"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