Frequency Modulation Using the Voltage Controlled Oscillator and a
Function Generator Circuit
Nur Syuhada Zakaria
April 4th 2012
Executive Summary
This application note explains how to use the ZX95-2536C-S+ Voltage Controlled
Oscillator around a certain frequency in between 2315MHz to2536 MHz by modifying the
function generator circuit that is built using the XR-2206cp. This method is used by Design
Team 5 from ECE480 to build a radar system with RF components that has a frequency of
2.4GHz.
Keywords
Modulate – Exert a modifying or controlling influence
Frequency modulation – The modulation of wave by variation of its frequency
Bandwidth – A range of frequencies within a given band
Federal Communications Commission (FCC) – A commission that regulates interstate and
international communications by radio, television, wire, satellite and cable in the US
Introduction
The voltage controlled oscillator (VCO) is a device whose frequency changes linearly
with an input voltage. It is used to perform direct frequency modulation on signals. VCO has
a center frequency fc and the input (control) voltage m(t) modulates the instantaneous
frequency around this center frequency.
m(t)
( )
( )
VCO
Center frequency, fc
Frequency deviation constant, fd
xc(t)
( ) – Instantaneous frequency of FM signal
(
∫
( )
) – Modulated signal
Ac = Amplitude of modulated signal
Zakaria 1
Figure 1. ZX95-2536C-S+ Voltage Controlled Oscillator from Mini-Circuits
Below is the list of electrical specifications of the VCO:
 Frequency range = 2315 to 2536 (MHz)
 Power output = +6dBm
 Voltage range = 0.5 – 5 (V)
 Sensitivity = 57 – 77 (MHz/V)
 Port capacitance = 13.6pF
 3dB modulation bandwidth = 70MHz
 DC operating power = 5Vcc and 45mA
Objective
To modulate signals using frequency modulation using the ZX95-2536C-S+ Voltage
Controlled Oscillator around a certain frequency in between 2315MHz to2536 MHz by
modifying the function generator circuit. The function generator circuit can be created
using the XR-2206cp Monolithic Function Generator.
Issues
The main issue is modulating the frequency around a center frequency that is within
the specifications made by the Federal Communications Commission (FCC) regulation to
avoid any electromagnetic compatibility issues. The frequency of this VCO ranges from
2315MHz to 2536MHz. A band of frequencies around 2400MHz has been designated as the
Industrial, Scientific and Medical (ISM) radio bands and the devices using this band does
not need to be licensed for instance, cordless phones, WiFi and routers. The design must
not exceed the allowed band frequency.
Zakaria 2
Steps
A signal needs to be fed to the input of the VCO by the function generator circuit. The
function generator circuit can output either a sine, square or triangle wave depending on
the designer.
Figure 2. Circuit for Wave Generation using XR-2206cp
Figure 3. Output Amplitude as a Function of R3 at Pin 3
Zakaria 3
Figure 2 shows the circuit for a sine, triangle or square wave generation with
minimum harmonic distortion. R1 at pin 7 provides the desired frequency tuning. R3
determines the output swing which is the positive and negative peak of the waveform. The
selection of R3 value can be done by referring to Figure 3. RA adjusts the sine-shaping
resistor and RB provides the fine adjustment for the waveform symmetry.
Figure 4. Performance Data and Curves of ZX95-2536C-S+ Voltage Controlled Oscillator
Figure 4 shows the performance and data curves of the VCO. All of the parameters
on the table are at 25°C (room temperature) unless mentioned otherwise. Voltage tune is
the output voltage of the waveform generator. The voltage tune corresponds to the desired
frequency. Therefore, R1 and R3 on the waveform generator circuit should be tuned to the
list of voltage tune listed on the table in order to get the corresponding frequency.
Zakaria 4
Example
This is an example of having the minimum voltage at 1V, average voltage at 2V and
maximum voltage at 3V. According to figure 4, 1V, 2V and 3V correspond to 2.334GHz,
2.408GHz and 2.481GHz respectively. Therefore the center frequency of the VCO is
approximately 2.4GHz.
Figure 5. ZX95-2536C-S+ Frequency and Tuning Sensitivity
Hardware Developed
A hardware that was developed using this method is the radar kit that was done by
a design team from ECE480 Fall 2011 and modified by Design Team 5 from ECE480 Spring
2012 (Figure). This kit produces a triangle waveform from the function generator circuit
using Figure. The potentiometer R3 is tuned to get 3V as the max voltage and 1V as the
minimum voltage and potentiometer R1 is tuned to get 2V as the average voltage.
Function Generator
VCO
Figure 6. ZX95-2536C-S+ Voltage Controlled Oscillator and XR-2206cp Function Generator
Zakaria 5
Figure 7. Radar Kit Developed
Design parameters of the kit are as follows:






Frequency = 2.4GHz
Bandwidth = 80MHz
Waveform = Continuous wave triangle
Antenna isolation = 50dB
DC power is less than 1W
RF power is less than 1W
Conclusions
The function of VCO is to do direct frequency modulation at a specific center
frequency that is determined by the waveform produced by the function generator. The
frequency of the VCO depends on the voltage tune input of the VCO. The voltage output
swing of the function generator depends on the value of potentiometer at pin 3. When
designing a device, it is important to ensure that the frequency band of the device is
permitted by the FCC regulation.
Zakaria 6
References
EXAR Corporation. (1997, July). XR-2206 Monolithic Function Generator Datasheet. EXAR
Corporation, 48720 Kato Road, Fremont, CA 94538
Mini-Circuits. (n.d.). Coaxial Voltage Controlled Oscillator ZX95-2536C+ Datasheet.
www.minicircuits.com/MCLStore/terms.jsp
Radha, H. (2012, Spring). FM Modulators & Demodulators. In Communication Systems (pp.
4-6).
Williams, J. H. (2011, May 12). Modular Description RF Design. In MIT Open Courseware.
Retrieved February 1, 2012, from MIT Lincoln Laboratory website:
http://ocw.mit.edu/resources/res-ll-003-build-a-small-radar-system-capable-ofsensing-range-doppler-and-synthetic-aperture-radar-imaging-january-iap2011/lecture-notes/MITRES_LL_003IAP11_lec03.pdf
Zakaria 7