[HN Gopher] Almost hardwareless RF transmitter for Raspberry Pi
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       Almost hardwareless RF transmitter for Raspberry Pi
        
       Author : _Microft
       Score  : 37 points
       Date   : 2022-04-27 08:09 UTC (14 hours ago)
        
 (HTM) web link (github.com)
 (TXT) w3m dump (github.com)
        
       | anfractuosity wrote:
       | Would I be right in thinking because you're twiddling a single
       | pin you wouldn't be able to do amplitude modulation?
       | 
       | Could you do amplitude modulation at RF frequencies with a
       | resistor ladder DAC using the pi?
        
       | pcdoodle wrote:
       | Very cool, opens up loads of possibilities for fun projects!
        
         | [deleted]
        
       | tommiegannert wrote:
       | Cute.
       | 
       | The posted link is to the sample applications repo. The actual
       | library is in https://github.com/F5OEO/librpitx.
        
       | phendrenad2 wrote:
       | > Never transmit on antenna without a bandpass filter!
       | 
       | What does this mean? And why does the github seem to skip this
       | step, despite the warning?
        
         | jdmoreira wrote:
         | I'm guessing this generates a really dirty signal, lot of
         | harmonics, etc...
         | 
         | So not using a bandpass implies that you are polluting the
         | spectrum which the FCC would obviously not approve of but that
         | can't be it because none of this would be something the FCC
         | would approve.
        
           | cf100clunk wrote:
           | The FCC and similar authorities worldwide provide slices of
           | RF spectrum for amateur experimenters. Even so, there are
           | rules that require your homework and attention.
        
         | dTal wrote:
         | This works by turning a pin on and off very quickly to generate
         | a carrier wave. However, this produces, not a sinusoid, but a
         | square wave. This means if you attempt to transmit on 1khz, you
         | will also transmit on 3khz, 5khz, 7khz etc. So you will
         | certainly end up transmitting outside of the band you were
         | intending to.
         | 
         | Of course the true advice should be "don't transmit on an
         | antenna _at all_ without the appropriate legal license ".
        
           | cf100clunk wrote:
           | Antennae tend to give a multiplication factor to almost any
           | input signal, no matter how low the initial power level might
           | be. As a crude example, a 1 watt input signal might suddenly
           | have an Effective Radiated Power in the hundreds of watts
           | (yes, there are a vast number of variables in such a formula
           | so take this as just a thought experiment).
           | 
           | A bandpass filter has the effect of only allowing a desired
           | sliver of the RF spectrum to be transmitted beyond it (i.e.
           | Block All Except For...). There are cheap, passive, dumb
           | analogue bandpass filters commonly available, and at the
           | higher end of the cost scale there are active, smart digital
           | circuits. Do your homework because not all bandpass filters
           | can provide the sharp shoulder needed to give a clean slice
           | of spectrum. A similar device is a bandstop filter, which
           | blocks a specific slice of the RF spectrum as specified (i.e.
           | Allow All Except For...).
        
       | akmarinov wrote:
       | Just went and posted all the RF stuff today huh
        
         | [deleted]
        
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       (page generated 2022-04-27 23:03 UTC)