Re: Transmission lines, Input match and Connectors #bestpractice #experiment

David Eckhardt
 

For the results of the tests presented:  The tests and data have nothing to do with input filtering or very little with input match.  Everything within each data set is within the same band.   Of course, any input filtering will pass all signals close to the target signal. 

Another point seen especially in data sets Airspy7_0dB.jpg and to a lesser extent in data set Airspy 9_0dB.jpg:  At issue may be digital saturation.  In an SDR when all available bits are utilized as with possible reception of a strong signal, the output becomes garbage and unpredictable.  This is highly evident in the Airspy7_0dB.jpg.  Attenuation between the antenna feedline and the input to the receiver, in this case, is mandatory.  I wrote an article some 4 or five years ago in the Journal of the Society of Amateur Radio Astronomers (SARA) addressing this issue for reception around 20 MHz (when solar conditions were pretty much the best of Cycle 24).

The issue of 'bogus' signals in the FM broadcast band:  This is common in nearly all receivers be they the older technology or the newer digital receivers like the Airspy and others.  The problem is that within the 88 to 108 MHz FM broadcast band, there are many truly strong signals, especially true in or near large cities.  This is required to minimize the noise and maximize the level of quieting in detection of a wideband signal.  The input stage(s) (front end) acts as a mixer as its driven into unintended non-linearities by all the strong stations within that 20 MHz of spectrum.  The strong stations mix with each other to produce bogus or false signals.  Again, too much signal requires either a stopband filter (if you're not interested in receiving FM), or insertion of an appropriate amount of attenuation between the antenna feedline and the input of the receiver.  Any input passband filter present for the FM band, specifically, will aim at passing the whole band on to the mixer or ADC.  Again, an attenuator is required due to too much in-band strong signal levels.

The suggestion has been made to understand how receivers are tested and the basics of their internal design.  The information is on the Internet, so you don't have to visit a library.  For proper evaluation of the newer SDR technology, to properly design tests and evaluations of the item, a knowledge far deeper than just the basics of the design will be required. 

In conclusion, your measurements and data do not address input filtering and/or input match.  To properly conduct the tests you desire, you must first understand what you are evaluating.  

Dave - WØLEV  
 

Virus-free. www.avast.com


On Fri, Oct 18, 2019 at 5:07 AM jdow <jdow@...> wrote:
Please look at "https://en.wikipedia.org/wiki/Radio_noise". It is a simple
discussion. Then explain to me why you insist on NOT using an attenuator at MW
as a matter of course? If you had a SMALL antenna like one of those telescoping
whips the low noise figure of the HF+ would be really good for you. But with an
antenna such as you describe in an environment with a "typical" level of man
made noise a 30 dB attenuator would do you no ill and probably a 40 dB
attenuator would do you no ill. Look at Wikipiddle er Wikipedia for a perhaps
over-simplified discussion of noise figure to see how it works.

Now, knowing the input impedance of the HF+ input may help you optimize a filter
to reduce signals in the MW broadcast band when you are working elsewhere. But
that would probably be for one tuning setting. The input impedance may vary with
the frequency to which HF+ is tuned.

My purpose in trying to drive this home is to help you optimize your time and
efforts to best effect. I am trying to boost your game not deride you. Too many
years in a very male oriented occupation has left me a tad abrasive. Please forgive.

{o.o}

On 20191017 10:23:46, Wes Stewart via Groups.Io wrote:
> On Thu, Oct 17, 2019 at 07:45 AM, prog wrote:
>
>     Let's be more pragmatic. What is the real world performance using an antenna?
>
> Thank you for asking.  Let me preface the following by explaining the
> environment.  The antenna is a 55 foot vertical over ground radials,  For the
> results that follow, the antenna is connected to the Airspy through a Telonic
> Industries Model TG-950 stepped attenuator.  Although not a traceable device I
> have verified its performance with my DG8SAQ VNWA. I have several plots, all
> taken during the same session, which are screen grabs of the SDR# program.  The
> model, serial number, firmware and software versions are all obvious.  This unit
> is as received a few days ago.
>
> I'm not sure I can intersperse comments between the attachments so I will
> summarize them here.  The image "Airspy_1_40db" is of a local MW station with
> the inline attenuator set to 40dB.  The image  "Airspy_2_20db" is the same as
> before except the attenuation has been reduced to 20 dB.  It was pointless to go
> to 0 dB.  Images "Airspy_3_40dB" and "Airspy_4_20db" are similar but of a
> different station.
>
> The next set, "Airspy_5_20db", "Airspy_6_10db" and "Airspy_7_0db" show station
> WWV being received on 10 MHz with 20, 10 and 0 dB attenuation respectively. 
> Note that with 0 dB attenuation, the Airspy is essentially useless.  The next
> set,  "Airspy_8_10db" and "Airspy_9_0db" show station WWV being received on 15
> MHz with 10 and 0 dB attenuation respectively.  Same situation.  Now you might
> understand my interest in knowing the performance of the input filtering because
> obviously it isn't up to the task.
>
> But it gets worse.  Image "Airspy_10_Spurious_of_88_1_0db" shows an FM broadcast
> signal, fully readable so I could identify it, that doesn't exist.  Image
> "Airspy_11_Spurious_of_88_1_20db" shows the same spectrum with 20 dB of
> attenuation inline. The non-existent station disappears.
>
>
> AIrspy_1_40db.jpg
>
>
> AIrspy_2_20db.jpg
>
>
> Airspy_11_Spurious_of_88_1_20db.jpg
>
>
> Airspy_3_40db.jpg
>
>
> Airspy_4_20db.jpg
>
>
> Airspy_5_20db.jpg
>
>
> Airspy_6_10db.jpg
>
>
> Airspy_7_0db.jpg
>
>
> Airspy_8_10db.jpg
>
>
> Airspy_9_0db.jpg
>
>
> Airspy_10_Spurious_of_88_1_0db.jpg
>





--
Dave - WØLEV
Just Let Darwin Work
Just Think

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