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 Post subject: AM Transmitter Design Using AD811
PostPosted: Jan Wed 24, 2018 8:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4255
FREQUENCY STABILITY: In another thread, regarding non-crystal controlled frequency stability, Eric Loepke brings a up a good point about beat frequencies.

Non-crystal means to limit frequency drift employ such techniques as low drift oscillator designs, (e.g. Vackar oscillator), the use of ovens to maintain stable temperature, the use of low drift capacitors and coil materials, physical stability, and alternative frequency stability means, such as huff-puff frequency stabilizers (which do use crystals or resonators).

I've fiddled a little with a Vackar circuit and there seems to be some promise there. Maybe it's possible to find some lucky combination of components that will always be readily available that interact together such that they are immune to the forces that cause drift. But so far, it looks to me that coming up with that magic combination will prove to be more expensive and a lot more difficult and than employing an $18 custom crystal oscillator or a three IC PLL synthesizer that provides all the frequencies.

SIMPLICITY: I'm still playing with a Class-E transmitter. I feel it's an inconvenience to have to use resonating coils, which is the case with my design and with most transmitters. If we could do away with coils and tuning, that would be a big step forward. Eric's design looks coil-less.

AD811 SOLUTION: One approach is to add a broadband RF amplifier to the output of a transmitter that can use a boost. Years ago when testing Q of coils, I needed a powerful MW generator. I came across the AD811 video amplifier IC. In a Wien-Bridge configuration, it comprised a coil-less transmitter that can output WATTS. The Wien Bridge oscillator proved stable enough for testing Q. Converting it to a transmitter could be done by feeding audio into a light dependent resistor inserted in the feedback loop, thus providing a modulated output. I've not tested it as an AM transmitter, mainly because it's an RC oscillator and I've become spoiled with the $12 PLL synthesizer.

Here's a link to the AD811 MF frequency generator.

Perhaps a better idea is to simply use one section of the AD811 as a filtered voltage follower after a PLL synthesizer. Additional filtering could be done with the other section to produce an acceptably clean sine wave. I see no reason why modulation couldn't be put into the feedback loop. The AD811 is designed to feed 75 ohm loads, so feeding an antenna directly just might work. I hate to stop mid-project with the Class-E work, but this looks pretty tempting.

 Post subject: Re: AM Transmitter Design Using AD811
PostPosted: Jan Thu 25, 2018 5:09 am 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 641
Location: Equinunk PA 18417
Do you think an LDR will be fast enough? I understand a "typical" response time is something like 5-10ms on, and longer to off. Would and LDR modulate content above a couple hundred Hz?


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