Ideas for "single digit ghz" fixed frequency IQ modulation - Page 1
Forum section: electronics / rf-microwave

author: forrestc
author_rank: Unknown
timestamp: April 03, 2024, 10:09:55 pm
content: I'm hoping someone can point me toward some semiconductors or a solution I'm not aware of.For lab use, I'm working on a project that needs to generate an RF signal in the 1-2.5ghz range (sorry, I can't be more specific here). I need to be able to build many of these so I want to use components instead of assemblies. We're currently using a couple HackRF one SDR and that isn't sustainable price-wise (with needing dozens or hundreds of them).Frequency stability over temperature is more important than frequency agility (although if the solution is agile, that is fine also). Generally, a given board I build would be at a specific frequency and never need to change. The labs all have high-precision clock sources, usually 10Mhz, but sometimes higher is available (a solution needing a higher reference clock up to around 100Mhz is definitely doable, although they'd all be disciplined from either the 10Mhz source or GPS). I guess what I'm trying to say here is that we have a good reference clock, and want to make sure we don't generate a not-very-stable carrier from it.The modulation itself is currently being generated as an I/Q stream, so that's easy to deal with. I don't think DDS in in the cards here due to the high frequency. Other options to generate a quite wide digital modulation would be of interest. I realize that without knowing the application it's hard to say whether a modulation method would work - in this case, it would probably be more a thing of me not knowing a method exists.The closest solution I've come up with so far is something like an ADF4360 "integrated synthesizer and VCO" (or a similar part) feeding a TI AFE7071. However, the 20mhz bandwidth of the AFE7071 doesn;t seem to have quite enough signal bandwidth to handle this application - according to the docs here the signal to be generated is 25mhz wide but I need to do analysis on the actual I/Q stream to figure that out. I'm also not sold on the ADF4360 specifically as there seems to be a whole bunch of similar parts and I haven't looked at all of them and right now I'm just looking at building blocks, not digging deep into datasheets other than "will this block possibly work". One other note here is that I'm drawn to the AFE7071 as the analog DAC->IQ modulator connection seems problematic as far as getting the analog levels correct goes.I'm mainly asking here as I want to make sure I don't miss an easy, obvious way to take an I/Q stream and modulate a ghz signal with it. Obviously an all-in-one chip where you feed 10mhz and the I/Q stream in and a nicely modulated signal comes out the other side on the right frequency would be perfect, although it doesn't seem that exists, at least not at any reasonable price.So, any ideas/suggestion/pointers/etc. would be helpful.

author: KE5FX
author_rank: Super Contributor
timestamp: April 03, 2024, 10:53:14 pm
content: Quote from: forrestc on April 03, 2024, 10:09:55 pmDon't neglect the possibility of DAC or DDS operation in higher Nyquist zones. An AD9957 might be a candidate, from what you're saying.Also you might consider SDR chips like the AD9361 and similar parts, although they tend to be a bit pricy.

author: langwadt
author_rank: Super Contributor
timestamp: April 03, 2024, 11:00:44 pm
content: https://www.analog.com/en/products/adrf6755.html?

author: forrestc
author_rank: Unknown
timestamp: April 03, 2024, 11:28:49 pm
content: Quote from: KE5FX on April 03, 2024, 10:53:14 pmFunny you should mention that... After I posted I ran across the TI versions of a similar part.Somehow I believed that DDS always required a stream directly from the processor/FPGA into the DAC high enough to generate the entire waveform in software. For Ghz ranges, that is an incredible sample rate to move around, let alone generate math on, even with a FPGA.It seems many of these parts do the "Digital LO" generation onboard, then the I/Q modulation is applied digitally and only then output to a DAC. From my perspective, that seems much easier to deal with since the carrier synthesis is done on-chip. The AD9857 looks like it works that way, based on an initial glance.I would also have never considered anything above the Nyquist frequency, but now I think about it, it makes sense. I'm always surprised by how many times things like this are the solution—that is, using something you'd normally consider trash to be eliminated as your solution.Quote from: KE5FX on April 03, 2024, 10:53:14 pmIf I could find one not-pricey I'd do that in a minute.

author: forrestc
author_rank: Unknown
timestamp: April 03, 2024, 11:37:56 pm
content: Quote from: langwadt on April 03, 2024, 11:00:44 pmIt seems like most of these parts combine some subset of the PLL, I/Q modulator and DAQ that you need for a complete solution. Some throw in a VCO for good measure. Unfortunately, the solutions that combine them all suddenly become very expensive parts.The ADRF6755 seems to combine the PLL, VCO, and I/Q modulator but is missing the DAC. This actually seems to be the likely way I'm going to end up going, I'll just have to figure out how to ensure the DAC is putting out the correct signal levels.But.. I think from a "likely issues with the analog part of the circuit" standpoint I'd rather do the DAC+I/Q modulator together, then put the PLL/synthesizer in an external device. That way I don't have to worry about the separate I/Q analog signals being the correct amplitude. BUT.. all of the solutions that do it that way are either way expensive or fairly narrow bandwidth.

author: KE5FX
author_rank: Super Contributor
timestamp: April 04, 2024, 12:27:09 am
content: Classically a DDS doesn't use an external data stream, just a clock and a programmed frequency tuning word to tell it what to generate. Some will accept PM or FM data input but that's a bell-and-whistle, not a mandatory feature. ADI calls the AD9957 a "DDS" in some of their charts but while it can be used as one, that's not all it is.Really, the AD9957 sounds like the right solution unless you actually need carrier frequencies across the entire 1 - 2.5 GHz range. That could make it harder to use undersampling, since some of your desired frequencies may be too close to the clock multiples. If you need a single-frequency output somewhere within that range, it's almost a no-brainer. It will certainly give you a cleaner signal than the ADRF6755, which requires external DACs and uses a poor excuse for an integrated PLL rather than a DDS core.