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Ioannis Makris makrisj at gmail.comAnd 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 at 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. > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.osmocom.org/pipermail/osmocom-sdr/attachments/20190525/75dab499/attachment.htm>