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DSB power generation

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neazoi

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Here is a quick and dirty power oscillator that can do CW, along with an AM modulator (discussed in another topic).
**broken link removed**
Now I want to experiment with generation of DSB out of this little rig. We are talking here on DSB generation with a high power oscillator.
Before any comments, I am aware this is a bad technique, nevertheless I want to experiment if this is possible, cause I haven't seen that in any books or magazines.

So I am thinking that this can be done simply by a separate bifilar winding on the same core (to act as a phased secondary) and two power diodes capable of operating on HF. Like the design attached.
No potentiometer will be used, so I will try to match the diodes.

Now what kind of diodes should I use? RF out of the oscillator is about 4W on AM and 10W on CW.
Any other ideas/comments having in mind this has to be kept minimal?
 

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I don't think that will work.
You won't find power rated diodes that work at 7MHz and I'm pretty sure you will need to provide equal modulation power to RF power as well.

Brian.
 
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    neazoi

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I don't think that will work.
You won't find power rated diodes that work at 7MHz and I'm pretty sure you will need to provide equal modulation power to RF power as well.

Brian.

How about these schottky diodes out of SMPSUs?
How about connecting several lower power ones (eg 1n4148?) in parallel?
How about these diodes used in multiplier stages?

For modulation I am thinking of using the AM modulator, after all it is an AF amplifier.
 

Even power Schottky diodes are not capable of switching hard on and off at 7MHz, they will work but be inefficient.
You could parallel lots of 1N4148s but you would need a lot and they need to be matched, not easy. Remember that as you parallel them their combined capacitance is also increased.

The modulation can't be applied to the supply in a DSB system, it has to 'unbalance' the mixer and applying it globally through the supply can't do that. I would be very cautious copying the schematic in post #1 too, it has some serious design errors. The 741 audio pre-amp should not be driving a capacitive load as it will almost certainly cause instability, there is no DC return for the mixer stage, no stabilization of the PA bias and no diode across the relay coil.

Brian.
 
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    neazoi

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Even power Schottky diodes are not capable of switching hard on and off at 7MHz, they will work but be inefficient.
You could parallel lots of 1N4148s but you would need a lot and they need to be matched, not easy. Remember that as you parallel them their combined capacitance is also increased.

The modulation can't be applied to the supply in a DSB system, it has to 'unbalance' the mixer and applying it globally through the supply can't do that. I would be very cautious copying the schematic in post #1 too, it has some serious design errors. The 741 audio pre-amp should not be driving a capacitive load as it will almost certainly cause instability, there is no DC return for the mixer stage, no stabilization of the PA bias and no diode across the relay coil.

Brian.

As far as concern the modulator, I was thinking of using it as an AF amp to unbalance the mixer of course. The oscillator will work in CW mode and the modulator will unbalance the mixer. So it will connected to another point on DSB, not like shown in my schematic.

What kind of power (currents) do I need to look for these diodes for handling 5-10w of RF power applied to them? Any hint?
I can't believe that with today's technology, there aren't fast switching power diodes out there to try, maybe I am looking for the wrong type of diodes.
 

The minimum turn on and turn off times of the diodes added together must be less than 1 cycle at 7MHz (142nS) for it to work at all and it should preferably be much less than that.

It is difficult to work out the voltage and current requirements, certainly voltage wants to be twice the supply at least and I would guess around 0.5A maximum current but without running a simulation I can't be sure. Pumping 10W of RF into a balanced mixer isn't something normally done!

Brian.
 
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    neazoi

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The minimum turn on and turn off times of the diodes added together must be less than 1 cycle at 7MHz (142nS) for it to work at all and it should preferably be much less than that.

It is difficult to work out the voltage and current requirements, certainly voltage wants to be twice the supply at least and I would guess around 0.5A maximum current but without running a simulation I can't be sure. Pumping 10W of RF into a balanced mixer isn't something normally done!

Brian.

I have 2 of these diodes. Are these anywhere close to what we are talking about?
 

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and also the sb3045st?
 

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Don't let me stop you trying them but I think you will be disappointed. They are SMPS rectifiers, intended for pulsed high current at relatively low frequencies (< 200KHz) so running them at >35 times their intended rating is pushing your luck to say the least.

Note the capacitance of these power rectifiers is large because of the large junction area.

Brian.
 
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    neazoi

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Don't let me stop you trying them but I think you will be disappointed. They are SMPS rectifiers, intended for pulsed high current at relatively low frequencies (< 200KHz) so running them at >35 times their intended rating is pushing your luck to say the least.

Note the capacitance of these power rectifiers is large because of the large junction area.

Brian.

I tried 2t bifilar twisted in the secondary. The end of the first winding connected to the beginningof the second. This common point goes to the GND.The other two wires ends go to two diodes (antiparallel) and connected together to the antenna. As simple as this.
But I do not think it worked. I even tested it with low power and 1n4148 which I know it should work. But no success.
So this first try was a failure. I may try more, but this made me thought of the next:

Why would I actually need DSB in this TX? The point of DSB is
1. PSU power efficiency
2. Bandwidth efficiency (carrier missing, but the same bandwidth occupied)
3. less loading of the final amps (not working at full time)

But my TX works (and must work, as its an oscillator) already at full power since it is a power oscillator. So I gain no PSU power efficiency and no less loading.
On the other hand simple AM sets (even as simple as crystal sets) can demodulate AM. Also folks with SSB sets can zero beat my carrier and receive either the LSB or the USB of my AM signal. In fact, this last bit is used in synchronous AM detection on commercial receivers to avoid fading and it works.

So I do not think that there is gain from DSB in my particular oscillator. If it was a two stage transmitter then yes it would make sense.
 

You are getting the idea!

The best you could achieve with the present circuit is a 50% reduction in power usage because as you now realize, the oscillator must always run. Even at half power with no modulation, your amplitude is restricted by the lower voltage the oscillator will run on, go below a certain voltage and it will stop dead. You can use reduced carrier DSB, but it has no advantage whatsoever unless the receiver has a synchronous detector, and that is fairly rare. I do have one here though!

Brian.
 

You are trying to make a linear balanced modulator at a carrier level that operates the diodes as on-off switches? Is this possible at all?
 

My thoughts exactly FvM. That and because the same device running as a high power oscillator has to work in linear mode as well.

Even the idea of balancing two 30A SMPS rectifiers at 7MHz is impractical.

Brian.
 

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