# Capacitance

Discussion in 'Electronic Basics' started by Bill, Sep 25, 2003.

1. ### BillGuest

In frequency terminology what means IF?
And how to calculate or measure bus capacitance and is 470pF is limit
capacitance for all buses or jut I2C?

Bill.

2. ### Jack AssGuest

Can't help you with the capacitance, but I have seen the "IF" term used for
frequency mixers. In this context it means "intermediate frequency". Mixers
have three ports, the two inputs signals are typically called "RF"
(radiofrequency) and "LO" (local oscillator), and the output "IF". The
reason for this notation is that the most common use of a mixer is to
demodulate a signal of lower frequency (IF) that rides on a much higher
frequency carrier (RF).

There might be other contexts in which "IF" means another thing, though.

3. ### Peter BennettGuest

Fine so far...
In a superhetrodyne receiver, the incoming RF is converted to a
(usually lower) intermediate frequency (IF), where it is amplified and
band-limited (to reduce interference from signals on adjacent
frequencies), then fed to the demodulator.

4. ### Jack AssGuest

You are of course right... That is how a superheterodyne works, and it has
to be the most widespread circuit in which a frequency mixer is used. My
apologies for the misinformation.

The one application I was referring to (direct demodulation) is the one I
have been using mixers for in the last years... I work with lab
instrumentation, and there it is very comon to use lock-in amplifiers and
mixers to directly demodulate a signal that rides a carrier of the same
frequency as the LO. Typically one uses some function generator to provide
the modulation signal for the experiment -could be modulation of a laser
beam, a microwave source...- and also to provide the LO signal for the
mixer, which is later use to demodulate and recover the signals obtained
from the experiment. Physicists use this kind of phase-sensitive detection
setup -old and well known in electronics- to increase the sensitivity of
their detection schemes.