G’day
For those topbanders, who have medium wave stations nearby and are thinking
about trying a DDC/DUC SDR, you may find the comments below from Phil
VK6APH/VK6PH (in answer to a query on the Apache Labs reflector a few years
ago) of interest.
Phil lives line of sight from the Australian Broadcasting Transmitter
installation in the northern suburbs of Perth. There are three AM transmitters
there which run 24 hours a day, seven days a week:
6PB at 10 kW on 585 kHz; 6RN at 20 kW on 810 kHz; and 6WF at 50 kW on 720 kHz,
so this is a very demanding situation for the various DDC/DUC transceivers,
including the HPSDR, that Phil uses/designs.
HPSDR users on topband include Greg ZL3IX and Luke VK3HJ.
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“Unlike conventional superhets the presence of strong in-band and
out-of-band signals is not necessarily a concern as long as these do not
instantaneously add and overload the ADC.
In fact signals below this level actually improve the overall performance
by adding "dither" to the ADC input.
However, as pointed out by others, once ADC overload has occurred then it
can't be undone with post processing.
The presence of very high signals in the MW broadcast band can be an issue
for some users. In general a HPF will provide enough attenuation to
prevent these signals overloading the ADC.
However, there are still a few issues. When developing the Alex filters
for HPSDR I found that at signal levels of > 0dBm even 2" diameter toroids
would produce measurable IMD products. In which case air wound inductors
would be necessary. The size of these would prevent them being included
in the same enclose as the rest of the radio. Placing these in a separate
enclosure also prevents clocks etc from the radio being picked up.
The issue of MW broadcast overload may not be such an issue when operating
on the HF bands. This depends on the performance of the HF antenna well
away from its intended operating frequency. In my case, I live in
line-of-site from a MW broadcast station. Unless I use a wide band active
loop antenna, or operate on 160m, then I don't require any filtering
before the attenuator/preamp/ADC.
The signals adjacent to the 40m band in Europe have incredible signal
strengths. Depending on the band noise floor an attenuator many be all
that is needed to prevent these signals from causing ADC overload and
still provide an acceptable S/N for the wanted signals. If not, then a high
Q preselector is required.
My experience with supporting hundreds of HPSDR users is that there is no
universal solution. Each users faces a different set of circumstances.
It would appear that the natural frequency response of the *majority* of
users antennas (plus ATU if used) allows operation on the HF Bands without
the need to use an attenuator or other filters.
Designing a front end that will be overload proof in all circumstances
would be not be cost effective. In which case providing flexible support
for the selection of external filters is the best approach and the one
that HPSDR and Apache have taken.
All of the software that supports HPSDR and Apache products provides an
indication of ADC overload. The KISS Konsole and cuSDR PC software also
displays the entire spectrum from 0-55 MHz so the user can see exactly
what frequency band(s) is responsible for any overload.
73 Phil...VK6APH
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Enjoyed the recent discussion about ADC overload with DDC/DUC SDRs, in
particular the number of contributors and the increasing degree of knowledge
about ADC overload that now exists.
Vy 73
Steve, VK6VZ
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