EDNE November 2012 - (Page 70)

designideas which in turn increases the frequency. This combination of responses generates the wide tuning range. The 0.01-μF coupling capacitors separate the varactor bias voltage from the operation of the converter core. Light varactor biasing Microsoft Excel. This information then can be used in the voltage-to-frequencyconversion equation for the converter. For the NTE618, the approximate relationship of capacitance to voltage is expressed as C=800E−10×e−0.46V. Figure 4 shows the similarity between the calcu1,200,001 lated and measured results. The higher frequencies differ more as the varactor capaci1,000,001 tance reduces to the order of the stray capacitance in the circuit and parts. Careful lay800,001 out can minimize this issue and increase the range. NTE618 VARACTOR-BASED FREQUENCY (Hz) CONVERTER Note that at low input 600,001 voltages, the varactor-based response and the fixedcapacitor converter response 400,001 are nearly identical because of the varactor’s inverse exponential relationship 200,001 to the voltage. One useful RC-BASED CONVERTER result of achieving this range is eliminating the need to 1 switch between converters 0.001 2.001 4.001 6.001 8.001 10.001 to extend the tuning range. INPUT VOLTAGE (V) You can explore other useful and interesting applicaFigure 3 Replacing the fixed timing capacitor with a voltage-variable capacitor results in a much tions using this approach wider tuning range. with phase-locked loops, modulators, or function generators. 1,200,001 Editor’s note: Figure 13 of the Analog Devices AD654 data sheet contains errors 1,000,001 (Reference 2). The 74LS86 EDN DI5276 Fig 3.eps DIANE and LM360 cannot be subjected to 15V, and R7 is most 800,001 likely 8.2k, not 8.2Ω.EDN FREQUENCY (Hz) 600,001 CALCULATED (RED) VERSUS ACTUAL with high-value, 1M resistors prevents additional loading of the oscillator. This behavior is calculable and predictable to some degree, even from the data sheets. The tuning curve for the varactor diode can be generated in RefeRences Williams, Jim, and David Beebe, “Switching-regulator supply provides low-noise biasing for varactor diodes,” EDN, Nov 9, 2000, pg 117, www.edn.com/4359609. 2 “AD654: Low Cost Monolithic Voltage-to-Frequency Converter,” Analog Devices, http://bit.ly/MMCdOE. 3 “NTE618 Varactor Silicon Tuning Diode for AM Radio,” NTE Electronics, http://bit.ly/NLnZMQ. 1 400,001 CALCULATED VERSUS MEASURED (OVERLAPPING ON THIS SCALE) 200,001 1 0.001 2.001 4.001 6.001 8.001 10.001 INPUT VOLTAGE (V) Figure 4 The calculated and measured responses are in close agreement. 70 EDN EuropE | NOVEMBER 2012 www.edn-europe.com http://www.edn.com/4359609 http://www.bit.ly/MMCdOE http://www.bit.ly/NLnZMQ http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE November 2012

Digi-Key
Cover
Agilent Technologies
Contents
International Rectifier
Microchip
Digi-Key
Masthead
EDN comment
Microchip
Pulse
Analog Devices
Maxim Integrated
Maxim Integrated
Texas Instruments
Silicon Laboratories
Test & Measurement World
FTDI
Digi-Key
Strategies for extending product reliability
Managing the 8- to 32-bit processor migration
PCIM
Pickering Interfaces
Mechatronics in Design
Baker's Best
Digi-Key
Emerson
30 years of DSP
Hirose
Vicor
The nuances of variable-frequency drives
Digi-Key
National Instruments
Volt’s battery-stack manager aids Chevy’s drive
Digi-Key
EDNE
Design Ideas
RS Components
RS Components
RS Components
Product Roundup
Tales from the Cube
RS Components

EDNE November 2012