🚀 go-pugleaf

In article <498@galadriel.bt.co.uk>, pcf@galadriel.bt.co.uk (Pete French) writes:
> 
> A simple problem - I need to make (or buy) tranformers to operate at 999kHz
> to split powqer R.F. from a main feeder into several sub-feeders in the 
> ration
> I want - this is normally 1 in with 4 out equally, but some points in the
> system need 1 in and 2 out split 2:1 between them.
> 
> Both the inputs and outputs must be matched to 50 ohms.

There are several ways you can go here...

Ground Rules:  Power level was not specified.  Moderate power is assumed,
range from milliwatts to tens-of-Watts.  Frequency range / bandwidth was
not specified.  Since the frequency "999 KHz" was mentioned, narrow-band
as well as wideband solutions are presented.  I ramble on about semi-
esoteric RF circuits because I like to hear myself talk, and I am
a semi-frustrated RF designer now doing gate-array design (but I like
that too).  I Take No Responsibility (tm) for ANYTHING I say here.  For
that, I like to get paid. 

1) Duplicate the existing design.  
The transformer core material is probably not critical in a 1 MHz 
splitter (assuming a broadband splitter of good design).  Any moderately 
high permability RF ferrite should do. A uL of 125 to 1000 should work, 
if the Xl of the windings is at least 5x the impedance level 
(5x 50 Ohms = 250 Ohms).  If the current design uses tuned circuits 
however, the core material becomes very critical.  Note that a splitter is
_not_ made by merely making a multiple-winding transformer.  As was mentioned
in the original question, matching impedances is a goal, and so the circuit
is necessarily more complex.

2) Buy ready-made splitters from Mini-Circuits Labs or Merrimac.
These and other manufacturers make broadband N-way splitter/combiners
at a reasonable price.  The performance is specified, which greatly
simplifes things.  This is probably the best solution.

3) Build a broadband splitter.
I recommend the Motorola RF Transistor databook app notes for design
info here.  Try to find AN749 (Broadband Transformers and Power 
Combining Techniques for RF).

4) Use resistive splitters and distribution amps.
The practicality of this depends on the power level and other things.

5) (My favorite, because I think it's neat): Build a Wilkinson power
divider.  This circuit is a narrow-band solution.  It provides good
port-to-port isolation and can be configured for any number of ports.
The component tolerance sensitivity is moderate, and it will operate
well over several percent bandwidth.  The termination resistors only
dissipate power in the case of a mis-match, so for reasonable splitting
applications, the power rating can be low.  There is no power lost
in the splitter (assuming ideal components and impedances).  Wilkinson
described this in "An N-WAY Hybrid Power Divider", IRE 
_Transactions_on_Microwave_Theory_and_Techniques_, January 1960.  The
circuit works as an N-to-one power combiner, as well.

The general prinicple is that for an N-way splitter (or combiner),
you take N quarter-wave impedance-matching sections, each matching
the source/load impedance Z to (N * Z) then parallel them at the
source.  The required characteristic impedance of the quarter-wave 
section is sqrt(Z1 * Z2), where Z1 = the source Z and Z2 = N * Z.
This of course reduces to sqrt(N * Z**2). Thus, for the 50 Ohm 2-way 
splitter, the  line Z = 70.7 Ohms, and for the 4-way, the line Z = 
100 Ohms.

Termination resistors are placed at the outputs to cancel any
load-to-load coupling (the load1 - source - load2 path has a 180 degree
phase shift, and the resistor path cancels this).

The quarter-wave sections can be replaced with equivalent 
lumped-impedance PI sections, where the reactance of the components
is equal to the line impedance.  The source-end capacitors can be
combined into a single capacitor (XCs = Zline / N).

For a 1 MHz 2-way splitter:
    Zline = 70.7 Ohms, Cs = 4.5 nF, Ls = 11.75 uH, Cl = 2.25 nF.

For a 1 MHz 4-way splitter:
    Zline = 100 Ohms, Cs = 6.37 nF, Ls = 15.9 uH, Cl = 1.59 nF.


A two-way Wilkinson power divider:
(XLs = XCl = 70.7 Ohms, XCs = 70.7 / 2 Ohms) 

50 Ohm in o--+----[ Ls ]---+----+----o 50 Ohm out
 source      |             |    |       load
             |                  |
             |             Cl   
             |                  Rt
             |             |  100 Ohm
             |            gnd    
             |                  |
             |                  |
             +----[ Ls ]---+----+----o 50 Ohm out
             |             |            load
                                        
            Cs             Cl
                            
             |             |
            gnd           gnd


                                

A four-way Wilkinson power divider:
(XLs = XCl = 100 Ohms, XCs = 25 Ohms) 


                                +---+---+---+
                                |   |   |   | 
                                |   |   |   | 
                                Rt  Rt  Rt  Rt (Rt = 50 Ohms)
                                |   |   |   | 
                                |   |   |   | 
                                |   |   |   | 
50 Ohm in o--+----[ Ls ]---+----|---|---|---+---o 50 Ohm out
 source      |             |    |   |   |          load
             |                  |   |   |     
             |             Cl   |   |   |     
             |                  |   |   |     
             |             |    |   |   |     
             |            gnd   |   |   |    
             |                  |   |   |    
             |                  |   |   |    
             +----[ Ls ]---+----|---|---+-------o 50 Ohm out
             |             |    |   |              load
             |                  |   |        
             |             Cl   |   |        
             |                  |   |         
             |             |    |   |         
             |            gnd   |   |        
             |                  |   |        
             |                  |   |        
             +----[ Ls ]---+----|---+-----------o 50 Ohm out
             |             |    |                  load
             |                  |            
             |             Cl   |            
             |                  |             
             |             |    |             
             |            gnd   |            
             |                  |            
             |                  |            
             +----[ Ls ]---+----+---------------o 50 Ohm out
             |             |                       load
                                        
            Cs             Cl
                            
             |             |
            gnd           gnd



I hope this has been at least amusing, if not helpful.



-- 
Paul M. Elliott      Optilink Corporation     (707) 795-9444
         {pyramid,pixar,tekbspa}!optilink!elliott
"I used to think I was indecisive, but now I'm not so sure."