dbm to volts (humax F2foxT)

On Fri, 03 Aug 2007 17:34:35 +0100, Andy Wade
mused:

The best approach, in my view, is to use a good quality crimp IEC plug,
such as this:
http://www.dastv.co.uk/ViewProduct.aspx?Product=455

The thing I like about the word professional is it makes many people
look like amateurs.
--
Regards,
Stuart.

"Andy Wade" wrote in message
...
mike wrote:

Conversely, to convert dBm to microvolts follow this example:
Snip

You're making hard work of this. To convert from dBm to dBuV in a
75-ohm system just add 109 (or 108.751 if you want to be fussy).
The
corresponding number for a 50-ohm system is 107 (106.990 if being
fussy).

--
Andy


Can we have yor maths for that please?

0dBuV is 1uV irrespective of impedence

0dBm/75R is 1mW across 75R which by my maths is 273mV

Que?


--
Woody

harrogate3 at ntlworld dot com

harrogate3 wrote:

Can we have yor maths for that please?

0dBuV is 1uV irrespective of impedence

0dBm/75R is 1mW across 75R which by my maths is 273mV

OK so far (although it's 273.86 mV, so 274 mV is closer, but that's
splitting hairs).

So in dBuV it's 20 * log(273,860) which is 108.751.

For 0 dBm in 50 ohms you get V = sqrt(P * R) = sqrt (0.05) volts =
223.607 mV or 223,607 uV. 20 times the log of that gives you 106.99 dBuV.

--
Andy

"Bill Wright" wrote in
:


And I don't expect to last to 2012 to see an improvement.
Well, Heaven will be line-of-sight to Crystal Palace I should think.
Let's hope you've been good eh?


Buggered if I'm going there; I'd be with the God-bothering idle smug
overpaid killer quack Cockcroft from Billericay Health Centre who makes
me come in to tick all the boxes to get his overinflated pay-packet, but
for the first time in years I went in on my own volition told me it was
nerves and refused a little light endoscopy, which only takes 15 mins.

When I collapsed and a real doctor found the truth, too late, he called
it an Adverse Outcome and said it was all my fault as he's an expert on
the disease and he's an excellent listener..

My next door neighbours on the other side will be there too, and they're
about as bad as god-botherers get.

I'm hoping the cats and dogs will let me in to one of their dives.

Or maybe you've got some idea, perhaps I could be an infernal aerial
apprentice!

mike

The message
from "harrogate3" contains these words:


"Andy Wade" wrote in message
...
mike wrote:

Conversely, to convert dBm to microvolts follow this example:
Snip

You're making hard work of this. To convert from dBm to dBuV in a
75-ohm system just add 109 (or 108.751 if you want to be fussy).

To be _really_ fussy, it's actually 108.7506126 (to 7 decimal places:-)

The
corresponding number for a 50-ohm system is 107 (106.990 if being
fussy).

To be _really_ fussy, it's actually 106.9897 :-)


--
Andy


Can we have yor maths for that please?

0dBuV is 1uV irrespective of impedence

0dBm/75R is 1mW across 75R which by my maths is 273mV

To 3 digit accuracy, it's actually 274mV (to 10 digits, it's 273.8612788mV)

Que?

What? Were you questioning the 273mV figure? :-)

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

The message
from Andy Wade contains these words:

harrogate3 wrote:

Can we have yor maths for that please?

0dBuV is 1uV irrespective of impedence

0dBm/75R is 1mW across 75R which by my maths is 273mV

OK so far (although it's 273.86 mV, so 274 mV is closer, but that's
splitting hairs).

Let's leave the 'Hair splitting' to the WPCs, shall we?

So in dBuV it's 20 * log(273,860) which is 108.751.

For 0 dBm in 50 ohms you get V = sqrt(P * R) = sqrt (0.05) volts =
223.607 mV or 223,607 uV. 20 times the log of that gives you 106.99 dBuV.

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

The message
from brightside S9 contains these words:

On Sat, 4 Aug 2007 16:24:58 +0100, Johnny B Good
wrote:

The message
from "harrogate3" contains these words:


"Andy Wade" wrote in message
...
mike wrote:

Conversely, to convert dBm to microvolts follow this example:
Snip

You're making hard work of this. To convert from dBm to dBuV in a
75-ohm system just add 109 (or 108.751 if you want to be fussy).

To be _really_ fussy, it's actually 108.7506126 (to 7 decimal places:-)

The
corresponding number for a 50-ohm system is 107 (106.990 if being
fussy).

To be _really_ fussy, it's actually 106.9897 :-)


--
Andy


Can we have yor maths for that please?

0dBuV is 1uV irrespective of impedence

0dBm/75R is 1mW across 75R which by my maths is 273mV

To 3 digit accuracy, it's actually 274mV (to 10 digits, it's
273.8612788mV)

Que?

What? Were you questioning the 273mV figure? :-)


Oh! It seems someone needs to understand significant digits.

ITYMM the difference between rounding to the nearest n significant
digits and truncating to n significant digits (the microsoft way :-)

Stop nit picking and take a look at

http://www.physics.uoguelph.ca/tutor...ig/SIG_dig.htm

The big difference here is that all the calculations were done by the
use of a scientific calculator which displays to 10 digit resolution
(which doesn't necessarily mean that that's the resolution limit of the
intermediate results produced by the calculator). This leaves the choice
of how many significant digits to quote as the final answer to the
user's discretion (applying the standard rounding rule to correctly
determine the least significant digit value).

That 3 digit value of 273mV is 'correct' only in the Microsoft sense
and wrong by the rules for rounding to the least significant digit in
the answer. As it happens, rounding either of those 3 digit figures down
to 2 significant digits will both, correctly, produce a value of 270mV
provided it is stated to have a 2 digit accuracy (otherwise a 3 digit
accuracy would be implied).

Normally, 2 digit accuracy for db figures suffices for most practical
purposes until we start dealing in +/- db figures greater than 100 where
we then need 3 digits accuracy to at least stay within +/- half a db of
the actual value.

When dealing with a logarithmic based scale, a +/- 5db error remains
equally significant regardless of how many tens of db we are displaced
from the reference level. However, a 2 digit accuracy for voltage (or
current) levels only represents a worst case decibel error of just under
+/- 0.5db (0.4455 db to 4 significant digits) whatever the value.

Just to put it all in perspective, the db error due to specifying 273mV
instead of the slightly less inaccurate 274mV figure represents a mere
0.0318 db ( that's to an accuracy of four decimal places or 4
significant digits, unless you prefer the form 3.18E-2, which then makes
it accurate to 3 significant digits :-) Either way, the error is
unlikely to be measurable on any analyser kit available to the aerial
rigging trade. :-)

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

Johnny B Good wrote:

That 3 digit value of 273mV is 'correct' only in the Microsoft sense

In what context do Micro$oft truncate numbers rather than round them?

--
Andy

On 05/08/2007 14:30, Andy Wade wrote:

In what context do Micro$oft truncate numbers rather than round them?

None (unless specified) but they are not consistant across different
products

http://en.wikipedia.org/wiki/Roundin...ming_languages

"kim" wrote in message
...
"Bill Wright" wrote in message
...
That's interesting because our communal system relies on a single
conventional Yagi array mounted on top of a 14-storey tower block followed
by huge amounts of amplification. Perhaps if they used a better aerial
followed by less amplification we would get a usable signal on more than
one Mux?

No, because the signal from the aerial will most likely have an excellent
c/n ratio. The extra gain you could get from a bigger aerial is
insignificant compared to the gain of the amplification within the
disrtibution system.

Bill