Bill, Thanks. Your reply was very informative. I understand the reason for using decibels for the applications you mention. However, I did not consider the output level of a non-RF signal to be in that category.
You are right, I have never seen a DMM reading in decimals. I know that digital scopes can do so (and understand how they work), but again I have never seen or used one. Given the zero point (finally figured that one out, I was missing the assumed impedance factor which was varying) I can convert levels - not quite in my head, since I can't do logs in my head - but close and can get a pretty good guestimate that way. Mike P.S. I also used punch cards. I kept my source library first on punch cards and then mag tape. I eventually bought my own IBM disk drive for the 360 at work. I started on an RCA 70/46 using punch tape in a time sharing environment. I had a trash-80 before S-100s. I even had a full size IBM tape drive and four hard disk drives. Never could do anything with them, I just couldn't get 3-phase 440 in an apartment complex and IBM used some very unusual logic levels. I eventually gave the hard disk drives away and abandoned the tape drive. I had the original CP/M source code from Gary Kildall written in PL/M. Threw all of that stuff out about 10 years ago and even more a bit more about a year and a half ago. But I still have the MASM Assembler manuals from Microsoft. Those were last published around 1995 - and I needed them just a few months ago. > Hi, MIke. I used the university CDC6400/6600 supercomputer while in > engineering school with punch cards or Teletypes and was familiar with S-100 > vintage equipment. Somewhere I may still have a MIcrosoft BASIC pre MS-DOS > (HDOS or CP/M) looseleaf manual. I haven't retired yet, but have been an > Application Engineer at Tektronix for nearly 30 years. So I can appreciate > your mindset. I'm going to only answer two of your questions: > 1. What is the zero value for voltage and watts using logarithmic > scaling (at least as used here)? Is there actually a consistent > underlying value across all applications? > 2. Why use it for specifying voltage or power in a limited range? Why > not just say that the output is 1.0v rms or 0.7v, or that it uses > 50mW? There does not appear to be any actual advantage to using a > logarithmic scale for a small range of values - and 1mV to 1kV IS a > small range. Especially when you have to convert the logarithmic > value to a "real" value to actually do anything with it. > RF and audio (including telephone) types have used logarithmic (dB) > units for many decades. There is often a need to discuss thermal noise > levels and transmitter power levels in the same circuit, which can lead > to very large voltage and power ratios. It's common to need to relate > voltages over a 10^10 range and powers over a 10^20 range > (1:0.00000000000000000001). Engineers and scientists like to use numeric > values which are easier to work with. You wouldn't want to specify a > hard drive size as 1,000,000,000,000 bytes but as 1 TB (ignoring the > power of 2 vs power of 10 issue). So we use "engineering units" (powers > of 1,000) for frequency (kHz, MHz, GHz, THz), voltage (pV, nV, uV, mV, > V, kV, MV, GV). > RF applications are more naturally dealt with in terms of power. The > noise generated in a resistor due to thermal agitation (Johnson-Nyquist > noise) is P = kTB, where P = power in watts, k = Boltzmanns constant > (1.28 x 10^-23 J/K), T = absolute temperature in Kelvins, and B is the > measurement bandwidth in Hertz. Many RF components are rated by power > dissipation. Historically it's been much easier to measure RF signals > levels by measuring thermal changes due to signal power. > The very large dynamic range required for characterizing sound and > telephone line levels and relating them to human perceived level change > led to the definition of the Bel (power ratio of 10, named for Alexander > Graham Bell). The decibel (1/10 Bel) is the logarithmic unit which is > used in practice. The noise delivered by a resistor to a matched load at > room temperature and normalized to a 1 Hz measurement bandwidth is about > -174 dBm/Hz. > The "m" in "dBm" stands for 1 mW (milliwatt). So 0 dBm = 1 mW. You > should read "dBm" as "decibels relative to 1 milliwatt". Since most > power levels in RF equipment tend to be within a couple of orders of > magnitude below or above a milliwatt, dBm is the main unit used for RF > equipment which can fit on your lab bench. In some cases it's convenient > to use logarithmic voltage units, and the common units are dBuV > (decibels relative to 1 microvolt), dBmV (decibels relative to 1 > millivolt), or dBV (decibels relative to 1 volt). But except for cable > television and a few other applications (including noise levels at low > audio frequencies), dBm rules the RF world. > You are correct that linear non-logarithmic units work well when a small > range of values are being used. Amateur Radio handheld transmitters have > power ratings usually given in linear watts (100 mW, 1 W, 3 W, 5 W, > etc.). A HF (high frequency 3-30 MHz) transmitter may have an output > power meter marked in linear units of Watts. But the received signal > strength meter is marked in logarithmic units, since with an automatic > gain control the receiver dynamic range is many orders of magnitude too > large to be shown with linear units. > You happen to have test equipment which measures linear units. Many > voltmeters designed to measure audio levels are calibrated in dBmV. Most > RF signal level measurement instruments (power meters and spectrum > analyzers) are usually used with dB or dBm scaling. Since the ratio of > RF signals is often the main measurement of interest (such as harmonic > or intermodulation level), spectrum analyzers are usually set up with a > a variable full scale value (reference level) and a dB ratio vertical > scale. The horizontal scale of a spectrum analyzer is usually linear > frequency, but in many cases can be changed to logarithmic frequency. > Modern oscilloscopes (those made in the past 20 years) digitize the > voltage waveform, and they can easily show RMS voltage and even power > levels. For many RF measurements the oscilloscope uses an FFT to create > a spectrum display scaled in dBm. A DMM is not used to measure RF > levels, but RF power meters directly display dBm power, as also used by > spectrum analyzers. > So the reason that your test equipment doesn't produce measurements in > the same units commonly used by RF engineers is that you don't have > modern RF test equipment, but rather old non-RF measurement instruments. > That oscilloscope was introduced 27 years ago and many repair parts have > not been available for many years. I own some much older Tektronix > scopes and other equipment, but don't use oscilloscopes for general > purpose RF measurements. > https://en.wikipedia.org/wiki/Decibel > -- > Bill Byrom N5BB > > > > On Tue, Jan 26, 2016, at 06:04 PM, [email protected] wrote: >> Hi, >> >> I am a newbie to this list. I have downloaded the archives and read about >> 5,000 of the past messages. I plan on building my own GPSDO, probably >> using a >> LEA-6T (but LEA-7T or LEA-M8T would be good if I can find one >> affordably). I >> have a MTI 260 on order (although it could wind up being a 261 since they >> all >> appear to ship one or the other randomly). >> >> Currently, my resources include a DMM (well, a couple) and soldering / >> desoldering stations and quite a few tools. I also have an oscilloscope >> that I >> am currently repairing - a 400Mhz Tektronix 2465BCT analog scope. I am >> waiting >> on the final parts from Mouser. Once that is done I need to get it >> calibrated. >> All of that will probably take me another month. I also need to finish >> fixing >> my cassette deck - and then to finish writing a special recording program >> to >> use raw device drivers to get around the fact that Windows is not real >> time. I >> interrupted that project to work on the scope. >> >> In the meantime, I am reading the time-nuts messages (and lots of other >> things) to gather information and ideas about how I am going to do this >> and >> generally to learn more. >> >> So, I have some questions. Let me tell you a bit about me, so that you >> know >> the context and my limitations. I am a retired programmer. I wrote just >> about >> everything including device drivers, operating systems, utilities, >> various AI >> programs, telephone systems, compilers, encryption, web applications and >> much >> more. If I need to throw 50,000 LOC at a project, no problem. I have used >> many >> languages including quite a few different assembly languages (I have also >> written an assembler). I consider myself a mathematician / programmer, >> although I haven't really needed Calculus or Differential Equations for >> decades, so I am pretty rusty in that area. I do more work in formal >> logic >> than higher mathematics. But, I THINK like a mathematician. Formalism and >> abstraction come naturally to me. >> >> During my career I also helped to debug hardware during S-100 days. I >> have >> sporadically messed with electronics off and on, informally, with no >> education >> in the area. Now that I am retired (and have more time, but less money - >> it IS >> a zero sum game!), I am trying to learn more about electronics and start >> doing >> hardware projects. I have never been into model building or anything >> similar, >> so my construction skills are lacking. I understand a lot of things in >> theory, >> but practice still eludes me. For example, knowing a part exists or >> determining which of 10,000 apparently identical parts is the "right" >> choice. >> It can hours or even days to find the "right" connector. In many cases, >> the >> names or descriptions are completely meaningless. That all appears to be >> an >> experience related issue, so I will (hopefully) overcome that in time. >> >> I have no problem with soldering / desoldering, but I haven't designed or >> built my own PCB yet. I have designed / redesigned some minor circuits, >> especially on the power supply side. I can follow schematics reasonably >> well, >> but I am not comfortable with Eagle or other PCB layout programs. Every >> time >> I have tried one of those programs, half of the parts I needed were not >> available. I have started using TinyCAD which is much easier to use. So, >> I >> have a lot to learn. But, that is basically what I do, all day, every >> day. I'm >> the type of person that gets bored easily and quickly. As #5 said "more >> input, >> more input"! 6.02059991327962 >> >> Paradoxically, I have no interest in time. As in time of day, day of >> week, >> etc.. I have never had a job where I got to work on time. My philosophy >> has >> always been "go to bed when sleepy, get up when not". I was notorious in >> high >> school for only showing up on test day. But, I am interested in being >> able to >> timestamp events accurately and in measuring time (and other things). I >> am >> also interested in how a very accurate frequency source can be used in >> other applications and test instruments. That brings me to my desire to >> build >> a GPSDO and my questions. >> >> I understand the logarithmic scaling used for voltage and power. I even >> understand why voltage uses a multiplier of 20 and power a multiplier of >> 10. >> It makes sense when working with a wide range of values. However, my DMM, >> my >> scope and generally schematics work directly with current, voltage and >> watts. >> So, I am constantly seeing statements like an output is 7 dBm or 13 dBm. >> If I >> knew the actual value for 0 dBm then the basic equations would resolve >> the >> values. However, I have not found a consistent answer for that. When I >> have >> attempted to work values backwards from various statements, again I don't >> get >> a consistent value (probably because those statements were approximations >> and >> not exact values). I always see statements that an increase of 6dBm >> doubles >> the value. It is used so often that most people forget it is an >> approximation. >> It is 6.02059991... and sometimes, it may make a difference. Worse, the >> zero >> value appears to be different for different applications. In some it >> appears >> to be completely arbitrary. So this leads to two questions... >> >> 1. What is the zero value for voltage and watts using logarithmic >> scaling >> (at least as used here)? Is there actually a consistent underlying >> value >> across all applications? >> >> 2. Why use it for specifying voltage or power in a limited range? Why >> not >> just say that the output is 1.0v rms or 0.7v, or that it uses 50mW? >> There >> does not appear to be any actual advantage to using a logarithmic >> scale >> for a small range of values - and 1mV to 1kV IS a small range. >> Especially when you have to convert the logarithmic value to a >> "real" >> value to actually do anything with it. >> >> I have also been researching GPS antennas. From what I can see there are >> two >> basic types - the flat puck and the helical. I have not seen anything to >> distinguish the two types based on performance or usage or to indicate >> that >> one or the other might be better for GPS timing. However, I have seen >> "GPS >> Timing Reference Antennas" advertised. Most or all of those appear to be >> helical. But, I have not seen anything that specifies the difference >> between >> an active GPS antenna and an active GPS Timing Reference Antenna. >> >> 1. What is the difference between a "normal" GPS antenna and a GPS >> Timing >> Reference antenna? What features are of interest? >> >> 2. Is there anything extra needed besides a GPS antenna to enable the >> use >> of WAAS or other services? Apparently the ubolt receivers can make >> use >> of some of that, but it is not clear what is needed to provide that >> information to them, or if they just pick it up automatically using >> a >> standard GPS antenna. >> >> Also, from what I have read, using carrier phase for timing is >> potentially >> more accurate by a couple orders of magnitude. Are there any GPS timing >> receivers available that use carrier phase? Or use both L1 and L2 for >> increased accuracy? I see that the ubolt receivers can report some >> carrier >> phase information, but that doesn't appear to translate to increased >> accuracy. >> And the LEA M8T use dual channels, but don't appear to mix GPS and >> GLASNOS to >> improve accuracy. Do any receivers do that? I suspect that building a GPS >> receiver is probably more complex than can be easily handled by an >> amateur so >> I am most likely restricted by available receivers. >> >> I have also read, more than once, statements in this forum that something >> or >> another could be had for some low, low price so why build it yourself? I >> think >> that there are several reasons, including but not limited to the >> following. >> >> 1. It is an interesting project. >> >> 2. It is an educational project. >> >> 3. You may have some ideas about how things could be done differently >> or >> better. >> >> 4. You may want some combination of features that is not commercially >> available or perhaps is not affordable even with a generous budget. >> >> 5. Many people on limited budgets are not limited by total cost, but >> rather >> by incremental cost. So, someone may not be able to afford several >> hundred >> dollars for a pre-built system. But, they may be able to afford $50 >> here >> and there. So, building it themselves is the only practical option. >> >> >> Mike >> >> >> >> >> >> >> _________________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- Best regards, Timenut mailto:[email protected] _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
