[sdiy] 4 channel mixer of signals with frequencies between 2Mhz and lower.. any circuits available.. ??

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Fri Sep 29 00:10:06 CEST 2023


This is a very off topic for Synth DIY...  However, having previously 
worked on RF power stuff, I can say that what you really need is called 
a diplexer, triplexer or "quadruplexer".  It is a frequency selective 
combiner.  It acts to combine many sources of RF power presented at the 
input ports into a final "mix" that can be fed to an antenna connected 
to the output port.  The actual design is quite complicated for several 
reasons...

1. Each transmitter will expect to see a "matched" 50 ohm load when 
looking into it's triplexer input at whatever frequency that the 
particular transmitter is running at.
2. Each transmitter will not like to have it's output driven with RF 
power from the other transmitters!  So these triplexers usually contain 
resonant "traps" to effectively block each transmitter from "seeing" the 
other transmitters even though their outputs must eventually meet at a 
common antenna.  (If the transmitters get back-fed significant amounts 
of the other transmitters output, intermodulation is highly likely!)
3. The impedance of the single common antenna will most likely be 
different at each of the frequencies that the transmitters are running 
at.  One of the jobs of the triplexer is to perform matching between 
whatever the antenna impedance happens to be at each frequency to 
50-ohms that each transmitter expects to see as its load at its 
operating frequency.

So you can see that this type of "power combiner" is quite a complicated 
device with lots of L's and C's in series/parallel resonant circuits.

For anyone interested, there's some cool pictures here of what such a 
device looks like in the broadcast industry, where several radio 
stations share one antenna and the power levels are in the tens or 
hundreds of kilowatts...

https://www.rfcafe.com/miscellany/cool-products/lba-technology-triplexer.htm

For a hobby project, you might get away with a lossy resistive combiner, 
but it might damage the transmitters and you might produce a lot of 
intermodulation products in the resulting spectrum.  Fortunately, with 
only a few milli-Watts of power fed into a lossy combiner and an 
electrically short antenna at these frequencies it probably won't 
radiate very far!

-Richie,



On 2023-09-28 22:02, Jean-Pierre Desrochers wrote:
> Here is what I'm trying to do as an experiment.
> 
> I have 4 small AM transmitters transmitting at 600, 900, 1200 and 
> 1600Khz.
> At about 10mW of power each.
> I was thinking about mixing the 4 frequencies as a single unit
> that I could adjust the final output transmitted on a small antenna.
> That's my idea so far..
> 
> 
>> -----Message d'origine-----
>> De : Synth-diy <synth-diy-bounces at synth-diy.org> De la part de 
>> Gordonjcp
>> Envoyé : 28 septembre 2023 16:46
>> À : synth-diy at synth-diy.org
>> Objet : Re: [sdiy] 4 channel mixer of signals with frequencies between
> 2Mhz
>> and lower.. any circuits available.. ??
>> 
>> On Thu, Sep 28, 2023 at 03:17:56PM -0400, Jean-Pierre Desrochers 
>> wrote:
>> > Hi all,
>> >
>> >
>> >
>> > This is an out off topics question but I will ask you anyway..
>> >
>> >
>> >
>> > I want to build a 4 channel mixer that would receive signal
>> > frequencies between 2Mhz and lower one channel..
>> > These 4 frequencies won't change. They will be fixed.
>> >
>> > Google would give me lots of Radio/Ham solutions that imply the use of
>> > tuning coils and such things I don't want to get into.
>> >
>> > The idea would be to use ready available opamps with high bandwidth
>> > with the use of resistors and caps only. Gain would be of one.
>> 
>> For the avoidance of the XY Problem, what exactly are you trying to 
>> do?
>> 
>> --
>> Gordonjcp
>> 
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