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


David Eckhardt
 

We are operating n a high RF environment with a 'tower from hell' cell installation.  Ask why there?  We desire the installation including the optical side (18 and 24" optics) available to  the students at the HS and the community of Berthoud, Co. and beyond.   At the time we were trying to answer where and if in the receiver system, gain compression was occurring.   

My own installation is in a valley and is relatively RF quiet (except for my own computers and laptops and new appliances......ggggrrrrr).  My strongest FM brtadcast station measures roughly -40 dBm on an FM antenna in the radio room window and I still need an FM stopband filter on any of the SDRs. 

Dave - WØLEV


On Fri, Oct 18, 2019 at 10:03 PM Marcus D. Leech <patchvonbraun@...> wrote:
On 10/18/2019 03:55 PM, David Eckhardt wrote:
We have fed the antenna port with broadband noise well below the 12-bit limit and well over the 12-bit limit for saturation.  Even with the low input we see raw I/Q values in excess of what should be spit out with a 12-bit system.  With the well into saturation, we see many more raw I/Q values well in excess of saturation for a 12-bit system.  These are raw I/Q values grabbed on the USB output.  We discovered this some 1.5 years ago when we were attempting to put the 'nominal' output from our H1 telescope no more than 3 to 5 bits above the bottom of the bit stack.  We asked more questions that we were able to answer, as a result, and are still scratching our heads as to how this can occur.  We were seeing values upwards of roughly 2X that of the +/- 33k values (for a 16-bit system).  If its a true 12-bit system without attempting to internally adjust the position of the signal within the bit stack in the airspy FW, how can this occur?   

Dave - WØLEV
I use a pair of AirSpy R2 for our hydrogen spectrometer, and I've never seen any hint of unexpectedly-large values coming out of it, although
  I don't use anything close to a "raw" interface, but use gr-osmosdr and Gnu Radio.

Do your results show a gain dependency?



On Fri, Oct 18, 2019 at 7:23 PM prog <info@...> wrote:
On Fri, Oct 18, 2019 at 09:14 PM, David Eckhardt wrote:
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 output is scaled to fill a 0 dBFS unit. It can be either float (-1 .. +1) or a 16bit signed integer (-32768 .. +32767). You also have some DSP running in the library to convert the RAW samples into IQ.
Speaking of the RAW samples, the 12bit data is stuffed in the LSB bits of the unsigned 16bit samples and no further processing is done. You can read the code of the library for the details.

Not sure what you are doing, but I recommend you check your math. You can't get more resolution than what the device is giving.


--
Dave - WØLEV
Just Let Darwin Work
Just Think



--
Dave - WØLEV
Just Let Darwin Work
Just Think

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