The AM Forum

I'm still pondering ideas for another project.  I'm leaning toward making a crystal oscillator out of an octal buffer chip and using that to drive an IXDD gate driver.  From there it's just the output RF device.

My question is about the developed square wave on the gate of the FET.  Could that be used for CW or would key clicks be a problem?  If they are a problem then modulating the carrier would be the next option.

I have a crystal for 3870 and 14060. 

Jon
KA1TDQ

I have no idea what you are talking about, but CW on 3870 is always fun.

Hi Jon,

I built the same simple line-up for a Class E transmitter to test single FETs for evaluation of operation at 40 meters a number of years ago; just a digital crystal oscillator i.c. driving a digital gate driver i.c. driving a single FET gate.

For a Class E transmitters you don't want to cut off the drive.  What happens then is the drain voltage swings quite high and then will probably exceed the voltage rating of the FET and cause the FET to fail. For Class E PA's, the drain modulation is the way to go for CW.

Class C solid state - cutting the drive is o.k.

That's more good info to go on.  Thanks!

I found this regulated power supply at the last hamfest for $5.  It's old so I'm going to rip the guts out and use it for my next project.  It already has a large 72vct transformer, meters and huge heatsink that I will reuse.  

I've found myself using 40 CW recently (using the rice box) as well as 40 AM (homebrew rig), so I might just make a class C 40 meter CW transmitter.  This way I won't be on the air using commercial equipment.  Here's what I have for numbers using 36 vdc on the drain:

145 watts output / 36 vdc = 4.03 amps

4.03 amps / 70% efficiency = 5.75 amps

(.5)(36 volts) / 5.75 amps = 3.13 ohms for the FET output

Using a 4:1 turns ratio on a FB-43-1020 gets me to 50 ohms output.

The FQA11N90 would have to dissipate 62 watts intermittently for CW (well within the 300 watts on the datasheet).

Jon
KA1TDQ

I just finished the oscillator this morning.  All 8 buffer inputs and outputs are paralleled.  The output doesn't have a DC blocking capacitor in the output, so the 5 volt swings will go directly to the input of the IXDD614. 

I don't have a scope, but I threw an antenna on it and listened on a receiver.  It works, and it does key fine.  I could use it for CW or AM. 

Jon
KA1TDQ

Jon

Don't forget that you will need some kind of harmonic filtering at the output of this transmitter.

See the attached JPEG

Some harmonic filtering can be provided by the total drain-to-source capacitance (intrinsic to the FETs, plus any added capacitance). The rest of the harmonic filtering will usually be provided after the step up transformer.

The FET's want to look into a load of around 3.13 ohms (as you calculated) at the fundamental frequency, but they want to look into substantially less than 3.13 ohms of load impedance at harmonics of the fundamental frequency. Otherwise, the Class C stream of pulses of drain current will produce a stream of (downward) pulses of voltage... from drain-to-source... that contains harmonics of the fundamental frequency that are almost as large as the fundamental... and you will have excessive downward swings of the drain voltage, that will reduce the achievable output power at the fundamental frequency.

To avoid radiating signals, that are less than 43dB down from the strength of the fundamental frequency signal, at harmonics of the fundamental frequency, you typically need some type of low pass or band pass filter between the output of the step up transformer and the antenna.

§97.307 Emission standards

(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission.

Stu