Re: Pushing the QCX (T1 question) #qcx

Hans Summers
 

Hi John, Alan
 
Hans makes his RX kits quite narrow compared with other SDRs that use
similar circuitry.

;-)   I think a narrow BPF improves the performance. Even with a strong mixer such as in the QCX (high dynamic range and IP3), it is good to keep strong out of band signals away from the mixer. Having a wide BPF which covers several bands makes the radio cheaper and less complex (fewer filters) but it is a compromise, in my opinion. 
 
As for optimising L4 and C30 for two bands I guess reasonable results
could be achieved.

The Class-E resonance is quite broad. There should not be any major problem, operating the Class E PA stage on two adjacent bands such as 20m and 30m. 
 
In view of a few reports of catastrophic failure due to PA breakdown I
would suggest isolating IC3 pin 3 with a capacitor. Use a resistor to
keep the gates low when not driven.

It won't work... if you capacitatively couple IC3 pin 3 to the PA gate, then the peak-peak squarewave will be 5V BUT, it will be centered on 0V. The resulting peaks at the BS170 gate will be only 2.5V above ground and will not drive the BS170 into full conduction. The power output would be greatly diminished. One of the key criteria for obtaining Class-E operation is to drive the PA device hard off/on as a switch. 

73 Hans G0UPL

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