[hpsdr] QSD architecture and image suppression

[Oleg Skydan]

Hi, Bob, Anti and Phil!

Thank you for your comments. And special thanks to
Phil for the numbers - that was exactly what I needed.

I was interested in those numbers as I have some plans to
start a new transceiver. So I had three alternatives:
1. Use traditional superheterodyne architecture with 
DSP at low IF (just as my current design).
2. Use QSD/QSE architecture for the frontend.
3. Use DDC/DUC architecture.

I have several goals for the new design. Particulary
the image rejection should be more then 90-100dB. 
It can be easilly achived with 1st and 3td architecture, but 
I can not use high speed ADC with more then 12 bits
(export regulations), so DDC architecture can not be 
used. 

It looks like I will use the traditional superheterodyne 
with low IF DSP. 

You can find my site here http://t03dsp.skydan.in.ua
The information is a bit outdated - there were many changes
since the last site update. The new DSP block with faster 
and better ADC (CS5381) and DAC (CS4392) and new 
processor (DSP56367). The FST3125 based first mixer. 
The new control block and front panel (you can see the 
picture of it here http://skydan.in.ua/downloads/NewTrcrv.JPG).

All the best!
Oleg
73 de UR3IQO

----- Original Message ----- 
From: "Phil Harman" <phil at pharman.org>
To: <hpsdr at lists.hpsdr.org>
Sent: Thursday, February 21, 2008 7:26 AM
Subject: Re: [hpsdr] QSD architecture and image suppression


> ***** High Performance Software Defined Radio Discussion List *****
> 
> 
> 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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