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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Fri 24, 2017 10:39 pm 
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Location: Arlington, TX, USA
Macrohenry wrote:
Tube Radio wrote:
Would be nice to have a part 15 transmitter that transmits farther while staying within the legal limits.


With Part 15 it's mostly about the antenna/ground system and optimum matching. I could hear my Wenzel transmitter a mile away when I mounted my antenna on the roof and used radials. Some say that roof mounting is impermissible under Part 15 rules, others say not, YMMV depending on the FCC inspector. Personally, I can't imagine FCC denying Part 15 use to an apartment dweller, but what's my opinion worth?

In any event, an efficient antenna within Part 15 rules usually will get you several hundred feet range, and at least 100 feet of strong signal.


That just confirms what I've been saying, namely that those getting greater than normal range are using fairly elaborate antenna systems within Part 15 rules. Things like loading coils, copper tubing for the antenna, and a ground radial system. The 10 foot wire hanging off the end of the transmitter won't do as well. Also having good ground conductivity helps, and that depends on where your antenna happens to be. Personally, in my own case I'm just looking to transmit to radios around the house, but I can see others who want or need more range for whatever reason. It's more in the antenna than the difference between 85mW and 95mW output power. Physics puts a limit on how much of that RF is going to get out of the Part 15 antenna which is far too short to be efficient.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Sat 25, 2017 5:21 am 
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Erich Loepke wrote:
If it works out, please post some scope screen shots of the circuit if you can. I'll do the same and start a thread about my low level modulation transmitter as soon as I get it working again


Sounds good, Eric. I'm back home now. Four hours ago I guess was just a few miles from you when we drove through Ft. Worth. Anyway, I think I may do away with the two diode AND circuit and just put second IRF510 in series with the first, driving one with the 1690 MHz square wave and the other with the PWM circuit. That way I can decide whether to add a low pass filter on the PWM, which wouldn't work in the AND gate.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Sun 26, 2017 2:46 am 
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I changed the PWM frequency to 105625 KHz. Nothing special about the exactness of that frequency, that's just what resulted when using a standard value timing capacitor.

Here are screenshots of the PWM comparator raw output with 0-2VDC being compared with a 0-2VDC triangle wave. Three's no modulation other than the DC input going from zero to 2 Volts. A photo for zero volts is not shown, as it's just a straight trace.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Sun 26, 2017 3:03 am 
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Here's the output of the AND gate that will feed the IRF510 transmitter final. The first five photos are of various audio levels of the 440 Hz sine wave input with the waveform taken from the AND gate. Inputs to the AND gate are 1690 KHz square wave and 0-1V 440Hz sine wave modulating signal. These five are with the PWM oscillator set at 27KHz.

The last photo has the PWM frequency changed to 105625 Hz. Coincidentally or not, 105625 KHz is exactly 1/16 of 1690 KHz, which you can see on the peaks of the waveform. There are exactly sixteen square waves between the PWM peaks.

I used an LM386 to boost the output of my sine generator beyond 1V. When the PWM duty cycle is greater then 50%, sine wave volume starts dropping. This is because of which comparator input I'm using. The sine wave is heard increasing less time as the duty cycle shortens the duration it can be heard. Reversing the inputs to the comparator will fix that, but then there's no carrier on silences and the radio gets noisy in times of low or no signal.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Wed 29, 2017 6:59 am 
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I broadcast the Beatles' "Yesterday" throughout the house tonight using PWM. The music itself sounded good, but there are two problems. The modulation envelope is quite limited and noise is getting into the signal. I don't know the source of the noise, but I suspect my rats nest of clipleads doesn't help. Next step is to build as much as I can onto a pc board with ground plane.

The two diode AND gate configuration was not as successful as putting two IRF510s in series, each driven respectively by the PWM signal and the 1690 KHz square wave. PWM at 27000 Hz was less noisy than at 105625 Hz.

I'd like to see if I can filter the PWM to turn it into modulated DC.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Wed 29, 2017 8:54 pm 
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105625 Hz PWM modulated by 440 Hz sine wave with scope probe on antenna wire
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Next four photos are of carrier modulated by 440 Hz sine wave, scope probe on antenna wire. The first is at minimum volume/modulation, followed sequentially by increasing volume. I want to understand why the change displays as varying the slope of the carrier between the peaks. I can't account for the double peaks.

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Next are photos of modulated carrier with photos taken at a different time. These look more like typical modulation waveforms. I can't account for the difference between them and the square envelopes shown above.

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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Wed 29, 2017 9:24 pm 
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Location: Arlington, TX, USA
What is the circuit you're using? I will try running it in LTSpice to see what is happening.

I should have replacement DDS boards soon, they finally made it into the country yesterday. Then I can continue with my own design.

I tried to get clarification from the FCC on transmitter kit sales; the official position is they need certification as well. Not worth doing for the kind of sales volume I think anyone is going to get for AM transmitters. It would probably more than double the cost of a kit, adding several hundred dollars to the price by itself as well as needing to use a supplied antenna system. Instead I will have to offer only construction plans. I know that uncertified kits remain available, but I'm not going to tempt fate.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Thu 30, 2017 3:42 am 
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I did some circuit simulations on what I think your circuit is, but I'd need to see what exactly it is you're using to be sure. The resonant series LC circuit at the output of the lower MOSFET (the one which has the carrier frequency as its input) absolutely HAS to be there or the output waveform is a mess. A low pass pi filter helps clean it up even more. If you have LTSpice I can give you the schematics I made so you can get an idea of how things work in both a simple modulated Class E stage and one with a PWM input.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Thu 30, 2017 5:30 am 
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The unmodulated carrier uses this circuit. Series inductance is 140 uH and the loading coil is 227- 270 uH. Loading coil is connected directly to the drain. Variable cap on the drain is 30-200 pF. It makes little difference, but there is a minor peaking in there somewhere.

Adding PWM, I tried two configurations. In the first, I added another IRF510 between the current one and ground. In the second, I added it between V+ and the 140uH series coil. Both configurations took as input the output of the PWM comparator. Each had a 100K resistor from gate to ground. Both worked. I don't have the notes, but I'm thinking that one configuration produced the squareish wave output, the other produced the more usual modulated waveform. Both were accompanied by noise, which I believe to be a result of my lashup. Next step is to build it properly with short leads and ground plane. Hopefully the noise will disappear.

Image


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Thu 30, 2017 6:07 am 
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For domestic tranquility, I had to temporarily dismantle the rats nest of wires on the kitchen counter. When I put it back together, I'll try connecting the PWM IRF510 to the modulation transformer in this ckt. Now that will be interesting.

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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Nov Thu 30, 2017 11:20 pm 
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Something else I forgot earlier... Class C or E needs a low pass filter at the output to get a good output waveform. Your schematics don't show any filtering at the output.


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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Dec Sat 02, 2017 2:14 am 
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What are you using for a modulation transformer?

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 Post subject: Re: Class E transmitter with $12 MW Frequency Synthesizer
PostPosted: Dec Sat 02, 2017 5:11 am 
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I used a small output transformer. It was in my junk box, unlabeled, but I expect it is on the order of 1000:8 ohms.


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