[time-nuts] Advanced 5 to 10 MHz doubler

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Feb 15 19:31:21 UTC 2011


Paramithiotti, Luciano Paolo S wrote:
>    http://www.timeok.it/files/5_to10_mhz_advanced_doubler.pdf
>
>    
>> This design appears to have gone somewhat astray.
>> high impedance unless of course the transistors enter saturation in which case the phase noise performance will be severely degraded.
>> The best place for a balance adjustment circuit is actually in the emitter circuit.
>>      
> *The collector balancing work correctly and is more simple to implement.
>
>    
I contend that the collector balancing technique you use only works 
because the doubler isn't operating correctly.
With a high impedance collector output it would be relatively 
ineffective unless the balancing resistance is increased to a level that 
degrades the phase noise performance or saturation occurs.
>> The description of the biasing is misleading in that the actual bias level that sets the crossover current is determined by the signal dependent voltage>across the two 0.1uF capacitors in the emitter.
>> With a 1:1 input transformer the quoted figure of 35 ohms for the input impedance seems excessive for large signal operation of the CB stages unless of>course they saturate.
>>      
> *the input impedance is 35 Ohms @ 0dBm as measured with network vector analyzer. It can be upgraded to 50 ohms adding resistance on emitters, with some gain reduction and probably less phase noise. I will do some modification in the next future, including an input 6 Mhz low pass filter. As you know, the input signal have to be pure sinewave to avoid unsymmetrical positive and negative half wave and obvious unbalaced output and high harmonics contens. I will test also the common emitter configuration to better isolate the doubler from the input impedance and level variations. Regarding the input level I have setup it's range, as my personal standard,from +7 to +13 dBm.
>
>    
I thought as much, the large signal input impedance (this is far more 
important than the small signal value) will be much lower.
Since the bias shifts with input signal level the small signal input 
impedance that you measured is of little value.

>> It would also appear that the 20MHz tank 5.6uH + 12pF as drawn is inappropriate in that it inevitably leads to saturated operation.
>> A series resonant 20MHz tank from the collector node to ground would be a better choice.
>>      
> * The LC on collector is to adapt the impedance between the doubler and the filter and to cut the higher harmonics. The filter itself contain trap for 15 20 and 30 Mhz.
>
>    
Maybe so, but the filter input topology adopted is inappropriate for low 
phase noise and avoiding saturation.
Attempting to match the (poorly predictable and varying - with 
temperature and input signal level) collector output impedance to the 
filter input impedance is misguided, just treat the output as a high 
impedance source. The 4:1 (impedance ratio) output transformer should 
suffice, if necessary you can add a 200 ohm resistor in shunt from the 
collector node to Vcc if you need a 50 ohm output impedance. In practice 
it may be better to buffer the output with a series transformer feedback 
stage with well defined output impedance. Series resonant LC traps from 
the doubler collector node to ground should be more effective than 
parallel resonant series traps in that the high frequency component 
amplitudes at the doubler collector will be significantly reduced rather 
than enhanced by the filter.

>> A snapshot or even a sketch of the collector voltage waveforms would be useful in showing that the transistors saturate or not.
>>      
> *Actually the prototype is gone to friend's home and I cannot do any more measure on it. My next prototype's pubblication will be complete of collector voltage waveform to better understand the working condition of the doubler stage. I think the 2N3904 is not the best solution, i will test some more devices and bias point.
>
>    
At 10MHz you will find that most wideband transistors will be noisier.
However using transistors with a lower base spreading resistance than 
the 2N3904 may be useful.
> Thank you
> Luciano
>
> note: I'm not a genius, I just try to enjoy myself. If someone follow me, is at his own risk.
>
>
>
> Luciano P. S. Paramithiotti
>
>
>    
Bruce




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