🚀 go-pugleaf

In article <1992Sep10.175850.24383@lonex.rl.af.mil>, Brian R. O'Hern (BRO'HERN) writes:
> Short of buying a pad, does anyone know a good (cheap) way of matching a
> cable tv coax (75ohms) to the input of a HP spectrum analyzer (50 ohms)?
> I'm interested in the 500MHz to 1GHz range.
> 
> Thanks
> BRO'HERN (Electronics Engineer)

The easiest way is to use resistors.  A two way matching network uses
a 43.3 ohm and 86.6ohm resistors.

                             	----------------
				|
				\
				/ 43.3 ohm
				\
				|
		-----------------		75 ohm end
				|
				\
	50 ohm end		/ 86.6 ohm
				\
				|
		---------------------------------	


To the 50 ohm end, it sees 86.6 // (43.3+75) = 50 ohm
To the 75 ohm end, it sees 43.3 + (50 // 86.6) = 75 ohm 
I calculate a dB loss of about 11.4dB (sounds right, but I could be wrong)


This allows you to use the same "matching transformer" for going both 
directions.  Since resistors are used, frequency has no effect (until
you start to get into those frequencies where wire placements, etc have an
effect).

Since you really only need to match it from the 75 ohm signal into the
50 ohm spectum analyzer, you could just put a 25 ohm resistor in series with
your load.  Again, you will have a drop in signal (about 3.52dB -- and again
I could be wrong on this), but their is still no frequency effect.  That's
cheap.

-- 
Thomas Hilbig,
hilbig_t@qis.dofasco.ca                 Telephone:  (416) 544-3761