fl2k output voltage level changes every few seconds

Ioannis Makris makrisj at gmail.com
Sat May 25 13:17:17 UTC 2019


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 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.
>
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