[time-nuts] Newbie questions

Bill Byrom time at radio.sent.com
Wed Jan 27 03:51:18 EST 2016


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
> 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
>  
>  
>  
>  
>  
>  
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