[hpsdr] Norton Amplifiers and Noise Performance

[KA2WEU at aol.com]

Phillips sells a + 50 dBm , low noise push pull module with extremely low  
S12.
Take a look at their web site.
 
73 de Ulrich N1UL 
 
 
 
In a message dated 26.07.2006 20:25:18 Westeuropäische Normalzeit,  
ray.anderson at xilinx.com writes:

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>>     Can anyone comment on what  attributes the Makhinson (N6NWP) circuit 
>> (which is essentially a pair of Norton amps  in push-pull) brings to the 
>> table that would make it preferable over the  Norton amp in this 
>> application. 
>The main advantage of a push-pull amplifier is  that you'll get vastly 
improved  
>second-order inter-mod. performance. The  third-orders will improve by about 
 
>3dB as well. 
>I have to be a little cautious about what I due to  professional 
confidences.  
>It's certainly possible to get around +50dBm IPI3  from a push-pull Norton  
>amplifier, and the BFG591 which has been  mentioned, is capable of 
approaching  

>this in the right circuit. There are better  devices... One of the problems  
>with the 'traditional' Norton amplifier is that  the load line for the 
active  
>devices doesn't coincide with the conditions for a  50ohm match at the sort 
of  
>currents and voltages necessary for the highest IM  performance. There's 
also  
>the problem that the transistor bias current flows  through the feedback  
>transformer, which, of course, causes  magnetisation of the core, and limits 
 
>the dynamic range. That usually means that the  ferrites specified in a  
>'traditional' Norton amp. have to be larger than  would be necessary from 
just  
>a signal handling viewpoint.   
>There are ways around both of these problems, but  I can't discuss them  
>further... 
>Incidentally, measuring +50dBm plus IPI3 is not a  trivial matter... 
>The noise figure required of a receiver is  ultimately limited by the noise  
>which comes down the antenna cable. At HF and VHF  this is a well known  
>quantity, and there are graphs in most of the  standard text books from 
which  
>it can be calculated. If you work on the basis  that the receiver system 
will  
>not degrade the available S/N by (say) 1dB, then  receiver noise figures of  
>10dB at 10m and 6dB on 6m are more than adequate.  I know that there's a  
>market for (allegedly) 0.3dB NF pre-amps for 6m,  but any system performance 
 
>improvement they bring is more likely to be  psychological than real... 
>Vy 73 
>Chris 
>GW4DGU 
--  
Chris- 
Thanks for your insightful comments  regarding Norton amps and noise 
performance. I fully understand your inability  to get into specifics due to 
proprietary  considerations. 
It is looking like we may be able to  obtain very good performance on the HF 
bands without the need for a preamp,  however on 6 meters some noise figure 
improvement may be beneficial to the  system performance. I may look into the 
feasibility of a switchable preamp so  that the circuit may be used when 
necessary, but may removed when  not. 
73    Ray    WB6TPU 



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