I have used VCC filtering on clocks for quite a while. However with the inclusion of multiple Ics that reconstitute other clocks or simply pass the main clock through, adding many ferrites seemed illogical. I then resorted to filtering the Ics that were "vulnerable" to Vcc noise...having a low impedance power distribution didn't hurt either :-) Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: charles.grasso@xxxxxxxxxxxx; Email Alternate: chasgrasso@xxxxxxxx -----Original Message----- From: pwelling@xxxxxxxxxxxxxx [mailto:pwelling@xxxxxxxxxxxxxx] Sent: Wednesday, November 13, 2002 8:35 AM To: bmgman@xxxxxxxxxx; Anand.Kuriakose@xxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Decoupling of Oscillator Anand, From an experience at a previous company, I'm reminded of the first PC EGA/VGA Video controller cards that were produced by many vendors. As some may recall they used to have multiple oscillators on them for the different video pixel clocks and horizontal/vertical refresh rates. The initial ones (and I evaluated many cards at the time) had serious FCC compliance problems which many vendors resorted to output filters to make compliance which affected video quality. A large majority of the vendors that didn't have problems had used Pi filters on their oscillators. This prevented the oscillator contamination of the VCC planes and prevented coupling into their RAMDAC output device. Many of the cards started to put filters on the RAMDAC power pins and that soon became a requirement from the RAMDAC vendors. The EMI profile was obvious when you looked at it, you could easily identify each oscillator (all of them were always enabled - and the one they wanted was selected by a mux in the video controller chip). The 3rd, 5th, and 7th (some up to 19th) harmonics were always visible. These harmonics plus the selected clock usually caused problems for compliance as they were impressed on the video cable signals or cable shields. The Pi filters helped with the power supply pin noise but, not the oscillator harmonics that were injected into the grounds. The harmonics on the ground planes would shoot through the RAMDAC too. That was when they started to use ground moats for the RAMDAC to provide isolation. This too became a standard in RAMDAC datasheets. Some provided Pi filters for the ground but that didn't work well logically because of the return path inductance and Signal Integrity (not even coined back then) induced problems. A real boon to the video controller market was the PLL programmable video clock generators (like ICS) that had the ability to programmed with multiplexed clock divide ratios to give them only the clock(s) they needed. This only had 2 frequencies to worry about, the 14.318 MHz reference, and the Video output clock frequency from the PLL. This reduced emissions dramatically as all of the separate free running video clock oscillators frequencies (some had up to 6 oscillators) had been reduced to 2 frequencies plus the oscillator for the video logic. Sometimes the video logic (serial shift register clock) caused problems too. My point is that Pi filters are good for oscillators because they keep the emissions off of the planes which might couple to I/O cabling which causes EMC failures. Since oscillators draw relatively little current, a high impedance bead can be used to provide high isolation. Remember that as current is pulled through surface mount beads, the impedance (isolation) is reduced nearly logarithmically. You are making a new supply for the oscillator. Use high frequency ceramic and low frequency Tantalum decoupling. The Tantalum will filter the lower frequencies that make it through the bead causing jitter that you may not be able to track out of your PLL. A caution here, be careful using PLL based clock oscillators (like JITOs) feeding a PLL circuit. Their jitter may be multiplied up in the PLL circuitry ie... don't cascade PLLs. The series resistor will reduce edge energy (besides providing termination) to the destination device which helps quiet the destination device noise too. Make sure that you provide a good return path back to the oscillator. The Pi filter not only prevents contamination of the planes, it provides a clean "living area" for the oscillator enabling high performance. Just my $0.03 (cost of living). Philip Ross Wellington Mgr. Signal Integrity & EMI L-3 Communications CSW -----Original Message----- From: Mike Brown [mailto:bmgman@xxxxxxxxxx] Sent: Wednesday, November 13, 2002 6:54 AM To: Kuriakose, Anand; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Decoupling of Oscillator Anand, It is relatively common for the oscillator to be treated as just another load, and omit everything in the filter except the HF capacitor. I think the main purpose of the dedicated pi filter for the oscillator is protection of the oscillator, because the other loads are relatively immune to LF/MF variation in their supply voltages. That doesn't say that the filter doesn't provide other functions as well. The pi network mentioned will certainly keep the oscillator current spikes out of the supply. Is that its main purpose? Every load driven by the oscillator is also drawing current at the oscillator frequency, or 2x the oscillator frequency. The other loads don't use a dedicated overt pi filter, but rely on HF decoupling. Why not the oscillator? Its current spikes aren't that much different than the other loads. I think the question here is "Who's being protected from what?", followed closely by "What's the most cost-effective way to provide the protection?" Regards, Mike Kuriakose, Anand wrote: >Mike, > >You said "that filter network is not to keep the oscillator spectrum >out of the power supply, but to keep power supply noise from modulating >the oscillator." > >I an addition to isolating the power rail noise from the VCC of the >OSC, the >current spikes generated on the VCC rail due to the OSC switching is >being decoupled by the decoupling scheme mentioned below. > >That is to say that the filter isolates noise originating from either >sides of the filter, which is precisely what one should do to make all >devices connected to the common power rail operate without causing too >much of noise. > >Please correct me if i am wrong. > >Regards, >Anand. > > > >-----Original Message----- >From: Mike Brown [mailto:bmgman@xxxxxxxxxx] >Sent: Tuesday, November 12, 2002 7:42 PM >To: zhang_kun@xxxxxxxxxx >Cc: si-list >Subject: [SI-LIST] Re: Decoupling of Oscillator > > > >Zhangkun, > >You are right - that filter network is not to keep the oscillator >spectrum out of the power supply, but to keep power supply noise from >modulating the oscillator. Jitter will be introduced into the system >timing if this modulation occurs. Some jitter will unavoidably occur >but the filter, including the tantalum cap, will minimize the amplitude. > The noise frequency will be determined by the timing of the loops in >the software, which change the power loading periodically. Any load >variation above the regulator cutoff frequency is a possible noise source. > >I prefer to isolate the power to the oscillator. Others don't, and >they >get away with it if their system is not jitter sensitive. I once built >a system with two oscillators and no isolation and found the resulting >jitter due to the asynchronous noise to be intolerable. Isolation >solved the problem. > >Regards, >Mike > >Zhangkun wrote: > > > >>Hi all >> >>I meet one question about decoupling of oscillator. In our design, the >> >> >power supply of OSC is always isolated by one PI filter. In the side >near OSC, there are always one tantalum capacitor of 10uF and several >ceramic capacitors of 0.1uF or 0.01uF. As we know the resonance >frequency of tantalum capacitor is about 3MHz. If the OSC is 50MHz, the >spectrum will be speaded at 0, 50MHz, 100MHz, 150MHz, etc. There will >be no power in the frequency range between 0 and 25MHz. Therefore, I >think I could remove the tantalum capacitor. Is there something wrong? > > >>I think it will have nothing to do with the affection from OSC to >>outside >> >> >circuits. I am worrying about the affection from outside circuits to >OSC. If >there is some noise of 2MHz and my OSC is of 50MHz, there will be modulation >between noise of 2MHz and clock signal of 50MHz. The output of OSC and >the clock signal will be affected by the noise of 2MHz. The bead will >not isolate the noise of 2MHz. > > >>I want to know I could remove the 10uF tantalum capacitor or not. Why? >> >>By the way, is there some people who do not use bead to isolate the >>power >> >> >supply of oscillator? > > >>Best Regards >>Zhangkun >>2002.11.12 >> >> >>------------------------------------------------------------------ >>To unsubscribe from si-list: >>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >> >>or to administer your membership from a web page, go to: >>//www.freelists.org/webpage/si-list >> >>For help: >>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >>List archives are viewable at: >> //www.freelists.org/archives/si-list >>or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >>Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> >> >> >> >> > > > >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >or to administer your membership from a web page, go to: >//www.freelists.org/webpage/si-list > >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu