From: "nanchez" on Fri 3 Jun 2005 14:59

I'm doing some RF experimentation and I need to know the "relation"
between dBm specificatons and voltage level for a signal.

I have a RF mixer with a specification that says:

LO drive level (50 ohm) = -16 dBm

And I have a LO source that give me an output of 2.5Vpp to a capacitive
load of 5pF at 40MHz.

How can I relate both items and design a circuit to connect LO source
to RF mixer ?

A basic definition that is industry-wide, government-wide,
has "dbm" as decibels of "0 dbm" related to a power level of
1.0 milliWatts in a "50 Ohm system." That has become so
widespread that specification writers don't always include
those words. It is implicit when referring to RF components.

The RMS voltage can be quickly calculated from some identities
on the basic formula for Watts: P = E x I. Knowing R (50 Ohms)
one can substitute Ohm's Law of Resistance of I = E / R into
that to get E = SquareRoot (P x R).

For 1.0 mW in a 50 Ohm system, P x R = 0.050 and the square
root of that is 0.2236 so 0 dbm has an RMS voltage of 223.6
milliVolts.

In your mixer specification, -16 dbm is equal to 35.44 mV
RMS across a resistance of 50 Ohms.

You can't DIRECTLY use your 40 MHz source value of 2.5 V
peak-peak across a 5.0 pFd capacitance because it does not
include the characteristic RESISTIVE impedance of 50 Ohms.
Power in Watts must be related to the impedance of a load
in order to perform "work." [a basic definition of power
in Watts is "a unit of work"]

Capacitance across a load will vary its impedance depending
on the frequency. For that reason the electronics industry
has long relied on a basic resistive impedance to measure
and characterize RF components. The result is the very
common "dbm" referred to 1.0 mW across a resistive 50 Ohm
load...or the characteristic impedance of the measurement
system, both source and load impedance.

To relate your mixer specification to your RF source, you
will have to put a 50 Ohm load across the source and
measure that. If you have some stray capacitance across
that load (inevitable) and know approximately what that is,
you can calculate its effect across a resistance. At 40 MHz
a 5.0 pFd capacitance has a reactance of 796 Ohms. That is
not much but it changes the magnitude of the parallel R-C
from 50.0 Ohms resistive to 47.0 Ohms slightly capacitive.
That's a small change and can generally be neglected for
experimental bench work.

If you have some web source to study about this items, I'll be glad to
hear about it.

It's in practically every textbook on the subject of RF
electronics. What can confuse newcomers to RF is the
implicit "standard" which is not always included in
specification sheets. The definition of "dbm" is arbitrary
and probably picked (way back in time) for sake of
convenience in measurement by all concerned.

The reason for picking "50 Ohms" in a "system" is more
obscure and ties into the physics of power transfer in
coaxial cables. That's a curiosity that some can look up
if they are interested but does not apply to how to USE the
"dbm" specifications. To use "dbm" one only needs to
remember the definition and apply simple forumulas for Power
and Ohm's Law of Resistance.

I hope that was of some help to you.

wrote:

A basic definition that is industry-wide, government-wide,
has "dbm" as decibels of "0 dbm" related to a power level of
1.0 milliWatts in a "50 Ohm system." That has become so
widespread that specification writers don't always include
those words. It is implicit when referring to RF components.
. . .

That's common in the RF industry, but many others also use dBm -- for
example, it's often used in video systems where the standard impedance
is 75 ohms, and others where the standard is 600 ohms. In all those
applications, dBm is universally defined and understood to mean dB
relative to 1 mW, regardless of the impedance.

Roy Lewallen, W7EL

From: Roy Lewallen on Sat, 04 Jun 2005 14:06:41 -0700


wrote:

A basic definition that is industry-wide, government-wide,
has "dbm" as decibels of "0 dbm" related to a power level of
1.0 milliWatts in a "50 Ohm system." That has become so
widespread that specification writers don't always include
those words. It is implicit when referring to RF components.

. . .

That's common in the RF industry, but many others also use dBm -- for
example, it's often used in video systems where the standard impedance
is 75 ohms, and others where the standard is 600 ohms. In all those
applications, dBm is universally defined and understood to mean dB
relative to 1 mW, regardless of the impedance.

Quite true, Roy. :-)

I restricted myself to "50 Ohms" for a couple of reasons:
As far as "pure" RF components go, 50 Ohms is the Z
characteristic; I didn't want to complicate my explanation.

The TV Cable industry is HUGE and they use 75 Ohms. However,
so many radio amateurs run around snarking on "TV" that it
could have raised a lot of unneccessary babbling in here. :-)
TV cable is digital in some locations (ours is in the SF
Valley of L.A.) and has more TV channels crammed into the
same VHF-UHF space than old analog TV. Don't know the
modulation of my digital TV cable signals, whether it is
wider or narrower than analog channels...but my TV service
crams over 400 channels into the same bandwidth. TV sure
isn't the narrow-band stuff that many hams are used to.

"dbv" (sometimes "dbu" but rarely) refers "0" as 1.0 microVolt,
almost any characteristic. Not seen much in specifications,
though.

The "VU" (for Volume Unit) is an old, old one in the audio
and telephone industry with "0 VU" being 1.0 mW into 600
Ohms impedance. Not only that, the "VU" industry standard
used to call out the indicating meter's ballistic (needle or
meter motor) characteristics! :-)

"dbc" is an often-used term on component specifications but
is still a relative term of db in regards to the Carrier of
a signal where the noise is called out. ["C" for Carrier]

"dba" is a legal term in the USA standing for "Doing
Business As" in local governments that require business
licenses. :-)

My apologies if some ISPs show two more postings of my
message. When I posted via Google on Saturday early
afternoon, Google was interrupting itself with lots of
users (?) or something that kept prompting "server error."
I've since removed redundant posts on Google, but some
other ISPs may be storing the multiples. Stuff happens.

wrote:
. . .
"dbv" (sometimes "dbu" but rarely) refers "0" as 1.0 microVolt,
almost any characteristic. Not seen much in specifications,
though.
. . .


Interesting. I've many times seen dB relative to one volt as dBV, and dB
relative to one microvolt as dBuV, but never dBv meaning dB relative to
one microvolt, or dBu at all. Do you have any references that show these
unusual usages?

Roy Lewallen, W7EL

Yes, I do. :-)

On 5 Jun 2005 17:16:10 -0700, wrote:
Yes, I do. :-)



Yes, you 'do' WHAT??
What are you nattering on about?!?

Oh, I see. Another sloppy groups.google poster.

I've never seen "dBv" refer to 1uv either. Always 1V.

Joe
W3JDR


"Roy Lewallen" wrote in message
...
wrote:
. . .
"dbv" (sometimes "dbu" but rarely) refers "0" as 1.0 microVolt,
almost any characteristic. Not seen much in specifications,
though.
. . .


Interesting. I've many times seen dB relative to one volt as dBV, and dB
relative to one microvolt as dBuV, but never dBv meaning dB relative to
one microvolt, or dBu at all. Do you have any references that show these
unusual usages?

Roy Lewallen, W7EL

On Mon, 06 Jun 2005 11:12:03 +0000, W3JDR wrote:

I think that would be dBuv not dBv which is Ref 1V


I've never seen "dBv" refer to 1uv either. Always 1V.

Joe
W3JDR


"Roy Lewallen" wrote in message
...
wrote:
. . .
"dbv" (sometimes "dbu" but rarely) refers "0" as 1.0 microVolt,
almost any characteristic. Not seen much in specifications,
though.
. . .


Interesting. I've many times seen dB relative to one volt as dBV, and dB
relative to one microvolt as dBuV, but never dBv meaning dB relative to
one microvolt, or dBu at all. Do you have any references that show these
unusual usages?

Roy Lewallen, W7EL

--
Korbin Dallas
The name was changed to protect the guilty.

This page: http://decibel.biography.ms/ mentions both dBv and dBu.
Says in "electrical voltage" (probably with reference to audio
industry), dBu and dBv both mean dB relative to 0.775V -- generally
0.775Vrms, but dB taken as 20 log(V/0.775), without reference to a
particular impedance.

But the same page says dBu or dB(lower-case Greek mu: "micro") as radio
power is dB relative to one microvolt per square meter.

Go figure. It all points out the need to be careful to define your
terms if there's any chance of ambiguity. If you're not careful, your
reader may think dBu refers to Dallas Baptist University, or Deutsche
Billiard Union, or Duluth Business University... though the
capitalization would be wrong for those.

Cheers,
Tom

Thanks, Tom. This newsgroup is truly educational. I'm slowly learning
that out there, somewhere, just about every possible convention or
nomenclature is used by someone for some purpose. I'll bet I'm the only
kid on my block now who knows what dBu and dBv mean. And I guess Len is
the only kid on his block that knows they sometimes mean dB relative to
1 uV, as well -- ah, what's a factor of 775,000 one way or the other,
anyhow.

dBu = dumb ******* unit.

Roy Lewallen, W7EL

K7ITM wrote:
This page: http://decibel.biography.ms/ mentions both dBv and dBu.
Says in "electrical voltage" (probably with reference to audio
industry), dBu and dBv both mean dB relative to 0.775V -- generally
0.775Vrms, but dB taken as 20 log(V/0.775), without reference to a
particular impedance.

But the same page says dBu or dB(lower-case Greek mu: "micro") as radio
power is dB relative to one microvolt per square meter.

Go figure. It all points out the need to be careful to define your
terms if there's any chance of ambiguity. If you're not careful, your
reader may think dBu refers to Dallas Baptist University, or Deutsche
Billiard Union, or Duluth Business University... though the
capitalization would be wrong for those.

Cheers,
Tom