[Hallicrafters] Revolutionary Signal Generator

[Troglodite at aol.com]

To all interested,  

There are two problems with the simple DDS approach which are difficult to  
solve. If it were easy, we would see inexpensive digital RF generators in  
profusion.
 
The first problem is that the signal as it comes from the chip is riddled  
with "spurs." This is an easy to fix for a narrow tuning range, but becomes more 
 difficult as the range increases.
 
The second problem is that if you expect to use this for receiver alignment  
and testing, you really need a calibrated output attenuator. You need to be 
able  to get down to the microvolt range. To do this, you have to put the noisy 
parts  in a can, and use feedthrough capacitors for all the control lines. 
This means  the processor too. The only RF exit should be the output. The 
attenuator should  be mounted in a shielded box as well. Great care needs to be taken 
to prevent RF  leakage.
 
You can live with minor impurities in the waveform - at least for alignment  
purposes. Ever look at the output waveform on the inexpensive commercial RF  
generators? Some of them deliberately distorted the waveform so that it's  
harmonics could also be used. But if you want to measure receiver sensitivity,  
you need that calibrated output, and to get it you will need good shielding.  
Without it, you will have an instrument no better than the inexpensive ones,  
except that it is more frequency stable and accurate.
 
Regards,
 
Doug Moore