Topband: SDR Mythbusters - ADC Overload myths debunked...

W5JR Mike w5jr.lists at gmail.com
Tue Oct 6 19:05:11 EDT 2015


Interesting. Here at my place - wires in the trees, I'm able to transmit on the K3 (100w, old SYN) within about 20 kHz of the frequency where the Flex 6300 (zero band filtering radio) is listening without any detriment to the Flex's ability to hear signals on the frequency it's listening to.

I admit I was surprised at the results when I did this test a year ago. 

I've done this on several bands, 160 80 and 40. 

I routinely run these radios in SO2R mode (and with 1kw+ amps) on different bands without the aid of any bandpass filtering on the Flex, although I now have some Dunstars and plan to install them anyway on both transceivers. 

Same for the reverse, transmitting on the Flex (100w) and listening on the K3 in the same band 20 kHz away. 

The antennas are a multiband OFC @ 50', a 160m Inv L and an 80/40 vertical, all within 100' of each other. Maybe they are all inadvertently placed in a way to provide a good bit of isolation between them, but it wasn't intentional. 

tnx
Mike / W5JR
Alpharetta GA

> On Oct 6, 2015, at 6:29 PM, Tom W8JI <w8ji at w8ji.com> wrote:
> 
> 
> From Stu, K6TU
> 
> As with any technological change, there are many myths, past truths or part truths that get repeated endlessly and out of context by those frightened or challenged by the change.
> 
> Software defined radios are no exception to this in the world of Ham Radio.
> 
> Steve Hicks, N5AC and the VP of Engineering at FlexRadio has posted an excellent explanation and bust of the ADC overload myth on the FlexRadio community.  You don’t need to be registered on the community to read this excellent write up:>>>>>>
> 
> Regardless of the fact they cannot possibly overload, they overload here.
> 
> I don't understand his explanation at all.
> 
> We had ONE transmitter on while trying to receive. The ADC could not handle the level of one transmitter.  Because there was no in-band front end selectivity, the spacing did not matter.  We could not get the strong local signal outside a roofing filter.
> 
> With two transmitters on different non-synchronized frequencies, there will be periods of time when the voltages add. There will be periods when the voltages subtract. Anything following the summing point has to handle the random peaks, even an ADC.
> 
> The Flex we tried here, using it to listen while ONE transmitter was running, just went nuts. It was equally bad no matter what the spacing. At least with a radio with roofing filter, we could move the radio up or down band and use it.
> 
> 73 Tom 
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband


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