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.


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.


In many other cases they are purposely used as an extra level of abstraction
to sort out the 'RF men' from the 'boys'.
Manufacturers still use the dB concept for historical reasons. It doesn't
effect their sales as the RF people buying their kit carry in their heads
instant dB-V conversion tables.
Don't know about everyone else but all my scopes and signal generators and
sources and dc-ac-voltmeters and DVMs and signal probes etc, are marked in
Volts and Amps. So that's what I use.
(Someday I'll get round to building a real world 1:2:5:10 50ohm attenuator.
I certainly can't buy one :-)
regards
john