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

David Eckhardt

My article in the SARA Journal was based on the Airspy 1.  A lot has changed in the interim.

I should comment that every time Terry and I try to "discover" what makes the Airspy tick inside, we're frustrated - completely.  It appears it is not a true 12-bit system as we are getting raw I/Q values well in excess of that permitted by a strictly 12-bit system.  The Airspy is utilized in our H1 line telescope at the Little Thompson Observatory where both Terry and I volunteer.  I would be "rather" useful in this application to take the lid off the little box!!!!!

Dave - WØLEV

On Fri, Oct 18, 2019 at 5:53 PM prog <info@...> wrote:
The internal IF AGC of the ADC will not allow any digital saturation to happen. It will silently adjust the level at the input of the ADC and keep the signals below 0 dBFS. The same applies to the RF AGC chain plus or minus the threshold level. This excludes many of the assumptions stated in the previous emails.
What could happen, however, is all these non-linearity problems that could be introduced by bad contacts (mostly in connectors), bad cables, and even rusty metallic structures surrounding the antenna that could mix signals and translate them into unexpected frequencies. Here's an example from the recent memory:
I have seen this happen too many times to ignore it. Many experienced ops overlook these details. Even Leif found some noise humps (similar to those we see in these screenshots which are unaffected by the attenuation level) when testing the original HF+ and he traced them back to a faulty (leaky) attenuator. Once he replaced the attenuator, things went back to normal. At the same time, I am being realistic by not expecting everyone to have Leif's experience. Someone else would have just trusted his apparatus and produced a bogus appreciation.
This is the main reason I do not take anyone's tests on their face value. Understanding the phenomenons involved in the "bad" results and being able to reproduce them is proper engineering. The rest is, well... just noise.

Dave - WØLEV
Just Let Darwin Work
Just Think

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