Topband: ADC Overload from MW transmitters

Mon Oct 19 13:01:33 EDT 2015

If there is one transmitter per band and no duplex, I think it would work 
fine. (If you can get old timers to use knobless radios.) External filters 
would easily correct any problems.

The issue is duplex on one band at high local signal levels, where an 
external filter would be much too complicated.

----- Original Message ----- 
From: "bruce whitney" <zuceman at yahoo.com>
To: <topband at contesting.com>; "Tom W8JI" <w8ji at w8ji.com>
Sent: Monday, October 19, 2015 11:59 AM
Subject: Re: Topband: ADC Overload from MW transmitters

This has been an interesting discussion.
I heard a rumor that a very prominent and successfully competent multi-multi 
contester in IL was going to an all SDR multiple computer control set-up. 
Which would seem to be problematic in light of this discussion?
Anyone else hear this?
Bruce W8RA

--------------------------------------------
On Mon, 10/19/15, Tom W8JI <w8ji at w8ji.com> wrote:

 Subject: Re: Topband: ADC Overload from MW transmitters
 To: topband at contesting.com
 Date: Monday, October 19, 2015, 10:58 AM

 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.
 >
 > ---
 > This email has been checked for viruses by Avast
 antivirus software.
 > https://www.avast.com/antivirus
 > _________________
 > Topband Reflector Archives - http://www.contesting.com/_topband
 >
 > -----
 > No virus found in this message.
 > Checked by AVG - www.avg.com
 > Version: 2014.0.4830 / Virus Database: 4365/10847 -
 Release Date: 10/18/15
 >

 _________________
 Topband Reflector Archives - http://www.contesting.com/_topband

-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2014.0.4830 / Virus Database: 4365/10849 - Release Date: 10/19/15