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.

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 at "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

"Wes Stewart" wrote in message
...
On Sat, 4 Jun 2005 14:53:43 +0100, "john jardine"
wrote:


"nanchez" wrote in message
oups.com...
Hi.
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 ?

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

Thanks

Hernán Sánchez

To rejoin the real world, take the "16" figure and divide it by 20.
get "0.8"
Then find the antilog of that 0.8 [use normal 'base10' logs]
get "6.31"
This number is a multiply or divide factor that is applied to a 50 ohm
0dBm
reference voltage.
So what is this god like reference voltage?. The 50ohm 0dBm reference
voltage is in actual fact 0.223Vac.
The original number was "-"16 dBm. Just read the minus sign as meaning a
voltage less than the 0dBm reference voltage.
So that 0.223Vac reference value is divided by your 6.31 factor.
get "0.035" Vac.
So "-16dBm" is really 35mVac. This means you have more than enough drive
voltage available from your 2.5Vpp (900mVac) local oscillator signal.

Except neither you or Hernan can be sure of this. His source is not
specified to work into a 50 ohm load or present a 50 ohm source
impedance to the mixer (what is really needed). Who knows what the
delivered voltage will be when driving the mixer port?

A measurement is in order. Terminate the source in 50 ohm and measure
the power and/or voltage. If it exceeds -16 dBm, attenuate
accordingly.

I imagine we would both have read Hernans post the same way. I.e that he has
some logic running at 40MHz and is seeing a rough 2.5Vpp sinewave on his
50-100MHz oscilloscope via a 10:1 probe. From this I'd also assume we both
knew that the logic drive impedance would be a couple hundred ohms at the
most and that serious mis-matching was not going to be a problem.
I'd though, suggest he just connects the parts together and see what
happens. He's experimenting. Monitoring the results (good or bad) is just
part of the due process. Doesn't look like he's got a spectrum analyser, so
how can he validate a 50ohm test measurement as exceeding -16dBm?.

His sticking point was specifically about the link between Dbs and Volts. A
common, basic electronics question yet surprisingly badly answered by the
original suggested link, which started off with the concept of a
"dimensionless gain" and went downhill from there using 3 pages of sums.
How many radio hams talk to each other about their TX powers in terms of
(ISO standard) units of dBs wrt one watt?. Why do the filter tables tell me
a Cheb' filter ripple is in dBs when all I want is percent values. Why do my
function generators handbooks tell me the sine THD is 0.2% upto 200kHz but
beyond that the distortion suddenly becomes an obscure "-30db wrt the
fundamental".
A recent EW magazine article for a AC mV meter said the response was flat
within 0.0024dB. How flat is that?.



Be very wary whenever you come across dBm figures. There is a minefield
of
disinformation out there.
In many cases they are intended purely to obfscutate the reader and
prevent
them clearly seeing that the described circuit is junk.

Spoken like a real expert on bafflegab.


Indeed, the word was intended that way (UK English).
regards
john

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. :-)

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 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.

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