[time-nuts] Newbie questions

timenut at metachaos.net timenut at metachaos.net
Wed Jan 27 17:41:23 UTC 2016


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.


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, timenut at metachaos.net 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
>> 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
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Best regards,
 Timenut                            mailto:timenut at metachaos.net

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