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Any D/A converter and amplifier for microwave frequencies?

  1. Jan 31, 2008 #1
    At a time when pc's come with microprocessors clocked at frequencies of the order of 4 GHz, it must be possible to drive a D/A converter at microwave frequencies.

    Anyone know of hardware to do this?

    Anyone know of suitable hardware to transmit the waveform at a power density comparable to a mobile phone's?
     
  2. jcsd
  3. Jan 31, 2008 #2

    chroot

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    These two forms of technology have nothing to do with each other. I actually don't know of any D/A converters that run that fast, nor any market for them. Furthermore, I don't know of any microprocessors which could actually deliver data onto an external bus that fast. Hell, you'd have to use exotic stripline techniques just to route the digital data on your printed circuit board.

    I'm sure there's some other way to accomplish your goal without needing a 4 GHz DAC!

    I don't even know what this means.

    - Warren
     
  4. Jan 31, 2008 #3

    berkeman

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    Maxim has a pretty good line of wireless discrete devices:

    http://www.maxim-ic.com/products/wireless/

    Keep in mind that you can't just build a transmitter and start transmitting in the licensed RF bands. You can listen to anything, but there are rules about transmitting -- very important rules to prevent harmful radio interference.
     
  5. Jan 31, 2008 #4
  6. Jan 31, 2008 #5
    Thank you guys.

    > Anyone know of suitable hardware to transmit the waveform at a power density comparable to a mobile phone's?

    Sorry about this confusing question, I meant just like in a cellphone the microwave's phase and amplitude are digitally switched, the same technology could be used to switch between 2048 amplitude levels, ie a D/A converter at 1.7 GHz. I want to transmit my arbitrarily modulated microwave like a cellphone transmits its signal, at the same strength.
     
  7. Jan 31, 2008 #6

    berkeman

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    Then you will need an FCC license. It is illegal (and a bad idea) to transmit in the licensed RF bands. The radio spectrum is considered a shared resource, and is regulated by the FCC in the US, and other government agencies in other countries. Do not experiment with transmitting RF signals -- it is way too easy for you to cause harmful interference with important radio communications.

    For instance, Tuesday evening we were having our weekly on-air HAM radio practice Emergency Net, broadcasting from our city's Emergency Operations Center (EOC), and an interfering signal clobbered us during the practice Net. Very obnoxious. We had to leave the EOC and use our portable HAM equipment outside, positioned to receive less of the interfering signal. If this had happened in a real emergency (yes, we do have those too) instead of just in this practice Net, people could have been injured or killed because of our loss of communication with other EOCs and with our CERT teams in the field. This is serious stuff.

    So we are organizing a transmitter hunt for this interfering source. If it comes back on, we will be prepared to track it down and report it to the FCC. The FCC takes a dim view of harmful interference with licensed bands.

    So, I would advise you to still pursue your interest in RF and radio, but you have to do it in ways that will not cause interference with licensed RF bands. If your main interest is hacking into cell phone conversations, or creating cell phone jammers, then you will not receive help from us here on the PF. If your interest is to some day be a mixed signal RF ASIC or system design engineer, then you are at the right place. You just need to learn about RF design in a more controlled way than trying to build a full-power cell phone transmitter and firing it up....
     
  8. Jan 31, 2008 #7
    > you can't just build a transmitter and start transmitting in the licensed RF bands... there are rules about transmitting -- very important rules to prevent harmful radio interference.

    Other than filtering out any harmonics and using the frequency bands allocated for each purpose, are there any other rules?

    What about microwave-oven frequencies, they must be free from licensing requirements.
     
  9. Jan 31, 2008 #8

    berkeman

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    Yes, the rules often require a license to use the band. You have to have a HAM radio license to use the HAM bands, for example. You have to hold a commercial license to use the commercially licensed bands (there is money involved in cell phone service, right?)

    Now, you raise a good point about the ISM band (microwave oven band). The rules there are much more relaxed, and it is much more practical for you to build your own transceivers in that band for experimenting and learning. I need to bail right now, but search on my user name and for the term ISM in my posts. That should get you a thread or two with more info. BTW, ISM = Industrial, Scientific, Medical.
     
  10. Jan 31, 2008 #9
    Now I see your longer message, alright fair enough. I want to test senator Nick Begich's claims about resonant frequencies of water etc.
     
  11. Jan 31, 2008 #10
    Microwave oven is not transmitting into the air, rather the the waves are contained and absorbed by the food. Of course there is some leakage but it's insignificant.

    You can freely experiment with any frequency you want as long the signal is contained.
     
  12. Jan 31, 2008 #11
    Thanks waht, I was looking for something much weaker than an oven's microwave source anyway.

    But know I got an idea. What happens if you fix the door latch and operate the oven open, against a fly or something. :devil:
     
  13. Jan 31, 2008 #12
    Microwave oven manufacturers are required by law to devise at least two independent failsafe systems in an event door is opened. But if you bypass those anyways and expose yourself to the intense radiation, you might suffer deep body burns and cataracts. Also, if the magnetron is not properly matched to it's waveguide, it will burn out quickly too.

    Microwave ovens operate at 2.45 GHz. If you send that out into the air, you might distrupt local wifi networks, and 2.4 GHz cordless phones.
     
    Last edited: Jan 31, 2008
  14. Jan 31, 2008 #13

    f95toli

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    But you don't need a fast D/A converter for that. Your A/D and D/A converters only need to be as fast as the bandwidth of the signal you want to transmitt. When we are talking about frequencies of cell phones etc we are refering to the CARRIER frequency, the bandwidths are in most applications very small. This means that we can use mixers to up/dowconvert the signal to the carrier frequency, i.e. the microwave part of the circuit (LO, mixers, amplifiers etc) is complettely analog; only the parts that handle the SIGNAL have to be digital.
     
  15. Feb 1, 2008 #14
    I was just kidding. I do appreciate your suggestion, to contain any experiments within a properly shielded enclosure.

    Any thoughts how to filter any cables going in?
     
  16. Feb 1, 2008 #15
    I understand what you're saying f95toli, sorry I forgot that. On the other hand, D/A conversion is very simple, it's just a passive ladder of resistors at exponentially increasing values - the output current is the analogue signal. So it should be available at really high frequencies, as long as the rise time of the digital waveform is small enough compared to the sampling interval.

    Actually I don't want to transmit any low-bandwidth information, I want a nice clean sinusoid at a high frequency that I can control accurately with a computer. That's to test senator Nick Begich's claims of resonances of water etc. A 4 GHz d/a converter would probably not make accurate enough sinusoids at microwave frequencies, but there are resonances at lower frequencies too.
     
    Last edited: Feb 1, 2008
  17. Feb 1, 2008 #16

    chroot

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    Totally false. If you designed a DAC with nothing more than passive resistors, you'd get fired and no one would ever buy your product. In reality, you can't manufacture chips with precise resistors (they can be off by as much as 20% from one unit to the next), or very large resistors. In practice, DACs are usually made with current-steering switches. They also often have to be calibrated to get good static and dynamic performance. People spend years designing good DACs, and it's not trivial at all. It's certainly not "easy" to create a 4 GHz DAC. Hell, it's certainly not "easy" to even develop a sense-amp cell that's capable of capturing 4 GHz digital data in the first place. Keep in mind that the transit frequency for today's CMOS processes are only about 10-15 GHz. There are two reasons you'll never find a 4 GHz DAC on the market:

    1) It would be insanely hard to make, and thus would insanely expensive.
    2) No one would buy it anyway, because no reasonable application actually needs 4 GHz of bandwidth.

    Then what you want is a frequency synthesizer. I'd look into PLLs intended for microwave use.

    - Warren
     
  18. Feb 2, 2008 #17
    > 2) No one would buy it anyway, because no reasonable application actually needs 4 GHz of bandwidth.

    On a lighter note, Bill Gates once said "640K of RAM ought to be enough for anybody". :smile: I guess you're talking about the market in which you're working, fair enough.

    Cause the military very likely have applications that need this sort of absolute control on a microwave, or even infrared light. They've always been far more than advanced than what is available for civilian applications. At the time of "640K of RAM" the military had radar-invisible jets. God knows what they have today.

    > Then what you want is a frequency synthesizer. I'd look into PLLs intended for microwave use.

    That's it. As long as they're precise, and can be switched on and off very fast, this is just what I need.
     
  19. Feb 2, 2008 #18

    f95toli

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    But again, 4 GHz is a HUGE bandwidh. Why and when would you need that? Imagine that you operated your device at 6 GHz, that would mean that the signal would cover the whole c-band (4-8 GHz)!
    We already have more or less "absolute" control over signals up to tens of GHz and there are generators and analyzers that work up to 110 GHz (if you can afford them, at these frequencies even the connectors are very expensive), the ones I i use at work operate up to 40 GHz and they are actually quite cheap nowadays (probably £20-30 000 depending on options).

    Also, in order to get "a nice clean sinusoid" you would need something like 14 bit resolution, and that might very well be impossible with existing technologies (there are fundamental problems with delta-sigma converters that make it difficult to build very fast high-resolution converters, even if the individual components work). There is absolutely no point in using a DA converter for that when a syntheziser works so well.

    Moreover, modern modulation techniques have very high spectral efficiency, e.g. WIFI operating at 2.4 GHz has a bandwidth of only about 20 MHz per channel which is why you can fit 14 WIFI channels in the 2.4 GHz band.
     
  20. Feb 2, 2008 #19
    8510?
     
  21. Feb 3, 2008 #20
    F95toli, may I ask what sort of application you're working on with such generators and analysers?
     
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