[hpsdr] QSD architecture and image suppression

[Phil Harman]

Hi Oleg,

I'd like to add my warm welcome to group as well. I've also followed 
your work for some time and it would be great to have you participate 
in the various HPSDR projects.

Bob, N4HY, has replied to your question regarding image suppression 
from a theoretical perspective so I would like to address what is 
presently being achieved in practice.

My own measurements on QSD based receivers indicates that it is 
possible to achieve >90dB of suppression over a narrow bandwidth if a 
manual or automatic adjustment is made at a single frequency. If such 
adjustment is made as say the edge of a ham band then the suppression 
drops to typically 40-45dB at the band edges.

Software that enables correction to be made across a band, such as 
Rocky, does much better than this. However, since this software is 
aimed at the SoftRock range of radios, that typically do not have any 
isolation between the QSD and antenna, then changes in antenna SWR etc 
can significantly degrade the suppression.  Also whilst this technique 
is effective over a single band I'm not sure how effective it would be 
for a multi-band radio or one that covered the entire HF range.

There was some work started some time ago to model the impulse response 
of the receiver by injecting a series of very narrow pulses into the 
antenna socket.

Given the impulse response I gather that it should be possible to 
accurately compensate for I and Q amplitude and phase errors.

I'm not sure what the current progress of this idea is and perhaps 
others can comment as to how successful this is/was.

 From my own perspective, since there does not so far seem to be an 
effective solution, I've been working on DDC and DUC radios since image 
suppression is usually not an issue with such technologies.

73's Phil...VK6APH




Quoting Oleg Skydan <oleg at skydan.in.ua>:

> ***** High Performance Software Defined Radio Discussion List *****
>
>
Hi, All!

I have seen a lot of different projects/radios based on QSD.
I understand that most parameters can be at a competitive level with 
traditional designs, but it is still not clear for me
how well the image suppressed?

I understand that you can get a very good image suppression (80-100 dB 
is real) for the limited time with stable temperature and etc. But I 
wonder how well the image is suppressed with all this parameters from 
the real life (at least with time/temperature influence)?

All the best!
Oleg
73 de UR3IQO



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