New Xmtr! Class E-Z (SIMPLE!) 500 watts, bandswitched

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New Xmtr! Class E-Z (SIMPLE!) 500 watts, bandswitched

Postby steve_qix » Wed Jan 18, 2006 10:49 pm

Here is a very simple, foolproof, high power (up to 500 watts carrier output), bandswitched (80/160 - 40 meters coming!) class E transmitter.

This is probably the easiest high power transmitter yet. I was able to construct two of these RF decks in one weekend! I run this
transmitter at 45 volts at 10 amperes. It will actually do 12 amperes, however my modulator will not 8)

There is exactly one tuned circuit in the whole thing - the output tank. Everything else is digital and broad band (100khz - 8 mhz), right
up to and including the gates of the final MOSFETs. The RF amplifier uses inexpensive IXYS MOSFET driver devices connected directly
to the gates of the final RF amplifier MOSFETs. The drivers take a standard TTL input at the operating frequency from 74 series logic,
and this signal is supplied from a simple circuit consisting of a quad NAND gate and an OP-amp. This takes 3 volts or so of RF at the
transmit frequency from a VFO or other RF source.

The design is a single ended push pull, resulting in low harmonic output and easier filtering.

This first picture is an overall view of the transmitter, clip leads and all. Note the RF tank components to the left; a variable
inductor, tuning capacitor and loading capacitor. I will use a switched coil in the actual implementation.


Image

The next picture is a close up of the RF amplifier components, mounted on the heat sink
Image

This thing is SIMPLE. I soldered it together, did a QUICK 'scope check of the stages, and went on the air (I used an already built modulator
and power supply). Works like a charm.

The goal of this project was twofold:

1) to come up with a very reproducible, uncomplicated, multiband RF amplifier - and -
2) to create a virtually indestructable transmitter by incorporating inexpensive protection devices in all sections of the RF amplifier,
and shutdown circuitry where appropriate.

The first goal has been achieved, and I am currently running the "torture tests" (shorting the antenna, disconnecting the antenna, mistuning (very badly), overmodulating (clipping the modulator at 150% positive modulation) while doing all of the aformentioned tests, etc). I will tollerate no device failures.

Assuming all goes as planned, I will be documenting this design - AND - this will be the basis of the QST article. Former designs (mine included),
required too much technical knowledge and "tinkering" to be suitable for a "first time" AM project. Now, I am reasonably confident that
anyone who has a relatively modest technical background, but who can otherwise solder components properly, and follow directions should be
able to build one of these things and get it on the air.

Oh well - another old buzzard transmission.

Regards,

Steve
Visit the class E web site at:

http://www.classeradio.com
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Postby ke1gf » Fri Jan 20, 2006 3:44 am

Cool Steve, as you know I've been working with drivers like what you are attempting for some time now. It will be interesting to see what your final design comes out like.

I'm very excited to hear that you're going to use this design in your QST writeup. You know how much I despise the voodoo magic twisted pair that that we've been fighting with for years now.

If you run into difficulties solving the shunt capacitor switching matrix, I'd be more than happy to send you my equations and calculations for my 160-80-40 RF deck to use as an example. I used MOSFETs to switch my shunt caps so the computations are pretty hairy, I don't think relays require any heavy calculations.

Cheers,
-Bill 'GF
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Postby frank carcia » Fri Jan 20, 2006 9:47 pm

OK now I see what you are doing. You have a driver on each fet. I suspect you will need a good resistive load on all the ttl inputs hanging off the coax. I got into trouble trying to resistor "or" a number of drivers in parallel. They ended up fighting each other. This method is much better and shifts the problem back a stage to a lower level. This would work well for my 160 meter rig if I removed a few fets from each driver. It is way over kill anyway and hard to drive. If I put 4 of these chips per phase and drove them with individual mos drivers I could eliminate my JS predriver.
I like it. BTW that twisted pair transmission line z is a lot higher than you think. I suspect around 150 ohms.

Bill It sure is a cooler way than using relays. I'm still into a final for each band so you never have to retune.
Are you doing the fet between the cap and ground? That makes sense to me since the drain would be clipped from going negative by the final fet diodes. The diode in the switch never conducts so there is never a problem with it. cool idea fc
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Postby ke1gf » Sat Jan 21, 2006 8:52 am

Yep, I'm going to give it a try.
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Postby steve_qix » Sat Jan 21, 2006 9:49 am

ke1gf wrote:Yep, I'm going to give it a try.


It will definitely work at some level. I did this a couple of years ago as an experiment, but got distracted and didn't keep up with the test.

One question remains in this and that is - what is the current flowing through the FET and the capacitor? I have modeled the output network, but I honestly don't remember the results :oops: If the peak current is reasonable with respect to one MOSFET, using a MOSFET for switching is probably an extremely viable option. If one switching FET is required for each output FET, relays might actually be better!

But, the modeling and experiments will bear this out!

Talk later and Regards,

Steve
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http://www.classeradio.com
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Postby frank carcia » Sat Jan 21, 2006 7:26 pm

I think one switch is required for each stage only but this switch (fet) needs to have a low RDS on. The series resistance will reduce the Q of the cap and there will be an I/R loss. The fet only needs to handle the voltage and the state only changes when there is a band change so slow is ok. It might be easier to get short leads with a fet compared to a relay.
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Postby frank carcia » Tue Jan 24, 2006 12:25 pm

A great design until 1 FET fails. Who would like to think up protection that limit a failure to 1 FET and one driver.
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Steve E-Z xmtr

Postby W3CRR » Mon Jan 30, 2006 5:02 pm

Well, Steve, your new design may be exactly what I've been looking for. Heretofore, I've been a boatanchor devotee and, thus, know NOTHING about Class E design or even solid state construction practice. I am, however, very anxious to embrace this new-to-me technology and get back on AM with a worthy successor to my late, lamented Collins 32V-2.

I'll soon be taking delivery of a PCS Electronics AM MAX II DSP 10 watt AM transmitter (http://www.pcs-electronics.com) and want to use it as an exciter for a 300-500 watt 40 meter rig. Allegedly, I can bolster this transmitter's little modulator with additional MOSFETs and tickle a Class E final with it. I certainly hope this is so. Your new circuit might be just the ticket. Please tell me more.
73,
Craig
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Postby W2INR » Wed Feb 01, 2006 2:33 pm

Any time frame on the documentation Steve? I would like on to use this new design in my E Class rig?.
G

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