[mythtvnz] Re: Conflicting voltage / signal on the same coax

[Steve Hodge]

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On 7/31/07, Sam Hadley-Jones <sam@samborambo.ws> wrote:
>
> Hey guys,
>
> This is still an issue even if both dvb-s receivers are requesting the
> same polarity. If the 22kHz signal is sent from both devices to the LNB,
> the signals may cause the LNB to misbehave (due to aliased low
> frequencies or harmonics). The tone sources are also fairly low
> impedance meaning that they may be damaged. In electronics you learn
> quickly that two low impedance (in other words, more powerful) sources
> don't like talking to each other. The problem would be a whole lot worse
> with 13/18V switching. Even a difference of a few millivolts is enough
> to cause high circulating currents on a short length of conductor.


All-ports-power-pass splitters are designed for this though. Are you saying
you know for sure that they have problems with more than one output driving
the polarisation signal?

I see your point with the 22kHz tone. I hadn't thought about it because I
don't have a universal LNB (or one of those multi-satellite Sky ones). But
again, I expect the splitter to be designed to handle it without causing
damage to equipment. My splitter is labelled "two tap, 5-2500MHz", which I
assume means that the "tap" outputs won't pass the 22kHz tone.

Steve

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On 7/31/07, <b class="gmail_sendername">Sam Hadley-Jones</b> &lt;<a href="mailto:sam@samborambo.ws">sam@samborambo.ws</a>&gt; wrote:<div><span class="gmail_quote"></span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hey guys,<br><br>This is still an issue even if both dvb-s receivers are requesting the<br>same polarity. If the 22kHz signal is sent from both devices to the LNB,<br>the signals may cause the LNB to misbehave (due to aliased low
<br>frequencies or harmonics). The tone sources are also fairly low<br>impedance meaning that they may be damaged. In electronics you learn<br>quickly that two low impedance (in other words, more powerful) sources<br>don&#39;t like talking to each other. The problem would be a whole lot worse
<br>with 13/18V switching. Even a difference of a few millivolts is enough<br>to cause high circulating currents on a short length of conductor.</blockquote><div><br>All-ports-power-pass splitters are designed for this though. Are you saying you know for sure that they have problems with more than one output driving the polarisation signal?
<br><br>I see your point with the 22kHz tone. I hadn&#39;t thought about it because I don&#39;t have a universal LNB (or one of those multi-satellite Sky ones). But again, I expect the splitter to be designed to handle it without causing damage to equipment. My splitter is labelled &quot;two tap, 5-2500MHz&quot;, which I assume means that the &quot;tap&quot; outputs won&#39;t pass the 22kHz tone.
<br></div></div><br>Steve<br><br>

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