[hpsdr] MDS and relatet architectures

[k3bu at optonline.net]

To add to the picture of super front end:
1. In case if there is a problem controlling the gain of RF preamp, the step and programmable attenuator should be used on the front as outlined by N1UL. 
2. Use the bandpass filters. They could be shared with TX in the transceiver arrangement, switchable/selectable when needed.
This is important in Multi-Multi station setup, where strong local signals (kW on the next table/antenna) from other bands are source of overload.
3. Use of tunable band filters on the front and behind of RF preamp. Hi-Q circuits or variation of Q multiplier/notch. Selectable if needed. 
They are needed especially on low bands 40/80/160, when very strong signals within the band are present and one is digging for weak ones in close proximity. 
4. RF preamp switchable in/out. In case signal levels are sufficient (high gain antennas) switch it out to improve IP3. Have items 1, 2, 3, available as needed.
5. AGC continuously adjustable - time constant (attack, release) and gain (DSP derived?). 
6. High level mixer with clean injection signal followed by selectable Xtal filters - steep skirted and/or low ringing banks. Option to take the wide band or filtered bandwidth for bandscope/waterfall or filtered signals.
7. Wonderful world of I-Q processing and controlling the above.  
Unless we have super overload mixer that can tolerate and eliminate the above, the preamp and filtering use if needed is highly desirable and should beat anything out there. Right now we have to use variations of outboard gadgets. Incorporating them into the RX front end design and be able to control switching and settings as needed by various antennas and situations would make the ride ultimate and not available presently in any of the rigs AFAIK.
Yuri Blanarovich, K3BU, VE3BMV


----- Original Message ----- 
N1UL writes: 
> 
> In principle, this is correct. There is no useful low noise ,high IP3 and  
> variable gain discrete ciruit know to do this. I would suggest to use a low  
> noise high IP3 GaAs amp, if necessary in pus-pull and  put an GaAs binary  coded 
> step attenuator ahead of this stage with 40 dB range and 0.2 dB minimum  loss 
> in 1 dB steps , connected to the main AGC, being activated after 50dB AGC  of 
> the IF AGC. This does NOT eliminate an high IP3 first mixer , second preamp  
> and second mixere. A similar system was implemented in the R&S ESH3   (10Khz to 
> 30 MHz )and ESVP (20 MHz to 1300MHz )  test and measurement
> receivers, using 
> a relay based attenuator. The ESH3 did not have a preamp, the  
> ESVP did have 
> one. 
> 
> The R&S XK2100L I am using has an optional preamplifier, which so 
> far i  have 
> only needed above 21 MHz.... 
> 
> 
> 73 de Ulrich, N1UL 
> 
> In a message dated 8/13/2006 9:02:12 P.M. Eastern Daylight Time,  
> k3bu at optonline.net writes: 
> 
> *****  High Performance Software Defined Radio Discussion List ***** 
> 
> Amen to  that! 
> 
> Another important requirement for superior RF preamp is  
> continuous gain 
> control and AGC control with possible programmable levels for  
> various antenna 
> selections. 
> 
> Continuous gain control is very important  for top notch RX 
> performance, the 
> gain of the preamp should be set just at the  treshold of band 
> noise, so you 
> know that there is enough gain to hear the weak  one, but not more 
> than needed. 
> 
> 
> 
> 
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