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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.

VCO

Center frequency, fc

Frequency deviation constant, fd

( ) ( ) – Instantaneous frequency of FM signal

( ) ( ∫ ( )

) – Modulated signal

Ac = Amplitude of modulated signal

m(t) xc(t)

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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.

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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

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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.

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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.

Figure 6. ZX95-2536C-S+ Voltage Controlled Oscillator and XR-2206cp Function Generator

Function Generator

VCO

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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.

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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-of-sensing-range-doppler-and-synthetic-aperture-radar-imaging-january-iap-2011/lecture-notes/MITRES_LL_003IAP11_lec03.pdf