Any D/A converter and amplifier for microwave frequencies?

In summary: But there are some bands where you don't need a license at all. For example, 2.4 GHz is a band where you can transmit without a license as long as you are not causing harmful interference to licensed users. There are a few other rules, like you can't transmit waves that are too short, too wide, or too intense. Also, you can't transmit if you are causing harmful interference to other users. All in all, it is a pretty complex system, and it takes a lot of knowledge and experience to be a safe and legal RF transmitter.
  • #1
Ulysees
516
0
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?
 
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  • #2
Ulysees said:
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.

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!

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

I don't even know what this means.

- Warren
 
  • #3
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
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.
 
  • #6
Ulysees said:
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.

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...
 
  • #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.
 
  • #8
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.
 
  • #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.
 
  • #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.
 
  • #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:
 
  • #12
Ulysees said:
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:

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.
 
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  • #13
Ulysees said:
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.

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 referring 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.
 
  • #14
waht said:
you might suffer deep body burns and cataracts. ... you might distrupt local wifi networks, and 2.4 GHz cordless phones.

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?
 
  • #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.
 
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  • #16
Ulysees said:
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.

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.

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.

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

- Warren
 
  • #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.
 
  • #18
Ulysees said:
>
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.

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.
 
  • #19
f95toli said:
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).

8510?
 
  • #20
F95toli, may I ask what sort of application you're working on with such generators and analysers?
 
  • #21
Various things, I mainly work with superconducting solid-state qubits (more recently qubits coupled to coplanar waveguide resonators) and they tend to have a level splitting of a few GHz. This means that the measurement systems I build and use also need to operate in that frequency range.
 
  • #23
It's still nowhere near your desired bandwidth. If you want a 4 GHz bandwidth, you need an 8 GS/s DAC. This part probably also costs $100/unit. You're also going to have a lot of trouble supplyling it with 4:1 multiplexed LVDS digital data. It's not quite a hobbyist part.

- Warren
 
  • #24
I wouldn't be surprised if some custom DACs have been made in this range. However, just like in the microprocessor case, the 4GHz clocks would never be exposed outside the chip. A practical case might be some collection of multiple lower rate streams that are then upsampled with internal clock multipliers or mixed to the higher rate. Such a device is really not just a DAC anymore, its become a hybrid digital & analog SoC or DSP.
 
  • #25
This one is more like it.

http://www.compactpci-systems.com/products/pdfs/30045.pdf

Can someone tell me, are the connectors the familiar ones of satellite tv that many houses have?

Anybody worked with CompactPCI? Seems dead easy. But can you plug it to a normal motherboard I wonder, through an adaptor perhaps?
 
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  • #26
> the 4GHz clocks would never be exposed outside the chip

I don't understand what you mean. The first solution I found, just the chip, accepts external clocking and its evaluation board does just that.
 
  • #27
Again, 2.3 GS/s is nowhere near the bandwidth you claimed to desire. We also carefully explained why you do not need high bandwidth at all, and simply do not understand your application. (High frequency does not imply high bandwidth.) Now you're looking at equipment that'll run you multiple thousands of dollars?

- Warren
 
  • #28
Can I suggest that you read more carefully. I don't remember saying anywhere how much signal bandwidth I need exactly, obviously the higher the better. 4GHz was only mentioned because top-of-the-range pc's are clocked about this fast, so 4GHz DAC's shouldn't be far down the line. How much do they charge for those? That remains to be seen, that's what we're here for.

You seem to be taking things too personally my friend. Please do not. It's not the end of the world if we say one thing that is proven wrong.
 
  • #29
Ulysees said:
I don't remember saying anywhere how much signal bandwidth I need exactly
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.

A "nice clean sinusoid" has only one frequency component. It effectively has zero bandwidth.

My problem with all of this is that you seem to have no idea what you're talking about, yet refuse to listen to people who do.

- Warren
 
  • #30
> A "nice clean sinusoid" has only one frequency component. It effectively has zero bandwidth.

The word bandwidth is also used for the various signals in an integrated circuit, that's what I was thinking of when I mentioned PC technology that should be followed by corresponding D/A converter technology sooner or later.

A PLL is the obvious solution, but not finding a simple enough one to drive, I gave up.

Later by chance I found the 2.3 GHZ D/A converter product and I thought I should mention it. Sorry if that made you feel offended. Shoudn't I have mentioned it? What is more important? Knowledge? Or our egos?

I later also found that it's not just resonances I need to experiment with. There are also other waveforms that seem to matter according to some references, saw-tooth waves etc. So it became hot again, the D/A converter idea.

> My problem with all of this is that you seem to have no idea what you're talking about, yet refuse to listen to people who do.

How does it follow, from posting the link to the (non-existent) 2.3 GHz converter, that I do not listen to people? Am I supposed to listen to you, or obey you? :)
 
  • #31
Ulysees said:
> the 4GHz clocks would never be exposed outside the chip

I don't understand what you mean. The first solution I found, just the chip, accepts external clocking and its evaluation board does just that.
The clock input for modern Intel CPU chip is the at the 'front side bus' frequency and is something like 233, 300, or 400 MHz. The chip takes that frequency and internally multiplies it by say, 17X, to get the GHz ratings you see them sold by. Thus the highest fundamental clock frequency running around on the PCB (mother board) in the 100's of MHz; the GHz clocks never leave the chip. One reason is that digital clocks in the GHz range are much more difficult to manage. So if you have in fact found a DAC that accepts a ~2GHz digital clock, I believe you'll find it difficult to deliver the clock to the DAC. The data sheet likely has notes on this, describing special layout procedures (perhaps strip line design is required as suggested by Chroot). Its highly unlikely that the usual inexpensive FR4 printed circuit will suffice. Also, I'm guessing that with a digital clock that high thermal management becomes highly problematic (power dissipated proportional to the clock rate) and you'll have to add heat sinks, etc. I think it highly unlikely you'll find a clock input all the way to 4GHz on any simple DAC as stated above; again that become more of a system than a chip and the cost will scale accordingly.
 
  • #32
Ulysees said:
>
How does it follow, from posting the link to the (non-existent) 2.3 GHz converter, that I do not listen to people? Am I supposed to listen to you, or obey you? :)

It is NOT a "2.3 GHz converter". It is a D/A converter that can handle inputs of 2.3 Gbps, which is why the bandwidth is 1GHz (which in this case means you can e.g. use to generate a signal between DC and 1 GHz, but not higher than that).
Now, this is still quite impressive; but it it is still nowhere near the performance you were asking for.

I think the point you are missing is that there is HUGE difference beween being able to handle PULSES at a rate of a few gigabits per second and handling SIGNALS with a BANDWITH of a few GHz; the latter is much, much more difficult.
 
  • #33
Thank you mheslep. Just out if interest, do you have any thoughts how accurate a clock is? (irrespective of whether it's multiplied internally or not). I think it doesn't matter for my application if a sinewave has some very weak harmonics, what matters is that the fundamental is sharp, ie that the clock does not wobble. Can clocks be accurate?
 
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  • #34
> It is NOT a "2.3 GHz converter". It is a D/A converter that can handle inputs of 2.3 Gbps

Yes, didn't mean anything different. Thought it was obvious we meant samples per second, not Nyquist frequency or anything like that.

> Now, this is still quite impressive; but it it is still nowhere near the performance you were asking for.

Is it? There must be some filtering, maybe sinewaves you make with it are not so bad 5 times below the 1 GHz, as I just said above it's probably the fundamental that needs to be super-pure when looking for those supposed very-sharp resonances.

> I think the point you are missing is that there is HUGE difference beween being able to handle PULSES at a rate of a few gigabits per second and handling SIGNALS with a BANDWITH of a few GHz; the latter is much, much more difficult.

I've always been aware of this. I have a degree in Electronic Engineering from a top university in the UK, and it was a First. So feel free to go into the maths of it, it's all familiar.
 
  • #35
Low-jitter 4 GHz clocks are indeed very difficult to generate and pass around a PCB. In most labs, you'd use a $50k frequency synthesizer to produce a highly-stable sine wave signal and use that for a clock directly, or pass it through some kind of a sine-to-square converter like an ECL gate. (The Maxim part appears to handle either sine or square clocks.)

The Maxim part has pretty good dynamic performance characteristics, with pretty high SFDR at low frequencies. I tried to look for the SFDR at higher frequencies, but they do not offer the datasheet to the public.

From Maxim's website:

This product is designed for a highly specific application. The detailed data sheet contains information about proprietary technology, and is only available to customers whose requirements closely match this application. To request the data sheet, please complete the following information detailing your requirements.

Unfortunately we are unable to respond to requests that do not meet our qualification criteria. If you do not receive a response within two business days, please select another part. Technical support for this product is NOT available from our normal Tech Support desk.

I'm guessing they won't actually sell you one, anyway.

- Warren
 

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