Electronic Design
  Click to view this week's ad screen

  
Reprints     Printer-Friendly    Email this Article    RSS   
 Submit
    Font Size     What's This?


[Ideas For Design]
Robust, low-cost continuous phase FSK modulator

Contributing Author  |   ED Online ID #6337  |   January 12, 1998


In bandpass data transmission, one common way to design low-cost frequency-shift-keying (FSK) analog modulators is to use two independent oscillators that have outputs switched according to the binary input wave to be modulated. While this approach is simple, phase discontinuities during oscillator switching unnecessarily increases the bandwidth of the modulated signal. On the other hand, any any solution based on the synchronization of the oscillator increases the modulator cost.

A well-known alternative is the use of classical oscillators such as opamp or transistor-based astables, CMOS oscillators, or “linear” oscillators (e.g., the Wien oscillator), where a capacitor value is switched under the control of the binary wave. Because the capacitor voltage can’t change instantaneously, the result is a continuous phase FSK (CPFSK). Another common solution involves the direct use of a VCO.

In both of these alternative cases, the design is very sensitive to powersupply variations and the tolerances among the IC’s fabrication series or manufacturers. This sensitivity could be negligible in small production runs, but leads to problems in large factory productions of FSK modulators, which are usually solved by adding some adjustable circuit component. Apart from the augmented production cost due to this adjustment, experience shows that the adjustment margin can be different if the oscillator’s IC is purchased from different manufacturers.

Shown here is a phase-locked loop (PLL) based alternative for CPFSK generation. It exploits the low sensitivity (or robustness) of the set composed of the phase detector and VCO with respect to certain parameter variations, mainly in the lower part of its frequency band (compared with the sensitivity of other solutions, such as the direct use of the VCO). However, the main problem in using a PLL for FSK generation is that the PLL input is designed for sinusoidal waves, not for the bit stream to be FSK-modulated.

The proposed solution benefits from the double-balanced mixer commonly used as a phase detector in PLL circuits. This mixer is based on a constant-current source for a differential amplifier (Fig. 1). By externally sinking or sourcing bias current at the transistor Q0, which acts as the current source, the conversion gain (KD) of the phase detector is changed. In a simplified X model of the PLL (Fig. 2a) with zero input (regulation model), the change of conversion gain is similar to having an offset in the output of the phase-detector model (Fig. 2b). This offset modifies the voltage input of the VCO of the PLL, so the VCO output frequency is varied under the control of the bit stream applied to the current source. This structure is similar to some phase-lock angle modulators. However, in the proposed idea, it’s unnecessary to break the PLL with additional circuitry in order to introduce the modulating signal, hence a single analog PLL can be used.

Figure 3 illustrates a circuit based on the analog phase-locked loop NE564. The IC 4016 is a switch controlled by the binary signal to be FSKmodulated, which commutes the voltage at pin 2 of th NE564 between ground and 5 V × 2700Ω/(2700Ω + 6800Ω) = 1.42 V. This voltage at pin 2 modifies the current bias of the current source in the phase detector, and consequently the VCO output frequency varies without phase discontinuities. The VCO center frequency is fixed by C0, and the margin between the two FSK frequencies is adjusted by R1 and R2. For an FSK modulator with frequencies of 390 kHz for “MARK” and 560 kHz for “SPACE,’ the component values are as shown in Figure 3.

In this design, the classical PLL low-pass filter has not been included, as its presence is unnecessary for FSK generation. In addition, the filter increases the transfer function sensitivity with respect to the VCO parameters.


Reprints   Printer-Friendly  Email this Article  RSS 
 Submit
  Font Size   What's This?


  		• 45-nm Via-Programmable ASICs Add High-Speed I/O Transceivers To Feature Mix
  		• Verification Evolves Into Lean, Mean Bug-Stomping Machines
  		• User Advisory Group To Guide Open Verification Methodology’s Evolution
  		• DDR3 and DDR2 Memory IP Bolsters SoC Designs
  		• PCB Tools Cross-Probe Between Layout And Schematic
  		• Constraint-Driven Flow Targets PCB High-Density Interconnects
  		• Cadence Abandons Its Bid To Buy Mentor Graphics
  		• Model Extractor For CMOS Sports Improved RF/DC Parameters
    1) Barbers, Adaptability, And Electric Pickup Trucks
    (740 views today)
    2) Low-Dropout (LDO) Linear Regulators
    (454 views today)
    3) Build A Smart Battery Charger Using A Single-Transistor Circuit
    (416 views today)
    4) SDR Transforms Amateur Radio
    (230 views today)
    5) Easily Convert Decimal Numbers To Their Binary And BCD Formats
    (227 views today)
    ALL TOP 20


    Reader Comments

    we are not alone but we are the bast

    mzaherm -June 30, 2008

    we are not alone but we are the bast

    mzaherm -June 30, 2008

    hi i realy need a circuit designe for amplitude shift keying modulator and a circuit that makes phase shift keying modulator.thanks for any body that can help.

    ahmed -June 29, 2008

    Please be good enough to send me a simple low cost detailed circuit diagram of FSK modem using PLL's since this is for my individual project component. Thanks in advance.

    MIA -June 27, 2008

    hello sir, i am a student of beng. and i am lokking for a circuet of fsk modulator which is design in matlab.so can you please help me out..thanks very much

    hemanshu -April 09, 2008

    sir, my name is raj. iam student of b.tech final year.please give me circuit diagram for 2 bit tansmitter & receiver.thankyou

    Anonymous -March 24, 2008

    projects about equalizers

    behzad_brs3000 -March 03, 2008

    Hello again , could you please tell me how to adjust this low-cost continuous phase FSK modulator to make its frequencies be around 10.8Mhz with max deviation 1 Mhz from this value? Thanks alot for your help.

    Salah -February 04, 2008

    Hello sir ,i am a college student and i am working on a FSK modulator-demodulator ,and i need a FSK demodulator for this low-cost continuous phase FSK modulator ,so could you please kindly send me a design with the design values as soon as possible ?? Thanks alot sir in advance .

    Salah -February 03, 2008

    hello sir, i am a college student and i'm working on a project,i have to use a fsk modulator and demodulator. i have to connect this to a PC with a baudrate of 9600bps. could you please tell me what should be the mark and space frequencies and also how to design the modulator ,demodulator and the FM transmitter and reciever. please help me.. thank so much before

    Anonymous -January 23, 2008

    hello sir, i am a college student and i'm working on a project,i have to use a fsk modulator and demodulator. i have to connect this to a PC with a baudrate of 9600bps. could you please tell me what should be the mark and space frequencies and also how to design the modulator ,demodulator and the FM transmitter and reciever. please help me.. thank so much before

    Anonymous -January 23, 2008

    is there any demodulator for NE564

    papargyris -December 17, 2007

    hello sir, i am a college student and i'm working on a project,i have to use a fsk modulator and demodulator along with FM transmitter and reciever,or for the alternative i want to use HT(handytalkie)as transceiver. i have to connect this to a PC with a baudrate of 9600bps. could you please tell me what should be the mark and space frequencies and also how to design the modulator ,demodulator and the FM transmitter and reciever. please help me..please thank so much before

    YuDha -November 02, 2007

    hello sir, i am a college student and i'm working on a project,i have to use a fsk modulator and demodulator along with FM transmitter and reciever,or for the alternative i want to use HT(handytalkie)as transceiver. i have to connect this to a PC with a baudrate of 9600bps. could you please tell me what should be the mark and space frequencies and also how to design the modulator ,demodulator and the FM transmitter and reciever. please help me.. thank so much before

    YuDha -October 31, 2007

    please send me a fsk schematic without a phase log loop

    conda -August 17, 2007

    I m 4th year EC student and require circuit idea or readymade modules using FSK technique for my project.plz send me some material or site links.... thank you.

    JINESH GANDHI -August 17, 2007

    How we can select the values of R1 and R2 TO DETERMINE THE MARK AND SPACE FREQUENCIES ?? Is there any specified formula to calculate the values of R1 and R2 in ref of mark and space frequencies?? I need it to know urgently as i am at critical stage of project. pls reply soon, thanks sanjay choudhary

    sanjay choudhary -June 26, 2007

    how does the I.C 4016,4051 work and there pin diagrams.THANK YOU

    Anonymous -June 10, 2007

    course of ask-psk fsk

    Anonymous -May 15, 2007

    please send me fsk,ask,psk,paw,ppm,pwm,pll circuit

    bahareh -April 28, 2007   (Article Rating: )

    Great deal

    Yousif -April 17, 2007   (Article Rating: )

    Dear Dearest, Me Praveen has been carrying a wireless project and am in a bad need of a FSK demodulator. I have already designed a mod using XR 2206 and demod using XR 2211. The prob is that the mod is working fine. But not the demod. Kindly help me with any other demodulator circuit of FSK. URGENTTTTTTTTTTTTTTTTTTTT

    Praveen Kumar -March 28, 2007

    Dear Dearest, Me Praveen has been carrying a wireless project and am in a bad need of a FSK demodulator. I have already designed a mod using XR 2206 and demod using XR 2211. The prob is that the mod is working fine. But not the demod. Kindly help me with any other demodulator circuit of FSK. URGENTTTTTTTTTTTTTTTTTTTT

    Praveen Kumar -March 28, 2007

    we build the schematics given here but it gives the ASK waveform so what can be the possible problem.

    asad -November 26, 2006

    Incredible, how can anyone know a to build an FSK mod/demod (me too). Im a student in a college en i really need de design please, or if someone have the design using de NE567 send it plz, i really appreciate. Another doubt, does anyone know a software to simulate it?

    Erwin -November 23, 2006

    hello sir, i am a college student and am working on a project wherein i have to use a fsk modulator and demodulator along with FM transmitter and reciever. i have to connect this to a PC with a baud rate of 9600bps. can you please tell me what should be the mark and space frequencies and also how to go about desigining the FM transmitter and reciever.Also plz let me know how to interface FSK demodulator to microcontroller n can you suggest me multiple error correcting code . please do help. Its urgent plz reply.

    Chintan -October 26, 2006

    Sir, i'm a college student. Here 's a questin i wanna ask.

    Is there any place/website where i can find backscatter modulator, ASK modulator and PSK modulator circuit?

    alex -October 11, 2006

    sir,im college student i need yr help, can u send me the circuit for transmiter & receiver of FSK. hopefully, can get yr feedback soon.TQ appreciate yr help.thanks a lot

    frm irene

    Anonymous -August 30, 2006   (Article Rating: )

    Alsalam alikom please sir, I ´d like to know how you choose the values of R1 and R2, and how they are related to mark and space frequences thanks

    reham -June 06, 2006   (Article Rating: )

    Hello sir, I ´d like to know how you choose the values of R1 and R2, and how they are related to mark and space frequences thanks

    reham -May 31, 2006

    I would like to know the same questions they asked before but I didn't found any answer, how you choose the values of R1 and R2, and how they are related to mark and space frecuency what are the exactly biascurrent what is needed for some frequency change. And also I would like to know why did u place a ic 4016 into the circuit Hopefully u could give me a answer thanks

    Wannaknow -May 22, 2006

    sir is there any special way to couple high frequency signals into the power line using FSK mod techs? please reply

    Sreeraj -May 12, 2006   (Article Rating: )

    very good! I want sir send for me about DSLAM (Digital Subscriber Line Access Multiplexer). Thank's

    Ardianto -May 08, 2006

    sir please give me your example circuit,mark freq,space freq,and mod.freq.

    eswaramoorthy.p -March 21, 2006

    hello sir, i am a college student and am working on a project wherein i have to use a fsk modulator and demodulator along with FM transmitter and reciever. i have to connect this to a PC with a baud rate of 9600bps. can you please tell me what should be the mark and space frequencies and also how to go about desigining the modulator ,demodulator and the FM transmitter and reciever. please do help.

    Anonymous -February 21, 2006   (Article Rating: )

    Sir , The output of NE564 is square wave , and my application is FSK Modulator , so is there's an easy way to obtain sinusoidal Wave instead of using LPF , thanx

    Abu Ammora -February 11, 2006

    Sir , The output of NE564 is square wave , and my application is FSK Modulator , so is there's an easy way to obtain sinusoidal Wave instead of using LPF , thanx

    Abu Ammora -February 11, 2006   (Article Rating: )

    Sir i want to know that how does FSK signal generate? And brief operation of generator circuit made by IC 555. And how we can select "mark" & "space" frequency?

    Kruti -February 09, 2006

    Sir i want to know that how does FSK signal generate? And brief operation of generator circuit made by IC 555. And how we can select "mark" & "space" frequency?

    Kruti -February 09, 2006

    sir i am an college student doing a research on topic continues phase frequency shift keying so please provide the latest technology and help me to draw transmitter and receiver for the same the tran receiver can be a simple one too please help me

    Anonymous -January 31, 2006

    sir i am an college student doing a research on topic continues phase frequency shift keying so please provide the latest technology and help me to draw transmitter and receiver for the same the tran receiver can be a simple one too please help me

    Anonymous -January 31, 2006   (Article Rating: )

    i need a modulator GFSK and a demodulator GFSK, or the same thing with FSK scheme (i will add a gaussian filter), thanks in advance.

    Regards ZINE

    ZINE -January 26, 2006

    Hi, I´m from Costa Rica, and I need a FSK modulator-demodulator circuit, because I´m tryng to connect two computer with RF by the serial port. Any help will be well received. Thank you

    David -January 21, 2006   (Article Rating: )

    sir,we r planning to make an fm tranmitter module along with the receiver to tranmit voice.the first stage consists of freuency modulator section(analog)followed by the fsk modulator circuit , finally the power ampliier stage.if possible please send the design and the circuit diagram for the tranmitter and receiver section.

    eeeeed -January 20, 2006

    please i would like you to send to me an article on how to design and basis of digital FM radio reciever . thank's

    Adejo -January 11, 2006

    please, Sir send me circuit of fsk modulator. thank you i need a circuit for commercial FSK modulator and FSK demodulator ......will any one send me for project purpose...... Thanx All

    Amro -January 01, 2006

    please, Sir send me circuit of fsk modulator. thank you i need a circuit for commercial FSK modulator and FSK demodulator ......will any one send me for project purpose...... Thanx All

    Amro -January 01, 2006   (Article Rating: )

    Hello sir I am doing project in RFID for transmitting data i am using FSK mod,can u please send me the fsk mod and demod circuit

    thank u with regards Fathima

    Fathima -December 24, 2005

    please send me circuit of fsk modulator. thank you.

    mik -December 18, 2005   (Article Rating: )

    I want electronic project report on data transmission through infra rays by using Frequency Shift Keying technique

    samarth -November 28, 2005

    I want electronic project report on data transmission by infra rays using FSK technique.

    samarth -November 28, 2005

    Hey there you!!! We need some help. Can anyone recommend a website where we can find an FSK and ASK modulator and demodulator circuits? HELP!!!!!!!!!!!!!!!! This is urgent!!!!!!

    Gregory -November 28, 2005

    i need a circuit for commercial FSK modulator and FSK demodulator ......will any one send me for project purpose......

    Anonymous -November 21, 2005

    i need a circuit for commercial FSK modulator and FSK demodulator ......will any one send me for project purpose......

    Anonymous -November 21, 2005

    hello sir, i am a college student and am working on a project wherein i have to use a fsk modulator and demodulator along with FM transmitter and reciever. i have to connect this to a PC with a baud rate of 9600bps. can you please tell me what should be the mark and space frequencies and also how to go about desigining the FM transmitter and reciever. please do help.

    Anonymous -October 28, 2005   (Article Rating: )

    Hi I ´d like to know how you choose the values of R1 and R2, and how they are related to mark and space frecuencies. I want to adjust free frequency of vco at 10.7Mhz.Does it relate to R1 &R2?Thanks

    nasrin -October 11, 2005   (Article Rating: )

    i want to know how ic 4016 is used as search logic to sychronize the clock of the transmitter and reciever pn sequence generator

    Anonymous -September 23, 2005

    with name of God With hello I hope,don't try and successful,else health. I have question,for example work frequency is 10 meg,but there is a 10.5 meg(for example) in input therfore vco will locked in 10.5 and no in 10. therfor data will recover in other frequency and no in desired frequency.am i true? thanks a lot Good bye.

    Rasool -July 30, 2005

    Sir, pls give m some circuits of FSK mod/demod, ASK mod/demod and two state TDM... i badly need it for our project...tnx

    kent -May 14, 2005

    how does ic-4016 work internally?

    Anonymous -April 02, 2005

    Hey people, I'm here...could somebody tell me about some FSK modulator and demodolator available in the market?

    Arshad -March 18, 2005

    What does digital data transmission mean?

    Anonymous -March 08, 2005

    Hello sir, I ´d like to know how you choose the values of R1 and R2, and how they are related to mark and space frecuencies. Thanks

    Fernando Tammarazio -February 25, 2005   (Article Rating: )

    hi ; now i do project about digital data transmission but i fase problem that i can not found the block diagram for tranmitter, channale and receiver can you help me .

    ibrahim -February 22, 2005

    Hi. Does anyone know how to build an FSK transceiver that can transmit data at 90/100 kHz at a baud rate of 2000? All the FSK chips I've seen only operate at 600, 1200, 2400, etc. baud.

    Graham Henshaw -November 10, 2004

    Please give us detail specification, and price leadtime. I will place order recently.

    Fred Shih -November 07, 2004

    Hello sir, my name is breenner. I am a college student at state polytechnic of jakarta (indonesia). Now i am have a final project to do. I must make a transceiver FM-FSK so i can transmit data. And when i see your idea in electronic design on 12nd january 1998 (circle 521) ,i decide to try your idea to make modulator FSK.Yes your idea is succeded. But the problem is come. That according all design of transceiver , modulator FSK must operate with "MARK" and "SPACE" frequencies under 16 KHz. I was trying to change the component of Co using 33 nF and 39 nF, also R1 and R2 . When I count the MARK" and "SPACE" frequencies, it is true that i could get "MARK" and "SPACE" frequencies under 16 KHz. But when i using it with the demodulator FSK, the result it failed. I check the output modulator (Vac), output at "MARK" frequency is different with "SPACE" frequency. And now decide to e-mail you to ask for help. Is it possible to design modulator FSK with "MARK" and "SPACE" frequencies under 16 KHz. If it is possible, can you show the circuit based on NE564. For your help and attention, i give you a lot of thanks before.

    breenner -September 17, 2004
    POST YOUR COMMENTS HERE
    Name:

    Email:
    Your Comments:

    Enter the text from the image below


    Please refresh the page if you have trouble reading this text.

    Search Electronic Design
         
      
     
    Email Newsletter
    Sponsored By:
    Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.

    Enter Email to Subscribe
      
    Electronic Design Europe Electronic Design China EEPN Power Electronics Auto Electronics Microwaves & RF RF Design
    Schematics Find Power Products Military Electronics Featured Vendors EE Events Free Design Resources



    		Planet EE Network | Contact Us | Editorial Calendar | Media Kit | Submit Articles | Headlines | Site Feedback | Subscribe
    Advanced Search | Help | Site Map
    Copyright © 2008 Penton Media, Inc., All rights reserved. Privacy