In message , Ralph
Mowery writes

"Ian Jackson" wrote in message
Strange as it may seem, if you use (for example) a TV 2-way '3dB'
splitter to combine two identical in-phase signals, you DON'T lose 3dB.
Apart from the unavoidable slight inherent losses of the two transformers
the circuit uses (a total of around 0.5dB at low VHF, increasing to 1dB at
high UHF), the splitter is lossless. Ignoring the transformer loss, the
3dB loss occurs simply because the power at each output port is half of
that at the input. You haven't actually lost anything.

If the splitter is now turned around to become a combiner, it doesn't
suddenly become more lossy. If you again ignore the transformer losses,
the two identical in-phase signals you feed into the 'output' ports are
added, and the result is a signal 3dB higher.



I don't have a TV splitter/combiner to play with. I do have a MiniCircuits
combiner I have been playing with.
http://www.minicircuits.com/pdfs/ZFSC-2-2.pdf

The spec says "• low insertion loss, 0.4 dB typ."
This is the real loss - the loss that will make the splitter get warm
when you feed a signal into it.

I am not sure what is in the combiner but it must be transformers of some
type as the resistance of the ports are near zero ohms.

The usual basic circuit is:
Input port to ground - a 2:1 impedance ratio step-down ferrite-cored
autotransformer T1 to ground.
T1 tap is connected to the centre tap of a second 1:1 ratio
ferrite-cored autotransformer T2.
Each end of T2 of connected to each output port.
A 100* ohm resistor R is connected between the ends of T2 (ie between
the output ports).
*R is 2 x Zo, so for a 75 ohm system, it will be 150 ohms. Note the
purpose of T2 and R is to provide isolation between the outputs (for
signals coming back into the output ports).

Their specs is for a 3 db loss and then an aditional loss of about .2 to
1.2 depending on the frequecy. Are you sure that is not the case where you
are saying you do not loose the 3 db ? That is the ratings is for the extra
.5 or so not counting the already 3 db of loss.

For forward-going signals, T2 and R play no part in the operation of
the circuit. The signal current from T1 tap enters T2 at its centre tap,
splits, and flows outwards in opposite directions to the ends of T2. The
magnetic flux created by the currents cancels out, so T2 presents no
impedance whatsoever. In effect, it isn't there, and the two outputs are
connected in parallel. The impedance presented to the centre tap of T1
is therefore 25 ohms, which is exactly what T1 is there for - to match
the 50 ohm input to 25 ohms of the two (effectively) parallel outputs.

So you see that apart from the unavoidable losses in the transformers
and in the copper wire, there are no losses in this circuit. However,
because the power emerging from each output is half the power of the
input, the loss measures 3dB (plus a bit). If you can devise a passive
2-way equal splitter with less than 3dB loss, you will make a fortune!

Maybe you can tell me if I am playing with the wrong type of combiner.. Here
are some results of my tests. using a HP 8924C service monitor and another
signal generator.

Theoretically, this type of circuit has no loss (whether used as a
splitter of an in-phase combiner). It's only the 'extra' losses that
will give you trouble. The same is true of any other type of combiner,
so which one you use may depend on which will have the least extra loss.
If you don't need high (or any) isolation between the 'output' ports
('input' if a combiner), you might do better with one of the
transmission line alternatives.

With one input port of the combiner having a signal and the other port
terminated with a 50 ohm load (the nominal impedance of all devices) there
is a 3 db loss (small values not being included) not the low values you
mention to the output port. When I hook up the other signal generator, I
get from almost a total of 0 db to 6 db of loss. I asume the spectrum
analizer going from 0 to 6 db is the phasing of the two generators.

Not quite sure what you're doing here. However, for this circuit to act
as a lossless combiner, it relies on there being no voltage across the
resistor R. This means that the two input signals must in phase and of
equal amplitude.

Typical measurements for this sort of device would be (with all three
ports correctly terminated):
In to Out: 3.5dB
Out to Out: 30dB
With one Out unterminated (o/c or s/c):
In to Out: 3.5 to 4dB
With In unterminated (o/c or s/c):
Out to Out: 7dB
--
Ian