No.

or a better way than looking at the carrier frequency change when the signal frequency is close (like when carrier=100MHz and signal=20MHz).
any better way?

My scope (a real 50Mhz 30year old item) can see the deviation on a 20Mhz carrier when modulated to 15kHz deviation with a 1khz tone.

BTW, I tried changing the modulating signal to 10MHz and 50MHz which should be visible in the carrier (spacing of the frequency), and still there seems to be no difference.. maybe Multisim doesn't modulate?
it couldn't be that all the "FM bugs" actually don't work, right?

More than likely that multisim does not understand that these toys work by changing the capacitance in the junctions of the oscillator transistor.
If you found a proper FM modulator circuit and modelled that, you may be in with a chance.

JimB

 

[Hero999]

I think the simulator software doesn't take into account the change in capacitance in a transistor with change in voltage. The models probely just list capacitances for a typical device rather than recalculating them when the signal changes. The only way to see the frequency change in such a circuit is to build it and measure it using a frequency counter.