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Simple DDS VFO for class E rigs




 
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Author Topic: Simple DDS VFO for class E rigs  (Read 1436 times)
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ka1tdq
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« on: January 13, 2017, 06:42:04 PM »

My 75 meter class E transmitter uses a DDS VFO connected to the IXDD drivers.  It works well, but it's a 2-step operation to switch between transmit and receive.  I need to turn on the VFO before I key the sequencer, and turn off the VFO after I key the sequencer.  I can't wire in the VFO to the sequencer since the VFO turns on/off too slowly.  If I don't turn the VFO off during receive, it wipes out the receiver.

So here's my plan...

I just bought two kits from WA1FFL.  They're the DDS VFO buffer kits used to drive the crystal socket of tube rigs.  You can inject about a .5V peak signal in and get 12-ish volts peak out.  The 12 volts peak would be perfect for driving the IXDD busses.  I would just need to vary the .5V input signal to adjust the duty cycle.  The good thing about this is that I could leave the VFO on all the time (the .5V signal probably wouldn't be heard on the receiver), and I can power the WA1FFL kits via the sequencer.  Then, when I use my 75 meter transmitter, it'll just be a simple one switch operation to go from TX/RX.

If this works, it'd be a simpler way to get into class E.  A person would just have to build the RF deck and buy the driver VFO and IXDD driver kit.

Jon


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KD6VXI
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« Reply #1 on: January 13, 2017, 07:17:32 PM »

Why not get a vfo with a receive offset on it?   Then move the carrier freq away a couple mhz on key up.

Bonus:  Ever get into building receivers you can use said vfo.

--Shane
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ka1tdq
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« Reply #2 on: January 13, 2017, 07:24:12 PM »

That would work too, except all the VFO's from what I've seen with that option have a very low output.  Not enough to drive an IXDD buss.  Right now I'm putting out a 6.20 volt signal for each phase.  That's significant enough to have an impact on my receiver.  My VFO is a Chinese model (high output and no offset).  

With this route, I can probably knock the output down to .3 volts, and that signal will itself be in a shielded box.  It should have little or no impact on the receiver.  

Jon
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W6TOM
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« Reply #3 on: January 13, 2017, 07:43:15 PM »

  I bought one of those WA1FFL VFO kits from him at Nearfest, I have yet to build the kit but as I recall he told me they could be programed so you had a RX offset.
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VE3ELQ
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« Reply #4 on: January 14, 2017, 09:20:27 AM »

N3ZI sells a DDS VFO kit that provides a programmable frequency offset when in receive. It has about 200mv output so requires an amplifier buffer co phase driver board for use with IXDD614 drivers.  If interested I can provide the schematic and PCB pattern for this board which fits on the back of the VFO PCB, but you will have to build it from scratch. This is what I am using.
73s  Nigel
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ka1tdq
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« Reply #5 on: January 14, 2017, 10:53:54 AM »

I would go that route, but I'm getting lazy as I grow older. Building from scratch is getting to be a bother, unless I have to. This kit that WA1FFL puts out is really well packaged. He separates the parts into little baggies, like Heathkit used to do. They're all labeled and there's no thinking involved.

Off topic, but I just picked these up this morning at today's hamfest. Class E is a disease.  Smiley

Jon


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« Reply #6 on: January 14, 2017, 11:48:50 PM »

You see class e.

I see legal limit tube linear.

Same same.

Lol


Actually,  I see a class e driving a California kilowatt.

--Shane
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ka1tdq
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« Reply #7 on: January 18, 2017, 09:37:11 PM »

I'm impatiently waiting for the second module to come in so that I can do break in operation.  I tried varying the input from .15 volts up to .6 volts and the output waveform stays linear and varies accordingly with amplitude.  I think it'll work great with the transmitter. 

Jon


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ka1tdq
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« Reply #8 on: January 20, 2017, 07:24:06 AM »

I have the second module built.  Now I'm just waiting on my pre-made short BNC jumpers to come in from China. They're so much better at building that stuff with their little hands.  

If this works, I'll build another one for the 24 FET rig.

Jon


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« Reply #9 on: January 20, 2017, 07:47:24 AM »

Jon,

   I wonder about those VFO amplifier boards. One constraint those boards were never having to meet was phase stability. Isn't that important when driving a class D/E rig?

Jim
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ka1tdq
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« Reply #10 on: January 21, 2017, 11:08:59 PM »

I'm still waiting on my BNC jumpers for a full test, but I was able to cobble together a test for one phase in the meantime.  Here's the resulting waveform as measured at the gate. 

I like it!  And, I couldn't hear the VFO during receive. 

Jon


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ka1tdq
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« Reply #11 on: January 25, 2017, 08:00:33 PM »

It works!  I now have full break in. 

Jon


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