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Maybe this 74ACT04N, $0.50 plus $10 shipping:
? ??

Ground pin 7 with a very short wire (or just bend the pin down to where you can solder it to a bare copper covered PC board)
Also ground the unused inputs at pins 3,5,9,11,13
Put 5.0 volts on pin 14, and add a 0.1uF cap from that pin to ground, keep the capacitor wires short.
Drive the Si5351 output into pin 1 using a twisted wire pair of that signal plus ground.
Try driving your transmitter with the 5.0 volt square wave coming out of pin 2 using another twisted wire pair.
Might work, especially if it almost works with the 3.3 volts from an Si5351.

The 74ACT family of parts is unusual in that it is CMOS but tries to be compatible with?
old school TTL, so the input threshold voltage is kept around 1.5 volts regardless of the power supply voltage.
That allows us to drive this part directly from the 3.3 volt Si5351, even when this part is powered from 5 volts.
And at 24ma, the output buffers are relatively hefty.
A bit more output could be had by bumping the supply up as high as 5.5 volts, beyond that is not recommended.
If you needed more current available at the output than 24ma (I doubt it) then multiple inverters could be wired in parallel.
A good idea to ground the inputs as they are high impedance and might decide to float into the input transition region,
at which point the part can get hot and possibly oscillate.

If you need more than a 5v square wave, an easy pre-packaged solution might be a comparator that can deal
with perhaps a 12v power supply.? But try this first.

Jerry, KE7ER


On Tue, Dec 17, 2019 at 01:07 PM, Don Richards wrote:

On a related topic, I built a vfo with a nano and a 5351 to replace the vfo in a tube rig but I need more voltage to drive it. It will operate the receiver but is just on the edge for the TX. It will only muster half power at best. I built the two transistor circuit that is commonly found online but it doesn't work that well. Does anyone have anything better? Since it is a square wave, would some flip flop chip work and go rail to rail?

Just thinking out loud

?

73 Don ve3ids