And another thing - the configuration demonstrated on the image you
provided is an actual Π (pi) type low pass filter composed by R-L-R
elements.
There, the individual resistor is 150 ohm, in DC they are parallel, in RF
they make a low pass filter along with the coil in btw.
Some implementations do not use any coil but a 0-ohm shunt resistor
instead. That means the resistors are parallel and they yield 75 ohms.
Cheers
On Sat, 25 May 2019 at 16:10, Ioannis Makris <makrisj(a)gmail.com> wrote:
I mean that each RGB analog output of the FL2k on the
PCB has permanent
fixed terminations of 150 ohm. Still, I need to apologise for having to add
a correction:
Some of the dongles having only analog output are being terminated @ 75
ohms at the factory.
Ref.1:
http://www.marsport.org.uk/smd/mainframe.htm
Ref.2: See attached image
This is normal. Those outputs are most probably open emitter outputs;
hence they need a termination in order to actually produce any current.
Leaving them floating in DC would most probably lead to unexpected and
unaccounted for results.
One could experiment by adding different values for termination or even a
330 ohms trimmer in rheostat configuration,while maintaining at least an 75
ohms resistor in series so as to give a range of 75-405 ohms of
termination.
What is to be monitored is the actual spectral behavior of the DAC rather
its maximum output, as there are unwanted spectral elements that could
register as power output on the wanted frequency in conventional needle
meters, when it actually that is far from truth.
Affecting terminations yields intermodulation in amplifiers and unwanted
spectral elements may occur due to several effects I can think of.
Always monitor spectral output if you want to transmit with this thing.
What I've seen is that if you just add 20dB of gain on its output without
applying heavy filtering beforehand you could easily end up to jail for
interfering aviation frequencies used at airports nearby.