Re: [time-nuts] IC for 1 PPS Output
Hi Rex: Yup. It's really hard to get 50 of those 2 Ohm cables. Brooke Rex wrote: On Thu, 06 Apr 2006 21:14:29 -0700, Brooke Clarke [EMAIL PROTECTED] wrote: to drive 50 a couple of Ohm cables he he. Interesting typo. I assume you really meant 2 50-ohm cables, rather than 50 2-ohm cables. I'll defer to others with more knowledge for a real answer to your question. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Hi Brooke, Most of the Video buffer/driver type IC's should work with a suitably scaled output resistor. Even a fast (high -slew rate) OP-Amp will work. The output impedance should be low enough to get a good 50 ohm match with just a couple of 100R in parallel. The out put impedance should be in the data sheet for the device. A good technique is to set the output of your buffer amplifier to a higher voltage than the desired output and use a resistive divider (50R output impedance) to attenuate the signal to the level you need, just don't forget about the other 50R load! This is sometimes called a soft output as the voltage will rise if it's not correctly terminated. See the TAPR TADD1 http://www.tapr.org/~n8ur/TADD-1_Manual.pdf , Maxim Data sheet for the MAX477 http://pdfserv.maxim-ic.com/en/ds/MAX477.pdf and application note http://www.maxim-ic.com/appnotes.cfm/appnote_number/1036 or National's LM6171 http://cache.national.com/ds/LM/LM6171.pdf for some ideas. With the MAX477 just using a 50R output resistor will give good results, not that it's not essential to have an exact resistance, many commercial designs use 52R (preferred value) output resistors with out problems, it's within 5%. HTH, Robert G8RPI. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brooke Clarke Sent: 07 April 2006 05:14 To: Discussion of precise time and frequency measurement Subject: [time-nuts] IC for 1 PPS Output Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Any opinions expressed in this email are those of the individual and not necessarily Genetix Ltd (Genetix) or any company associated with it. This email and any files transmitted with it are confidential and solely for the use of the intended recipient. If you are not the intended recipient or the person responsible for delivering to the intended recipient, be advised that you have received this email in error and that any use is strictly prohibited. If you have received this email in error please notify Genetix by telephone on +44 (0)1425 624600. The unauthorised use, disclosure, copying or alteration of this message is strictly forbidden. This mail and any attachments have been scanned for viruses prior to leaving the Genetix network. Genetix will not be liable for direct, special, indirect or consequential damages as a result of any virus being passed on, or arising from alteration of the contents of this message by a third party. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Dear Time-nuts, From a more theoretical point of view: The source impedance is not such a big issue in a one way distribution system. The most important thing is the load resistance (or impedance). Reflections in a cable will be caused by the load-mismatch not by the source. As power transfer is not the primary goal in a timing distribution system, a voltage source or low-ohmic source will perform just as well. (Due to the fact that optimum power-transfer will occur when the load and the source impedance ar the same). Reasons why you would like to have a resistor in your source output: * current limiting, in case of a short (allthough, if you use an OpAmp as buffer it usually has a current limiter inside) * to damp out reflections caused by the mismatch of the load. A reflection caused by the load can be absorbed by the source if it is well matched to 50 Ohms. If not, the reflected wave, will bounced again at the source, and again at the load, and again, and again. (OK, it will stop due to cable-damping ;) ). Anyway, if your load is well matched to 50 Ohms, any resistor close to 50 Ohms will perform well. You could even consider using a variable resistor in order to regulate the signal level. Good luck with your project Brooke, best regards, Jeroen Robert Atkinson wrote: Hi Brooke, Most of the Video buffer/driver type IC's should work with a suitably scaled output resistor. Even a fast (high -slew rate) OP-Amp will work. The output impedance should be low enough to get a good 50 ohm match with just a couple of 100R in parallel. The out put impedance should be in the data sheet for the device. A good technique is to set the output of your buffer amplifier to a higher voltage than the desired output and use a resistive divider (50R output impedance) to attenuate the signal to the level you need, just don't forget about the other 50R load! This is sometimes called a soft output as the voltage will rise if it's not correctly terminated. See the TAPR TADD1 http://www.tapr.org/~n8ur/TADD-1_Manual.pdf , Maxim Data sheet for the MAX477 http://pdfserv.maxim-ic.com/en/ds/MAX477.pdf and application note http://www.maxim-ic.com/appnotes.cfm/appnote_number/1036 or National's LM6171 http://cache.national.com/ds/LM/LM6171.pdf for some ideas. With the MAX477 just using a 50R output resistor will give good results, not that it's not essential to have an exact resistance, many commercial designs use 52R (preferred value) output resistors with out problems, it's within 5%. HTH, Robert G8RPI. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brooke Clarke Sent: 07 April 2006 05:14 To: Discussion of precise time and frequency measurement Subject: [time-nuts] IC for 1 PPS Output Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke -- Ing. Jeroen Bastemeijer Delft University of Technology Department of Electrical Engineering Electronic Instrumentation Laboratory Mekelweg 4, Room 13.090 2628 CD Delft The Netherlands Phone: +31.15.27.86542 Fax: +31.15.27.85755 E-mail: [EMAIL PROTECTED] ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Dear Robert You can either drive from A to B with the world finest German Maybach car or also with a British Mini Cooper. May be the SN74128 50ohms coax-cable driver is rather matched for this simple application ! Regards Hans ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Even more El cheapo (and also more ailable from your spare parts drawer): A fast hex inverter (74F04/74HCT04 etc...) wired in a 1-5 configuration (one input inverter driving five output inverters) and then having each of the five output inverters driving a separate 240 (or 220) ohm resistor (keep the leads short). Then, simply connect the five 240 ohm resistor unused leads together and BINGO! You have a 50 (ok, 48 ohm!!!) driver! And having 240 ohm to drive, the inverters are not overloaded. 73 de Normand VE2UM --- Hans H. Jucker [EMAIL PROTECTED] wrote: Dear Robert You can either drive from A to B with the world finest German Maybach car or also with a British Mini Cooper. May be the SN74128 50ohms coax-cable driver is rather matched for this simple application ! Regards Hans ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Brooke Clarke wrote: Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke Hi Brooke -- The circuit I use is stolen from Tom Clark's TAC -- paralleling several sections of a 74AC04 hex inverter through 47 ohm resistors. For the current TADD projects, I'm using three sections that way, so each chip gives me two outputs, and I use a 47 ohm SIP resistor pack to keep the board space down. Tom came up with the 47 ohm value (though IIRC he used four sections in parallel) based on compromising for maximum voltage into a 50 ohm termination, and best rising edge. The 74AC chips don't have internal source resistors, but I think some of the op amp distribution amp chips might. For square waves at these frequencies, I'd use 74AC parts over an analog circuit. John ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Brooke Clarke wrote: Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke Why do you need to drive the cable with a 50 Ohm source. You need a 50 Ohm load on the receiving end to reduce reflections. But that is not a problem for the drivers. I would use a low impedance driver to minimize losses in the driver. Just my approach. Bill K7NOM ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
As I mentioned about 6 months ago, I think the Extron ADA 6 300MX is a very handy and ready to go distribution amplifier. It has a switch so it can be DC or AC coupled. It has a built in PS. As it was intended for video applications it is currently set up for 75 ohms, which as mentioned previously should not cause any problems, and it has not for me. It has 4 sections with each section providing 6 outputs. So, there are a total of 28 BNCs on the rear panel. The 75 ohms is determined by fixed 75 ohm resistors, so, if one really wants 50 ohms, they just need to change the resistors. I am using one stock to distribute a rubidium to about 15 instruments on my test bench without a problem. I have bought a few for as low as $9.99 plus shipping on ebay. Personally, I highly recommend it. They seem to be very prevalent on ebay. The following URL is the specification: http://www.extron.com/product/product.asp?id=ada6300mxhvsubtype=32view=spe c#tabs Regards - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brooke Clarke Sent: Friday, April 07, 2006 12:14 AM To: Discussion of precise time and frequency measurement Subject: [time-nuts] IC for 1 PPS Output Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Hi Brooke -- I meant to say that there was a separate 47 ohm resistor on each of the inverter outputs, with the far end of those resistors tied together. Last time this came up, there was an argument about why you wouldn't use resistor values that in parallel would equal 50 ohms (e.g., 150 ohm if three gates were used), but Tom Clark jumped in to explain the basis for using the lower values -- I think it was mainly to ensure that you still have a valid TTL level into 50 ohms, coupled with looking at the internal resistance of the gate, and minimizing the ringing of the edge. In any event the scheme I described has been used with good success in the TAC-2 for a bunch of years. When I get home tonight, I'll post photos of the risetime that I'm seeing. John Brooke Clarke wrote: Hi John: I think you mean by -- paralleling several sections of a 74AC04 hex inverter through 47 ohm resistors directly connecting the outputs then using a series 47 Ohm resistor. But the post by Normand makes the good point that by using a seperate resistor on each gate the loading on the gate is reduced. So I could use a single 74AC04 chip with 150 Ohm resistors on each gate. I'll try that when I'm finished reading the mail. Have Fun, Brooke John Ackermann N8UR wrote: Brooke Clarke wrote: Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke Hi Brooke -- The circuit I use is stolen from Tom Clark's TAC -- paralleling several sections of a 74AC04 hex inverter through 47 ohm resistors. For the current TADD projects, I'm using three sections that way, so each chip gives me two outputs, and I use a 47 ohm SIP resistor pack to keep the board space down. Tom came up with the 47 ohm value (though IIRC he used four sections in parallel) based on compromising for maximum voltage into a 50 ohm termination, and best rising edge. The 74AC chips don't have internal source resistors, but I think some of the op amp distribution amp chips might. For square waves at these frequencies, I'd use 74AC parts over an analog circuit. John ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
Paralleling CMOS inverters to drive time critical signals is not a good idea because the inverters have skews between them, even if they are on the same chip (die). The skew for an improved version of the 74AC04 part (the 74LVC04) is specified as up to 1.5ns! Interesting enough, this parameter is wrong in the Philips datasheet, they say 1.5ps (impossible, since the difference in lead size of a couple of mm would already cause 1.5ps skew). I confirmed with Philips that it's supposed to be nanoseconds, not picoseconds: _http://www.semiconductors.philips.com/acrobat_download/datasheets/74LVC04A_6. pdf_ (http://www.semiconductors.philips.com/acrobat_download/datasheets/74LVC04A_6.pdf) This 1.5ns skew would cause all sorts of issues on the edges since the rise/fall time I measure on these parts is 1ns, so theoretically you could have one output drive high, and the adjacent one drive low for up to 0.5ns causing a short circuit over the termination resistors. It's better to use a single, faster, 5V compatible, LVC logic driving through a single series termination resistor. This series termination resistor is adjusted so that an open-ended cable shows a perfect square wave on its end. The cable has to be about 2-3 feet long going into a 1MOhm or higher fast oscilloscope or FET probe input. BTW: the value of the series terminator doesen't matter when the cable is 50Ohm end terminated since there will be no reflection from the end of the cable. Thus the value of the series termination resistor will determine the peak-to-peak voltage into a 50Ohm end terminated cable. The gate needs to be choosen so that it can drive down to 50 Ohms without damage of course. There is one advantage to using multiple gates: they will share the energy of ESD pulses, or external DC voltages applied to the connector, and thus have a higher survivability under stress conditions. bye, SJ ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
When I use 3 x 47 Ohm resistors and 3 gates the rise time seems about the same, but the voltage into a 50 Ohm load increases from 2.5 V p-p to 3.6 v p-p, so I tend to agree that's the way to go. One thing was missing from your original question. What sort of receiver are you going to use? Do you want an input to more digital logic or are you going into lab gear with an adjustable trigger threshold? If you are trying to preserve timing on a cable going from box to box, differential signaling avoids (most of) the noise due to ground not being the same on both boxes. LVDS is the modern version. 26LS31/32 and friends (RS-422) is the old low-tech stuff available in DIPs. Normal ECL/PECL chips are happy with 50 ohm loads. You might not like the DC offset if you want to receive it with something else. It should be easy to AC couple if you are sending clock signals with a 50% duty cycle. (You may need to bias the receiver.) You might find good chips intended for use as clock buffers. They probably won't come in DIPs so wiring them up as a hack will be a pain. They usually come with good data sheets and app-notes. I'm pretty sure I've seen an app-note saying it was OK to parallel drivers in the same package. I wonder what the technology was. One disadvantage of the op-amp or video buffer approach is that analog chips often need supply voltages that are not common on digital logic. They are often slew rate limited for large signal swings which doesn't give a great rise time. I'm slightly surprised there isn't a stock answer to this question. Seems like it should be in a FAQ someplace. If I was sending in an order to Digikey, I'd probably order a few of the old TTL 74128s just so I could put them on a scope. It's probably worth using as a straw man. -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
HI everyone: last year I did a design requiring a full 5V out (higher than TTL) into 50 Ohms with real 50 Ohm source Z and 1 nS risetime. For that I made an output driver using 15 V supply and high speed op amps, two in parallel. But that would be an overkill for what you are doing. See some comments inserted below. At 04:17 PM 4/7/2006, Hal Murray wrote: You might find good chips intended for use as clock buffers. They probably won't come in DIPs so wiring them up as a hack will be a pain. They usually come with good data sheets and app-notes. There are also many high speed line drivers which are multiple to a package and typically have good section-to-section matching. I'm pretty sure I've seen an app-note saying it was OK to parallel drivers in the same package. I wonder what the technology was. This may have been for one particular IC. Do not directly connect multiple outputs, even in same package. Phase difference in the risetime of sections can cause very high instantaneous currents between sections. Even though the durration is short, and therefore no heating problems, the high current causes glitches within the IC. This can mess up the timing of the rise, which is important for a clocking application. I put a nominal R in each output path, 22 Ohms in my case, to limit current from section to section. Then at the junction of all these output resistors, I put a single R to the output connector to set the desired output Z. Tom Buehl ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
John Ackermann took my name in vain: Hi Brooke -- The circuit I use is stolen from Tom Clark's TAC -- paralleling several sections of a 74AC04 hex inverter through 47 ohm resistors. For the current TADD projects, I'm using three sections that way, so each chip gives me two outputs, and I use a 47 ohm SIP resistor pack to keep the board space down. Tom came up with the 47 ohm value (though IIRC he used four sections in parallel) based on compromising for maximum voltage into a 50 ohm termination, and best rising edge. The 74AC chips don't have internal source resistors, but I think some of the op amp distribution amp chips might. For square waves at these frequencies, I'd use 74AC parts over an analog circuit. John One of my desires was to have a source impedance BELOW 50 ohms so that the driver would push at least 3½ volts into a 50 ohm termination. So in later versions of the circuit, I have used as small as 22 ohms for the series resistors. The main reason for using series resistors to to allow the output current to be shared between all the gates; although I have seen people put CMOS gates directly in parallel, it never seemed aesthetically right. The output impedance of an AC series gate (i.e. 74AC04) is in the 5-10 ohm range. With several stages in parallel, the source impedance of the gate array is then ~10 ohms. So when feeding a 50 ohm load (with logic running at 5v), you will get ~5v*50/(50+10) = 4.16v which met my 3½ volts criterion. It also turns out that with 22 or 47 ohm series resistors, a shorted load will not cause the chips to become instant crispy critturs ;} The 74AC gate array source doesn't need to be matched to the 50 ohms, since the terminated cable will have no reflections from the load. Regards, Tom ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] IC for 1 PPS Output
Hi: I've got the TVB 1 PPS PIC circuit working and would like to add an output IC to drive 50 a couple of Ohm cables. Need two outputs, the normal 1 PPS and also the 10 kHz output since I think it would work with the SRS app note for making 1,000 TI measurements in one second to better see small offsets. I seem to remember that there may be an internal resistance already in some chips so putting a 47 or 50 Ohm resistor in series may result in an impedance above 50 Ohms. Any thoughts? Thanks, Brooke Clarke -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] IC for 1 PPS Output
On Thu, 06 Apr 2006 21:14:29 -0700, Brooke Clarke [EMAIL PROTECTED] wrote: to drive 50 a couple of Ohm cables he he. Interesting typo. I assume you really meant 2 50-ohm cables, rather than 50 2-ohm cables. I'll defer to others with more knowledge for a real answer to your question. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
