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Author Topic: Do solid state amps need be push-pull?  (Read 8220 times)
N3QE
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Posts: 2190




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« Reply #15 on: November 29, 2012, 03:41:37 PM »

OOK CW is keyed on and off. The RF waveform from the exciter has rise/fall times that are intentionally about 5 ms. It is amplitude modulated. If you view this in the frequency domain there is a carrier, an USB, and a LSB.

A non-linear amplifier will decrease this intentional rise/fall time and in the frequency domain will widen the signal. How far one runs an amplifier into Class-C will depend on how wide a CW signal can be tolerated.  

It's a mistake thinking that shaping must be done only at low levels. Look in any 30's/40's/50's ARRL handbook; shaped keying done at the last stage was state of the art, through either cathode or grid-block keying, and some designs with a clamp tube do it at the screen grid. In fact there's a whole chapter for a couple decades just on CW transmitter keying, and preventing click and chirp.

All modern "clean" QRP transmitters do the same by shaping the collector voltage at the output amp. There's a near-universal circuit (I first saw it in W7EL's "Optimized QRP" design although I've been told it predates Roy) that uses RC networks and a PNP transistor to do this very nicely. Again, very intentional shaping done at the PA.
« Last Edit: November 29, 2012, 03:50:41 PM by N3QE » Logged
WX7G
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Posts: 6035




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« Reply #16 on: November 29, 2012, 05:16:43 PM »

The seemingly high efficiency of shaped class-C amps isn't so high in actual use. In the 1990s I built a 7 MHz, 25 watt class-C final amp having an efficiency of 95%. With the power consumed by the driving stage the efficiency dropped to 90%. To follow the ~5 ms rise/fall time of the exciter the RF output was rectified and fed back to the driver stage. When keyed at 60 wpm about 30% of the ON time was spent in the linear region (the ~5 ms rise/fall times) and if I remember correctly the real efficiency was around 75%.  

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K1DA
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« Reply #17 on: December 05, 2012, 10:00:42 AM »

Isn't "CW bandwidth" a problem with REAL low frequency systems of the type the Navy used. 
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K8OT
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Posts: 22




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« Reply #18 on: December 21, 2012, 03:48:19 AM »

hard keying has fast rise and fall times generating a square wave. A square wave is by definition an infinite number of odd harmonics.
Go figure.
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