Post by thread:[OT] PLL's

> Can anyone tell me some websites with good introductions to PLLs on them? And for a purely hypothetical practical example, let's say that you wanted to design a FM Broadcast transmitter kit which covered from 88 to 108 MHz.......... :-) You first design a nice stable Voltage Controlled Oscillator which covers the required band and is tuned via DC onto a vari-cap diode. You decide on 100 kHz channel spacing, so you arrange to divide a convenient crystal down to generate a 100Khz reference (say divide a 4Mhz Xtal by 40). Next the VCO is divided down to 100Khz through a Programmable Divider chip (to Transmit on 88 MHz you divide by 880 and on up to 1080 to get 108 MHz). The two signals are then compared in a phase/frequency comparator, which generates an error voltage which steers the VCO onto exactly the right frequency (and phase). The only tricky bit is getting the time constants of the control loop correct. The maths of this can get nasty. It is basically a PID, with just the right amount of leading and lagging phase to overcome the mechanical and thermal instabilities in the VCO. Most people cheat and tweak the values empirically. Oh yes, and to Frequency Modulate this beast you could feed a little audio into the control line to waggle the VCO frequency. These days all the divider chains and comparator etc are available in a single tiny SM chip which you control over serial lines via a PIC. The PIC Xtal probably also generates the original 4 Mhz reference. The PIC picks up the channel number from a dip switch or rotary encoder, etc. See I just knew that we would get back on topic eventually!!!!! .................................... Zim

>From: Peter Crowcroft <.....peterKILLspam.....KITSRUS.COM> >Can anyone tell me some websites with good introductions to PLLs on them? Peter, 1. Philips have a PLL design program for the 74HC/HCT297 Their order number 9398 650 00011 This was May 1989 so they may well have newer stuff too. 2. Philips EC&A Vol 9 No 2 (1989?) pp 66 to 89 "All digital phase-locked loops using the 74HC297" 3. **** Exar have an old but good treatment of PLLs (not too complex but not totally simplistic either and probably all most people need) "Phase-Locked Loop Data Book" Exar, 1982, 80 A4 paages. Includes datra sheets, design guides, ... Good Also "XR-212As modem design guide" Exar, 1986 70 odd A5ish pages. Not nearly as useful for general purpose design as the above book. 4. Electronics and Wireless World August 1997 pp642-646 "Understanding phase noise" - which you will want to do if you want to get complex with PLLs. I imagine there is a host of stuff on the web on PLL design. Maybe not. I'd try a good topic concentrating serach engine like eg http://www.google.com and just use eg PLL design as search terms Russell McMahon _____________________________ >From other worlds - http://www.easttimor.com http://www.sudan.com What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-))

On Fri, 17 Mar 2000, Peter Crowcroft wrote: > Can anyone tell me some websites with good introductions to PLLs on them? Peter, Do you have a specific application in mind or are you just wanting to learn about PLL's? If you have a specific application, then I'm sure we can give you specific answers and suggestions. Instead of websites, you may wish to check out a book - there are several books devoted to the subject (Floyd, as I recall, has written one that's widely respected). But here's a quick tutorial (that overlooks many of the essential details...) A PLL is fed a signal of unkown but banded frequency. The PLL measures the frequency indirectly by generating a separate waveform at the same frequency. The algorithm for generating this separate frequency is simple: If the incoming frequency is higher than the one synthesized by the pll, then increase the frequency of the synthesized waveform other wise decrease it. The difficult part of course is implementing this. How do you 'synthesize a waveform'? How do you increase or decrease its frequency? How do you compare two signals for differences in frequency? These three questions are often answered in the classical PLL block diagram: IN +---------------+ |-------------+ +-----+ OUT ---> | phase compare | -> | Loop filter | -> | VCO | ----+--> | | |-------------+ +-----+ | +->| | | | +---------------+ | | | | | +------------------------------------------------------+ The frequency comparsion is performed in the first block. As its name suggests, the phase between the two signals is compared. This implies that the two signals are the same frequency. It just doesn't make sense to speak of the phase difference between two signals of differeing frequency. So why have a phase comparator? Well what you can do is compare phase on a per cycle basis between the two wave forms. For example, if the incoming signal is a sinewave that makes a zero crossing at t = t1 then the next zero crossing of the output wave form may be expressed in terms of a percentage of the period of the incoming wave form. The goal of the PLL is to get those zero crossings to align. Skipping the Loop Filter for the moment, the VCO or Voltage Controlled Oscillator, produces a waveform whose frequency is directly proportional to a voltage. You double the voltage and the VCO will double its frequency. The response of the VCO, or the at which it responds to voltage changes, is a strong function of the requirements of your system. But typically, the control to the VCO is dithered around a DC voltage. This DC voltage is the one that causes the VCO to produce the frequency that matches the incoming signal. But why dither? Well, it's similar to the way an opamp works. An opamp has a positive and negative input. If the positive input is greater than the negative, then the opamp's output is driven high otherwise it's driven low. Without any feedback, the output would be driven towards one of the supply rails. With feedback, the output is driven to the voltage that minimizes (or attempts to minimize) the difference in voltages between the positive and negative inputs. The exact same thing (conceptually) happens here. The difference is that instead of voltages, frequencies (or more accurately, phases) are being compared. However, this frequency difference is translated into a (DC) voltage. But from a time point of view, frequency is changing and DC is constant - something's got to give. Here's where the loop filter comes in. So far I've tried to be vary abstract and not say that the signals are sine waves or square waves or whatever. But it's convenient to imagine the phase comparator producing pulses (they don't all do this...). If the phase comparator's output is +1 then this may indicate that the incoming frequency is higher than the VCO's and consequently the DC control voltage to the VCO needs to be increased. If the phase comparator output is a 0 then the opposite case is true and the VCO needs to be decreased. But 0's and 1's need to be converted into a suitable DC control voltage for the VCO. Yhat's the job of the loop filter. In this example, you could say, "oh, that's easy - all you need is a low pass filter!" But in reality it depends on your VCO and on how you design the loop filter. The problem is that you have mixed constraints. You want the PLL to accurately lock onto a signal and you want the PLL also to respond to changes in the incoming frequency. The accuracy constraint means you'd like the changes in the VCO control voltage only slightly affect the VCO frequency. On the other hand, the response to a change in frequency constraint means you'd like the VCO to be sensitive to changes in the control voltage. This mixed constraint problem is what typically makes PLL's a challenge. Again, I've overlooked many of the details. In fact some of what I said may be totally inappropriate for the PLL you need. But the concepts are generally correct. In fact, these concepts may be applied to digital phase locked loops as well. It all depends on what you're trying to do. So, Peter, what are you trying to do? Scott

> And for a purely hypothetical practical example, let's say that you wanted > to design a FM Broadcast transmitter kit which covered from 88 to 108 > MHz.......... :-) > The two signals are then compared in a phase/frequency comparator, which > generates an error voltage which steers the VCO onto exactly the right > frequency (and phase). > > The only tricky bit is getting the time constants of the control loop > correct. The maths of this can get nasty. It is basically a PID, with just > the right amount of leading and lagging phase to overcome the mechanical and > thermal instabilities in the VCO. > Most people cheat and tweak the values empirically. There are a few other considerations if you want to use this for broadcast audio. Essentially you have a conflict between loop settling time and audio quality. Too short a time and the thing risks unlocking on bass audio signals. Many professional transmitters (and even a few of the "unofficial ones") use a dual time constant which runs fast when the VCO is a long way off frequency but switches to a very slow rate when it is close. Think 4066... Cheers, Andy.

from http://techref.massmind.org http://www.microlet.com/yam/. Schematics for their early FPGA, which includes a Digital PLL implementation. All digital PLL is generally not a good choice to reduce jitter (unless your jitter's real bad to begin with). Jitter attenuators do the job, though http://www.dalsemi.com/DocControl/PDFs/2188.pdf... The Bell Labs tech journals from the mid 1970's have tons of articles on jitter in communication systems. The bible on analog PLLs theory is "Phaselock Techniques by Gardner (1979)" "Phase Locked Loops : Design, Simulation, and Applications" by Best has a very basic intro to DPLLs "Monolithic Phase-Locked Loops and Clock Recovery Circuits : Theory and Design" from the IEEE is more current, and has a good PLL theory section. http://www.infosite.com/%7Ejkeyzer/piclist/2000/Feb/1777.html Using a PLL and an integrator to measure the amplitude of a signal at a given frequency an reject noise (a one channel spectrum analyzer) --- James Newton EraseMEjamesnewtonspam_OUTTakeThisOuTgeocities.com 1-619-652-0593 http://techref.massmind.org NEW! FINALLY A REAL NAME! Members can add private/public comments/pages ($0 TANSTAAFL web hosting) {Original Message removed}

How do you actually make a transmitter TRANSMIT?!? I have made many kits in my time, but never understood this. Suppose there is an oscillator running at say 100MHz, which you modulate with your input signal... Now how you convince the high frequency signals in your circuit to just shoot off into free air? I mean, HOW DO YOU TRANSMIT?!? At 09:27 PM 3/17/00 +1100, you wrote: {Quote hidden}>> Can anyone tell me some websites with good introductions to PLLs on them? > >And for a purely hypothetical practical example, let's say that you wanted >to design a FM Broadcast transmitter kit which covered from 88 to 108 >MHz.......... :-) > >You first design a nice stable Voltage Controlled Oscillator which covers >the required band and is tuned via DC onto a vari-cap diode. > >You decide on 100 kHz channel spacing, so you arrange to divide a convenient >crystal down to generate a 100Khz reference (say divide a 4Mhz Xtal by 40). > >Next the VCO is divided down to 100Khz through a Programmable Divider chip >(to Transmit on 88 MHz you divide by 880 and on up to 1080 to get 108 MHz). > >The two signals are then compared in a phase/frequency comparator, which >generates an error voltage which steers the VCO onto exactly the right >frequency (and phase). > >The only tricky bit is getting the time constants of the control loop >correct. The maths of this can get nasty. It is basically a PID, with just >the right amount of leading and lagging phase to overcome the mechanical and >thermal instabilities in the VCO. >Most people cheat and tweak the values empirically. > >Oh yes, and to Frequency Modulate this beast you could feed a little audio >into the control line to waggle the VCO frequency. > >These days all the divider chains and comparator etc are available in a >single tiny SM chip which you control over serial lines via a PIC. The PIC >Xtal probably also generates the original 4 Mhz reference. The PIC picks up >the channel number from a dip switch or rotary encoder, etc. > >See I just knew that we would get back on topic eventually!!!!! > > >.................................... Zim > >

??? That's what the antenna's for! {Quote hidden}> -----Original Message----- > From: David Lions [dlionsspam_OUTMIVA.COM.AU] > Sent: Sunday, March 19, 2000 9:31 PM > To: @spam@PICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: [OT] PLL's > > > How do you actually make a transmitter TRANSMIT?!? I have > made many kits > in my time, but never understood this. > > Suppose there is an oscillator running at say 100MHz, which > you modulate > with your input signal... > > Now how you convince the high frequency signals in your > circuit to just > shoot off into free air? I mean, HOW DO YOU TRANSMIT?!? > > At 09:27 PM 3/17/00 +1100, you wrote: > >> Can anyone tell me some websites with good introductions > to PLLs on them? > > > >And for a purely hypothetical practical example, let's say > that you wanted > >to design a FM Broadcast transmitter kit which covered from 88 to 108 > >MHz.......... :-) > > > >You first design a nice stable Voltage Controlled Oscillator > which covers > >the required band and is tuned via DC onto a vari-cap diode. > > > >You decide on 100 kHz channel spacing, so you arrange to > divide a convenient > >crystal down to generate a 100Khz reference (say divide a > 4Mhz Xtal by 40). > > > >Next the VCO is divided down to 100Khz through a > Programmable Divider chip > >(to Transmit on 88 MHz you divide by 880 and on up to 1080 > to get 108 MHz). > > > >The two signals are then compared in a phase/frequency > comparator, which > >generates an error voltage which steers the VCO onto exactly > the right > >frequency (and phase). > > > >The only tricky bit is getting the time constants of the control loop > >correct. The maths of this can get nasty. It is basically a > PID, with just > >the right amount of leading and lagging phase to overcome > the mechanical and > >thermal instabilities in the VCO. > >Most people cheat and tweak the values empirically. > > > >Oh yes, and to Frequency Modulate this beast you could feed > a little audio > >into the control line to waggle the VCO frequency. > > > >These days all the divider chains and comparator etc are > available in a > >single tiny SM chip which you control over serial lines via > a PIC. The PIC > >Xtal probably also generates the original 4 Mhz reference. > The PIC picks up > >the channel number from a dip switch or rotary encoder, etc. > > > >See I just knew that we would get back on topic eventually!!!!! > > > > > >.................................... Zim > > > > >

> > Now how you convince the high frequency signals in your circuit to just > shoot off into free air? I mean, HOW DO YOU TRANSMIT?!? > Thats a good question... Radio is my brackground... qualifications and all that... and its a bit of a mystery to me to. What you are doing with a transmitter is taking electricity from your power source and putting it into the air/ space whatever. The device we use for this is an aerial which is an efficient interface between the air and the circuit. Maximum power transfer from the aerial to the air will occur when the aerial is matched to the impedence of the transmitter (with losses). Radio waves are both electric and magnetic so if you use an open ended wire you are utilising the electric component to match with, if you use a loop you are matching with the magnetic component. So there are certain lenghts of aerials that we can use dependent on the frequency we are transmitting and how efficient we want our aerial to be. ..... its a big topic that is covered by a huge array of books but a good start is to connect a 1/4 or 3/4 wavelenght aerial to your output. Wavelenght(metres) = speed of light(300,000Metres/Sec) / frequency. _____________________________ Lance Allen Technical Officer Uni of Auckland Psych Dept New Zealand http://www.psych.auckland.ac.nz _____________________________

some other little tidbits to add to this discussion.... The match between the aerial and the surrounding space is also important to the effeciency of the aerial. The effeciency of a given antenna (aerial) can be improved to a point by making the elements as large as practical given the constraints of the frequency and the bandwidth allowable. Also, the height above the terrain is an important factor. Generall speaking, the higher, the better. But, there is a practical limit. And, the material the antenna is made of is somewhat important to effeciency. And, I agree with what Lance said below. There are many many volumes of information and antenna theory out there. The web is a good place to look. Especially at HAM Radio sites. You'll find a wealth of information here. Good Luck, Jim On Mon, 20 March 2000, Lance Allen wrote: {Quote hidden}> > > > > Now how you convince the high frequency signals in your circuit to just > > shoot off into free air? I mean, HOW DO YOU TRANSMIT?!? > > > > Thats a good question... > Radio is my brackground... qualifications and all that... > and its a bit of a mystery to me to. What you are doing > with a transmitter is taking electricity from your power > source and putting it into the air/ space whatever. > The device we use for this is an aerial which is an > efficient interface between the air and the circuit. > Maximum power transfer from the aerial to the air will > occur when the aerial is matched to the impedence of the > transmitter (with losses). > Radio waves are both electric and magnetic so if you use > an open ended wire you are utilising the electric > component to match with, if you use a loop you are > matching with the magnetic component. > So there are certain lenghts of aerials that we can use > dependent on the frequency we are transmitting and how > efficient we want our aerial to be. ..... its a big topic that > is covered by a huge array of books but a good start is to > connect a 1/4 or 3/4 wavelenght aerial to your output. > Wavelenght(metres) = speed of light(300,000Metres/Sec) > / frequency. > _____________________________ > > Lance Allen > Technical Officer > Uni of Auckland > Psych Dept > New Zealand > > http://www.psych.auckland.ac.nz > > _____________________________ KILLspamjimKILLspamjpes.com

You wrote.. > some other little tidbits to add to this discussion.... > The match between the aerial and the surrounding space is also > important to the effeciency of the aerial. The effeciency of > a given antenna (aerial) can be improved to a point by making > the elements as large as practical given the constraints of the > frequency and the bandwidth allowable. Also, the height above > the terrain is an important factor. Generall speaking, the > higher, the better. But, there is a practical limit. And, the > material the antenna is made of is somewhat important to > effeciency. And, I agree with what Lance said below. There > are many many volumes of information and antenna theory out > there. The web is a good place to look. Especially at HAM > Radio sites. You'll find a wealth of information here. > > Good Luck, > > > Jim > It is such an involved topic that I didnt know how much to cover as an ' intro '. I tried to stick to the basic concepts. Being a UHF specialist I have to conclude radio is bordering on a bit of a black art, I love it when a transistor in the front end is short/dead/missing in a 800MHz RX and it just lowers the s/n ratio a bit but essentially the RX keeps working. _____________________________ Lance Allen Technical Officer Uni of Auckland Psych Dept New Zealand http://www.psych.auckland.ac.nz _____________________________