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Re: Topband: ADC Overload from MW transmitters

To: "topband@contesting.com" <topband@contesting.com>
Subject: Re: Topband: ADC Overload from MW transmitters
From: MIKE DURKIN <patriot121@msn.com>
Date: Mon, 19 Oct 2015 08:47:51 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
SPAM !!!!!!

> From: w8ji@w8ji.com
> To: topband@contesting.com
> Date: Mon, 19 Oct 2015 10:58:44 -0400
> Subject: Re: Topband: ADC Overload from MW transmitters
> 
> I think the problem here is some people read this as a SDR radios never 
> overload, or are superior in every case.
> 
> Apparently one person thought they were junk because multiple modest 
> strength signals would add up to overload them, and that triggered the 
> response that was misinterpreted to mean they never overload under any 
> condition or were always superior to roofing filtered systems common in 
> standard receivers.
> 
> In the case I had here, a *single* transmitter totally wiped the SDR out. 
> The overload was nothing like the desense or noise in a traditional 
> receiver. It just was totally useless. It was useless at any signal spacing, 
> because it had no front end selectivity at all that would reduce levels.
> 
> For my application, it was useless. It was far worse than a K3, which a few 
> kHz spacing would duplex on most antenna combinations. When the K3 (or 
> FT1000MP MKV's)  did overload, the overload was a desense or composite noise 
> type sound. It would take out noise floor signals worse, be progressively 
> less problem for stronger signals, and never be bothered with any antenna 
> combinations with strong signals. When the SDR overloaded, it was just 
> totally gone for everything, and wider frequency spacing with the local TX 
> made absolutely no difference like it does with a normal receiver. I assume 
> this was from overflowing the ADC, but it was a very dramatic sounding 
> overload.
> 
> That, coupled with the fact it did not have a traditional knob and panel 
> system and had some transmitter spurs, made it useless here. But that was 
> this setup and this application, where a local 1500 watt transmitter within 
> a few thousand feet of the RX antennas was being used while receiving. This 
> was a single transmitter multi-op, where one TX signal was allowed on the 
> air at a time but two or more operators were making contacts.
> 
> I still never find any SDR I listened to, even that one without a 
> transmitter running, better than analog detection for my ears on 
> "in-the-noise" signals.
> 
> 73 Tom
> 
> 
> 
> 
> 
> 
> 
> 
> ----- Original Message ----- 
> From: "Steve Ireland" <vk6vz@arach.net.au>
> To: <topband@contesting.com>
> Sent: Monday, October 19, 2015 8:07 AM
> Subject: Re: Topband: ADC Overload from MW transmitters
> 
> 
> > Hi Jim
> >
> > Clearly in a large US city, there is going to be a whole larger degree of 
> > difficulty than here.
> >
> > Perth is still pretty much a small city in world terms, with a population 
> > of about 2 million. In addition to the ABC transmitters, we have about 
> > half a dozen other transmitters, but only two of these have signals of any 
> > size – 6PR (10kW) and 6IX (2kW), with the former of these putting in the 
> > largest signal to me, with its transmitter/antenna on the banks of the 
> > Swan River estuary about 15km away.
> >
> > When I used my HPSDR, originally I had no filtering in front of the ADC 
> > and had some overload problems on 160m from the local BC stations. 
> > However, a simple Chebyshev HPF got rid of this. Later when I added the 
> > Alex bandpass filters, which are part of the HPSDR design, there was no 
> > longer any need for the HPF.
> >
> > The main point, as Phil says in his post, is that the amount of protection 
> > an ADC is going to need will vary widely, depending on factors such as 
> > local AM BC transmitters and how strong they are. In my case, all I had to 
> > do was to use the general coverage facility of the SDR to look at the 
> > medium wave here, see which of the signals were largest and look for a 
> > suitable HPF design accordingly .
> >
> > Vy 73
> >
> > Steve, VK6VZ
> >
> >
> >
> >> That's typical of medium-size cities in the US for high power 
> >> broadcasters, but major cities typically have twice as many. Both large 
> >> and medium-size cities, as well as smaller ones, typically have 6-10 
> >> stations in the 5kW range, and more in the 1kW range. Chicago is typical 
> >> of a large city (like New York, Boston, Los Angeles, San Francisco) -- it 
> >> has 50kW on 670 kHz, 720 kHz, 780 kHz, 890 kHz, and 1,000 kHz. There's 
> >> also a daytime only station with 50kW on 1160 kHz. Cincinnati is typical 
> >> of smaller cities like Indianapolis, Detroit, Minneapolis, Cleveland, St 
> >> Louis, and New Orleans, with 2-3 50kW stations and many smaller ones. 
> >> Cincinnati 50kW stations are on 700 kHz and 1530 kHz.
> >
> > I grew up in a small town in WV, with three 5 kW stations within two miles 
> > on 800 kHz, 930 kHz, and 1470 kHz.
> >
> > Bottom line -- there's a lot more broadcasting in the US than in most 
> > countries.
> >
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