[Milsurplus] [Boatanchors] Calling Smart People: Class D Transmitter

[Peter Gottlieb]

As mentioned, you need more than just some logic output to drive a FET at higher 
frequencies.  This is because the gate is relatively high capacitance and you 
will need significant drive capability to get anything like sharp transitions at 
high frequencies.  If you under drive it, you will burn it up and have terrible 
efficiency.

Peter



On 2/1/2015 10:49 PM, hwhall at compuserve.com wrote:
> You will need a more complex arrangement
> to divide an 8 MHz oscillator output to get 7 to 7.3 MHz, perhaps even
> choose a different chip that works at 30 MHz, and divide by 4. The RF
> amplifier is being fed essentially square-wave pulses.
> Seems like you could use nearly any sort of oscillator if you passed the output through some sort of solid state switch IC to square up the output.
>
> Wayne
> WB4OGM
>
>
> -----Original Message-----
> From: Brian <brianclarke01 at optusnet.com.au>
> To: David Stinson <arc5 at ix.netcom.com>; milsurplus <milsurplus at mailman.qth.net>; boatanchors <boatanchors at mailman.qth.net>
> Sent: Sat, Jan 31, 2015 6:41 pm
> Subject: Re: [Boatanchors] Calling Smart People: Class D Transmitter
>
>
> Hello Dave,
>
> The chip is providing divide by 16. You will need a more complex arrangement
> to divide an 8 MHz oscillator output to get 7 to 7.3 MHz, perhaps even
> choose a different chip that works at 30 MHz, and divide by 4. The RF
> amplifier is being fed essentially square-wave pulses. So, it relies on high
> Q in the PA to attenuate the predictable output from square waves - all the
> odd harmonics. I have no idea whether the active devices there will work at
> 7 MHz; you'ld need to check that.
>
> 73 de Brian, VK2GCE.
>
> On Sunday, February 01, 2015 9:49 AM , you said:
>
> Yes, this is boatanchor related.  I promise.
>
> Hello Smarter-Than-Me-People:
> Take a look at this Class D 630 Meter transmitter:
>
> http://njdtechnologies.net/wp-content/uploads/2014/08/Schematic-for-25-watt.jpg
>
> If I scale all the parts at RF in the Drain circuit
> by matching reactances (assuming IR factors are not
> material), can I make this transmitter work on 40 Meters?
>
> Thanks,
> Dave S.
>
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