Post by thread:Oscilator problem

Hi all! I am having problems with an 20Mhz oscillator that does not want to oscillate at 20Mhz. Something like 20,080,000Hz. I am using the "usual" oscillator construction with two fixed caps to ground, one at each end of the crystal (120pf in this case). The problem is that the damn circuit is oscillating at slightly above 20Mhz, so I changed one of the fixed caps (the OSC1/ClkIN) for a variable cap. The only one I had was a 5-20pf part. I was able to reduce the oscillation frequency to 20,035,000Hz but no less than that. I am using a temperature-compensated frequencimeter for measurement (using the OSC2/ClkOUT pin) that is very exact. What can I do to reduce it further? I was thinking about changing the (now only) fixed cap for another one, but with LARGER or SMALLER capacitance? Could someone help me here? Thank you very much! Regards, Andres Tarzia Technology Consultant - SMART S.A. email: spam_OUTatarziaTakeThisOuTsmart.com.ar

the caps should be 20 pf from memory stu Andres Tarzia wrote: {Quote hidden}> Hi all! > > I am having problems with an 20Mhz oscillator that does not want to > oscillate at 20Mhz. Something like 20,080,000Hz. > > I am using the "usual" oscillator construction with two fixed caps to > ground, one at each end of the crystal (120pf in this case). > > The problem is that the damn circuit is oscillating at slightly above 20Mhz, > so I changed one of the fixed caps (the OSC1/ClkIN) for a variable cap. The > only one I had was a 5-20pf part. I was able to reduce the oscillation > frequency to 20,035,000Hz but no less than that. > > I am using a temperature-compensated frequencimeter for measurement (using > the OSC2/ClkOUT pin) that is very exact. > > What can I do to reduce it further? I was thinking about changing the (now > only) fixed cap for another one, but with LARGER or SMALLER capacitance? > Could someone help me here? > > Thank you very much! > > Regards, > Andres Tarzia > Technology Consultant - SMART S.A. > email: .....atarziaKILLspam@spam@smart.com.ar

Possible answers: 1)Choose your capacitors to match your crystal. 120 pF is way off. Parallel resonant crystals are never in that range. 2)You need a parallel resonant crystal, maybe you have a series resonant one. Cheers, Bob >I am having problems with an 20Mhz oscillator that does not want to >oscillate at 20Mhz. Something like 20,080,000Hz. > >I am using the "usual" oscillator construction with two fixed caps to >ground, one at each end of the crystal (120pf in this case). http://www.bobblick.com/

While this doesnt help your accuracy problem, one good way to measure the frequency of a micro controller is to make a program which basically divides the clock by some value (i personally use 30) and outputs it to a pin. This is better than measuring directly at the crystal, both because the capacitance of the measuring instrument can, from what ive heard, load the oscillator, and moreover, i think frequency counters are more accurate at lower frequencies. ------------------------------------------------------------------------------ A member of the PI-100 Club: 3.1415926535897932384626433832795028841971693993751 058209749445923078164062862089986280348253421170679 On Wed, 30 Jun 1999, Andres Tarzia wrote: {Quote hidden}> Hi all! > > I am having problems with an 20Mhz oscillator that does not want to > oscillate at 20Mhz. Something like 20,080,000Hz. > > I am using the "usual" oscillator construction with two fixed caps to > ground, one at each end of the crystal (120pf in this case). > > The problem is that the damn circuit is oscillating at slightly above 20Mhz, > so I changed one of the fixed caps (the OSC1/ClkIN) for a variable cap. The > only one I had was a 5-20pf part. I was able to reduce the oscillation > frequency to 20,035,000Hz but no less than that. > > I am using a temperature-compensated frequencimeter for measurement (using > the OSC2/ClkOUT pin) that is very exact. > > What can I do to reduce it further? I was thinking about changing the (now > only) fixed cap for another one, but with LARGER or SMALLER capacitance? > Could someone help me here? > > Thank you very much! > > Regards, > Andres Tarzia > Technology Consultant - SMART S.A. > email: atarziaKILLspamsmart.com.ar >

Does anyone have a method for finding the specified load capacitance for a given crystal? aside from just using variable capacitors? ------------------------------------------------------------------------------ A member of the PI-100 Club: 3.1415926535897932384626433832795028841971693993751 058209749445923078164062862089986280348253421170679 On Wed, 30 Jun 1999, Dave VanHorn wrote: > > I am using the "usual" oscillator construction with two fixed caps to > > ground, one at each end of the crystal (120pf in this case). > > > Try using the actual value of load capacitance specified for that crystal. > No spec?... Well, buy one with a spec? > They are extremely accurate, in a properly constructed circuit. >

> While this doesnt help your accuracy problem, one good way to measure the > frequency of a micro controller is to make a program which basically > divides the clock by some value (i personally use 30) and outputs it to a > pin. This is better than measuring directly at the crystal, both because > the capacitance of the measuring instrument can, from what ive heard, load > the oscillator, and moreover, i think frequency counters are more accurate > at lower frequencies. You can scope it by probing the can of the crystal, or you can get the freq by tuning it in on a shortwave receiver. WWV is a nice calibration standard at 10.000000 MHz (and 2.5, 5, 15, and 20)

Hi, On 30 Jun 99, at 23:30, Andres Tarzia wrote: > I am having problems with an 20Mhz oscillator that does not want to > oscillate at 20Mhz. Something like 20,080,000Hz. This is 4000ppm, way way off. Even if this is a series resonant crystal it shouldn't have been more than 1000ppm off. > I am using the "usual" oscillator construction with two fixed caps to > ground, one at each end of the crystal (120pf in this case). This presents a load of 60pF to the circuit. Typically crystals in this range will a be specified for load capacitance of around 20pF. But increasing the load capacitance should *decrease* the frequency not increase it. You should be using two 33pF capacitors or so. > I am using a temperature-compensated frequencimeter for measurement (using > the OSC2/ClkOUT pin) that is very exact. When was it last calibrated? Like someone else said, to get the most accurate measurements you should make sure you are not loading the pin with your measurement. It may not be as accurate as you believe it to be. > What can I do to reduce it further? I was thinking about changing the (now > only) fixed cap for another one, but with LARGER or SMALLER capacitance? > Could someone help me here? Larger capacitance will lower the oscillations frequency, however you are already in the way to large area so start with 33pF and move a few pF's around. This could be a start-up problem where the crystal never properly starts up. It could be that your crystal is bad or just a cheap low tolerance one. Observe the waveform at clkout with a scope, does it look like a clean sinewave? Is this on a solderless breadboard? If yes this might be the source of your problems. Otherwise make sure the oscillator is as close as possible to the PIC, that the caps ground return directly to the PIC ground pin, that the PIC is properly decoupled with a 0.1uF between its ground and power and that the power is clean and starts up properly. Check all your connections with an ohm-meter. Check http://www.foxonline.com/ for some app notes on crystal oscillators. Let us know what the problem was... Hope this helps, Guy - .....mlsirtonKILLspam.....inter.net.il

THe method I use is to place 2 variables one on either pin and adjust for 1.correct frequency and 2. equal amplitude on both pins... Requires a VERY low cap scope probe and sometimes a buffer amp (for the counter (HP5315A)) I use a 100/1 divider probe. This is a procedure I do EVERY time I use a new type or frequency crystal... seems to be stable... It works really well for ceramic resonator types. Robert K. Johnson EraseMErkj1spam_OUTTakeThisOuTix.netcom.com At 09:53 PM 6/30/99 -0700, you wrote: {Quote hidden}>Does anyone have a method for finding the specified load capacitance for a >given crystal? aside from just using variable capacitors? > >------------------------------------------------------------------------------ >A member of the PI-100 Club: >3.1415926535897932384626433832795028841971693993751 >058209749445923078164062862089986280348253421170679 > >On Wed, 30 Jun 1999, Dave VanHorn wrote: > >> > I am using the "usual" oscillator construction with two fixed caps to >> > ground, one at each end of the crystal (120pf in this case). >> >> >> Try using the actual value of load capacitance specified for that crystal. >> No spec?... Well, buy one with a spec? >> They are extremely accurate, in a properly constructed circuit. >>

Should there really be equal amplitude on both pins? Considering that one is driving the xtal andthe other is not (only an input), I would naievely assume that the amplitude on the driving pin should be slightly higher. In fact, I never have problems with 4MHz,10MHz,or 20MHz xtal startup in my projects,and my scope usually shows one pin at about 3v p-p and the other at 4v p-p If I recall correctly. True, my scope probe is not very low cap (its a 10/1 divider 100MHz probe with about 15pF of capacitance). Sean At 03:59 AM 7/1/99 -0700, you wrote: {Quote hidden}>THe method I use is to place 2 variables one on either pin and adjust for >1.correct frequency and 2. equal amplitude on both pins... Requires a VERY >low cap scope probe and sometimes a buffer amp (for the counter (HP5315A)) >I use a 100/1 divider probe. This is a procedure I do EVERY time I use a >new type or frequency crystal... seems to be stable... >It works really well for ceramic resonator types. > > Robert K. Johnson > rkj1spam_OUTix.netcom.com > >At 09:53 PM 6/30/99 -0700, you wrote: >>Does anyone have a method for finding the specified load capacitance for a >>given crystal? aside from just using variable capacitors? >> >>-------------------------------------------------------------------------- ---- {Quote hidden}>>A member of the PI-100 Club: >>3.1415926535897932384626433832795028841971693993751 >>058209749445923078164062862089986280348253421170679 >> >>On Wed, 30 Jun 1999, Dave VanHorn wrote: >> >>> > I am using the "usual" oscillator construction with two fixed caps to >>> > ground, one at each end of the crystal (120pf in this case). >>> >>> >>> Try using the actual value of load capacitance specified for that crystal. >>> No spec?... Well, buy one with a spec? >>> They are extremely accurate, in a properly constructed circuit. >>> > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 @spam@shb7KILLspamcornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html