On 01/01/2011 04:23 AM, Piero Soldi wrote:
Il 01/01/2011 3.36, Joel Koltner ha scritto:
I'm designing an IF filter for a receiver that's placed at the output of
a passive (diode-type) mixer. While the mixer's RF input is being fed
with a (nominally) 50ohm source, is it correct to think that "looking"
back into the IF port the filter is going to "see" a 50ohm impedance as
well? (Normally I'd have a buffer stage right after the mixer to
re-establish impedance levels, but in this case I'm trying to keep
things low-power and hence directly feed the mixer's output into the
filter.)

Thanks,
---Joel


Yes,but only if the other two ports are closed with 50 ohm...

Happy new year,

Piero.

P.S: Beware of filters, they are highly reactive outside their
resonant frequency, so (they) reflect back signals with
worsening behaviour of mixer ( spurs ).
P.


Put a diplexor between the output of the mixer and the filter. Use a
matching transformer between the filter and the mixer so the mixer will
see a 50 ohm impedance. In my case I assumed the filter had an
impedance of about 500 ohms so I am going to use a 1:9 (1:3 turns ratio)
between the mixer and the filter. This transformer will be a trifiler
wound toroid and will be wired as an autotransformer. (the three
windings in series connect to the filter and ground, a tap on the first
winding up from ground will go to the mixer). The diplexor consistes of
a series resonant circuit between the mixer and the filter. In parallel
with this series resonant circuit are two 51 ohm resistors in series
with a parallel resonant circuit between the two resistors and ground.
Select the L&C to resonant at the IF frequency with a Q of 1.