Topband: ADC Overload from MW transmitters

[Tom W8JI]

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 at arach.net.au>
To: <topband at 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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