[[RF.Spice]]'s FM modulator takes an arbitrary input signal and generates a frequency-modulated sinusoidal signal at its output.
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To generate a binary sequence for this project, , you will use a "Digital Source" followed by a DAC conversion bridge. The data rate will be 1GHz or 1G sample/s. In other words, we assume a clock rate of 1GHz with a period of 1ns and a pulse with of 500ps, The input binary sequence is:
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0101 1010 1101 0001 0000
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[[File:SysTUT3 31.png|thumb|550px|The property dialog of the digital source.]]
The digital source device itself doesn't have an explicit clock input. You set the timing through its '''Time-Value Array''' table as shown in the table below:
{| class="wikitable"
|-
! Time !! State
|-
| 0 || 0
|-
| 1ns || 1
|-
| 2ns || 0
|-
| 3ns || 1
|-
| 5ns || 0
|-
| 6ns || 1
|-
| 7ns || 0
|-
| 8ns || 1
|-
| 10ns || 0
|-
| 11ns || 1
|-
| 12ns || 0
|-
| 15ns || 1
|-
| 16ns || 0
|-
| 20ns || 0
|-
|}
Place and connect all the parts as shown in the figure. The DAC conversion bridge generates a pulse train of binary voltage levels 0V and +5V based on the input bit sequence. For the QAM modulator, you will use a 3GHz carrier frequency with a carrier amplitude of 5V. For the signal splitter, you will use the Wilkinson power divider you explored earlier in RF Tutorial Lesson 12. Set the center frequency of the Wilkinson power divider to fc = 3GHz and set its power ratio to K =1 for equal split between the two output ports.
</table>
<table>
<tr>
<td>
[[File:SysTUT3 30.png|thumb|720px|The QAM modulator circuit.]]
</td>
</tr>
</table>
Run a Transient Test of your modulator circuit with the following [[parameters]]:
|-
! scope="row"| Stop Time
| 25n4u
|-
! scope="row"| Linearize Step
| 10p1n|-
! scope="row"| Step Ceiling
| 10p1n
|-
! scope="row"| Preset Graph Plots
| a1v(Digital1), v(62)
|}