5 ms·

Radio Architecture Matters: A Review of RF Sampling vs. Zero-IF

40 points by teleforce 3y ago

jhoechtl 3y ago
While this article is about frequencies you typically use in Wifi communication, it touches another point. I am a ham radio operator and with many radio devices delivered these days I witness the following. Mostly high quality components are used but they are assembled in a way that lacks knowledge of harmonics, interference, cross-talk, parasitic effects in general. PC board design makes it obvious and results in a sub-quality end product. It seems like a basic engineering craft which got lost in the last 30 years.
- willis936 3y ago
  I don't consider RF PCB layout basic engineering and I don't think it's lost. Plenty of us EEs have read Pozar and could make a competent RF layout given time and resources. I'd first guess that bad layouts are a management decision.
  - felipemnoa 3y ago
    This pozar? https://www.amazon.com/Microwave-Engineering-David-M-Pozar/dp/0470631554 https://www.amazon.com/Microwave-Engineering-David-M-Pozar/d...
    - mwbajor 3y ago
      Yes. But if you really want to get into it you need to do it. You could make some cheap PCBs and measure them with a cheap network analyzer. RF design has never been more accessible. The books will get you started and you will start to think about things like de-embedding circuit elements which is very important and not covered in books. You will also start to see why people consider RF design challenging; e.g. curves in the books will depict a "trend" in the circuit and in real-life, everything is a capacitor and/or inductor which causes the real life measurements to deviate substantially from the ideal for reasons that are not always obvious. An even more cheaper accessible intro to RF is just plain old analog electronics. Building amplifiers ,etc.
      - lb1lf 3y ago
        I'd upvote this ten times if I could. I had the good fortune of being exposed to both Pozar, an excellent lecturer and a wizard of an RF lab postdoc when studying RF engineering, and the utility of simply spending lots and lots of time in the lab, trying all sorts of implementations of filters, transmission lines, antennas and whatever caught our fancy, prototyping with wild abandon, verifying (or getting stymied by) the results, refining our measurement skills along the way... Priceless. Then all RF design jobs disappeared overnight and I ended up doing PLC coding instead of working with radio - but, hey, University was great fun, and RF design definitely is one of the fields where there's still room for intuition and a bit of artistry in addition to just knowing the hard theory of the subject.
        edrxty 3y ago
        Where are all the jobs going? There are a lot of RF things going on at an ever increasing pace but I don't see a lot of people hiring in the field. Do all the defense primes just have a silo of gray beards they keep in a dungeon somewhere?
        mwbajor 3y ago
        Yes. Many analog design groups in college went from 10's of students 10-15 years ago to just a few now, not due to lack of funding but due to a lack of interest. Our interface to nature is through analog signals but due to the heavy reliance on HW intuition and its relative inflexibility (but more 'elegant solutions' if you ask me) analog electronics continues to grow in unpopularity in industry and academia follows. Yes, there are just some older guys updating old designs in many instances. The push for RFSOCs and similar products is partially to reduce reliance on the greybeards. Its a shame because there are very interesting areas of analog electronics such as continuous time signal processing. TLDR: To answer your question, to reduce the reliance on a relative few engineers, the market solved it with a few products that are 'just good enough'. This has become a self perpetuating cycle, race to the bottom. There will still be many analog/RF EEs but relegated to a few niche sub-industries serving mainly the DoD and RFIC industries. Even the telecom industry is planning to pipe back raw, downconverted baseband data to a central location to do SDR DSP rather than rely on a modem at the basestation. All for flexibility in lieu of power/efficiency, NRE etc.
  - gorkish 3y ago
    > bad layouts are a management decision. That's an interesting way to describe AliExpress shovelware, but OK, yeah it's a "management decision"
    - willis936 3y ago
      Management chooses to make and sell a bad product. It's not that RF design is somehow arcane wizardry lost to time.
- gorkish 3y ago
  Oh god, the receivers being shipped with the LTC2208! They do not even deserve to have supply of that chip. What a disservice.
  - edrxty 3y ago
    Can you expand on this? You have my interest...
    - gorkish 3y ago
      If you want to dig in just have a look at the RX-888 and it's ilk. Pretty much a case study in an effort to hook a cypress usb3 chip up to an ADC with the lowest BOM cost possible. Forget the matched predriver, forget buffers, forget analog, forget the clock, forget the reference designs and application notes altogether really. But oh yeah make sure you spec the 16 bit part-- we'll have none of that 14 bit shit thank you.
cozzyd 3y ago
Analog pushing back against "fuck it, let's use an RFSoC" ?
- spacecadet 3y ago
  Not push back to me. As an "analog", I love SDR-SOC but what this is saying is that people may lack a fundamental knowledge when designing hardware devices that is contributing to these challenges. Id also push back against the article in the sense that I think the industry shot itself in the foot trying to provide too many options that all overlap. But, I do agree with the sense that minimizing the radio architecture to only what it needs to operate is smart.
  - mwbajor 3y ago
    Not only that, but it also stems from "management" or "customers" unable to contemplate tradeoffs in the design. The reason FPGAs and RFSOCs are pushed so much in certain industries (we all know which one) is because the customer wants everything reconfigurable because they can't make decisions now, or in most cases, not competent enough to make decisions now. However, they are OK with buying a $25K+ FPGA or RFSOC which is why the market is there and also why few understand the actual art of RF design and radio architecture anymore. RF budgeting is almost non-existant. Most Jr. engineers have never seen a level diagram and the only places you will find them is in old watkins johnson app notes. SDR design is ultimately a race to the bottom in most cases in terms of good design but not necessarily cheap design.
cushychicken 3y ago
Man I love ADI’s tech notes. They’re some of the best in the business at technical marketing.
Scene_Cast2 3y ago
Here's a super crude ELI5, feel free to correct me. Signals crash course: Radio comms have a channel they need to fit into (let's say 1.1GHz - 1.2GHz as an example). This means that what you send over the air is basically a high frequency (~1.15GHz) sine wave. You encode data by slowly varying this sine wave (amplitude and/or frequency). The variations are going to be super slow, as fast changes in either amplitude or frequency will make your signal go outside the frequency limits. It's only the sine wave (aka the "carrier frequency") that's high frequency (1.15GHz in our case). There are several ways to digitize this signal. You can do "direct RF sampling" - you need a very high sampling rate (2.4GHz+ due to Nyquist in our case) and just sample the high frequency signal. You can multiply (in analog domain) the RF signal from the antenna by another frequency to keep all the slow changes but change the carrier frequency. This means that you don't need such a high lower sampling rate (you can get away with much less than GHz). If your new carrier frequency is just some low but non-zero frequency, that's super-heterodyne (IF). If it's zero, that's Zero-IF.
- mwbajor 3y ago
  Yes all correct, you didn't capture any of the tradeoffs but thats the basic idea.
- gorkish 3y ago
  Just have to say this is the first time I've seen someone introduce the concept of an RF mixer "backwards".
- kurthr 3y ago
  I was surprised they didn't use the word homodyne. One issue they didn't mention, but probably shows up in the noise figures is that homodyne receivers suffer from 1/f noise in the relatively wide band "direct conversion" amplifiers. Of course all of the heterodyne receivers suffer from 1/f up-conversion at the mixer.
  - teleforce 3y ago
    It is rather a soup of terminologies namely homodyne, zero-IF, and direct conversion architecture are all referring to the same thing. It seems they are now settling on RF sampling for the newer transciever architecture technology but sometime direct RF is also being used for example this latter terminology is referred by Intel for their new line of FPGA and RF transceivers product integration.
spr-alex 3y ago
I am looking for PCB designers with RF experience. If this is what you're good at, please reach out
teleforce 3y ago
For both RF sampling and Zero-IF the two of the most popular analog manufacturers are TI and Analog Devices. It's rather intriguing that for RF sampling technology, Intel has recently join the bandwagon and called it a game changing technology when the RF sampling is integrated with high speed FPGA [1]. Intel Delivers a Game-Changing, Analog-Enabled Direct RF FPGA Portfolio: https://community.intel.com/t5/Blogs/Products-and-Solutions/FPGA/Intel-Delivers-a-Game-Changing-Analog-Enabled-Direct-RF-FPGA/post/1417363 https://community.intel.com/t5/Blogs/Products-and-Solutions/...
James2233 3y ago
[dead]

40 points by teleforce 3y ago

While this article is about frequencies you typically use in Wifi communication, it touches another point. I am a ham radio operator and with many radio devices delivered these days I witness the following. Mostly high quality components are used but they are assembled in a way that lacks knowledge of harmonics, interference, cross-talk, parasitic effects in general. PC board design makes it obvious and results in a sub-quality end product. It seems like a basic engineering craft which got lost in the last 30 years.

willis936 3y ago
I don't consider RF PCB layout basic engineering and I don't think it's lost. Plenty of us EEs have read Pozar and could make a competent RF layout given time and resources. I'd first guess that bad layouts are a management decision.
- felipemnoa 3y ago
  This pozar? https://www.amazon.com/Microwave-Engineering-David-M-Pozar/dp/0470631554 https://www.amazon.com/Microwave-Engineering-David-M-Pozar/d...
  - mwbajor 3y ago
    Yes. But if you really want to get into it you need to do it. You could make some cheap PCBs and measure them with a cheap network analyzer. RF design has never been more accessible. The books will get you started and you will start to think about things like de-embedding circuit elements which is very important and not covered in books. You will also start to see why people consider RF design challenging; e.g. curves in the books will depict a "trend" in the circuit and in real-life, everything is a capacitor and/or inductor which causes the real life measurements to deviate substantially from the ideal for reasons that are not always obvious. An even more cheaper accessible intro to RF is just plain old analog electronics. Building amplifiers ,etc.
    - lb1lf 3y ago
      I'd upvote this ten times if I could. I had the good fortune of being exposed to both Pozar, an excellent lecturer and a wizard of an RF lab postdoc when studying RF engineering, and the utility of simply spending lots and lots of time in the lab, trying all sorts of implementations of filters, transmission lines, antennas and whatever caught our fancy, prototyping with wild abandon, verifying (or getting stymied by) the results, refining our measurement skills along the way... Priceless. Then all RF design jobs disappeared overnight and I ended up doing PLC coding instead of working with radio - but, hey, University was great fun, and RF design definitely is one of the fields where there's still room for intuition and a bit of artistry in addition to just knowing the hard theory of the subject.
      - edrxty 3y ago
        Where are all the jobs going? There are a lot of RF things going on at an ever increasing pace but I don't see a lot of people hiring in the field. Do all the defense primes just have a silo of gray beards they keep in a dungeon somewhere?
        mwbajor 3y ago
        Yes. Many analog design groups in college went from 10's of students 10-15 years ago to just a few now, not due to lack of funding but due to a lack of interest. Our interface to nature is through analog signals but due to the heavy reliance on HW intuition and its relative inflexibility (but more 'elegant solutions' if you ask me) analog electronics continues to grow in unpopularity in industry and academia follows. Yes, there are just some older guys updating old designs in many instances. The push for RFSOCs and similar products is partially to reduce reliance on the greybeards. Its a shame because there are very interesting areas of analog electronics such as continuous time signal processing. TLDR: To answer your question, to reduce the reliance on a relative few engineers, the market solved it with a few products that are 'just good enough'. This has become a self perpetuating cycle, race to the bottom. There will still be many analog/RF EEs but relegated to a few niche sub-industries serving mainly the DoD and RFIC industries. Even the telecom industry is planning to pipe back raw, downconverted baseband data to a central location to do SDR DSP rather than rely on a modem at the basestation. All for flexibility in lieu of power/efficiency, NRE etc.
- gorkish 3y ago
  > bad layouts are a management decision. That's an interesting way to describe AliExpress shovelware, but OK, yeah it's a "management decision"
  - willis936 3y ago
    Management chooses to make and sell a bad product. It's not that RF design is somehow arcane wizardry lost to time.
gorkish 3y ago
Oh god, the receivers being shipped with the LTC2208! They do not even deserve to have supply of that chip. What a disservice.
- edrxty 3y ago
  Can you expand on this? You have my interest...
  - gorkish 3y ago
    If you want to dig in just have a look at the RX-888 and it's ilk. Pretty much a case study in an effort to hook a cypress usb3 chip up to an ADC with the lowest BOM cost possible. Forget the matched predriver, forget buffers, forget analog, forget the clock, forget the reference designs and application notes altogether really. But oh yeah make sure you spec the 16 bit part-- we'll have none of that 14 bit shit thank you.

cozzyd 3y ago

Analog pushing back against "fuck it, let's use an RFSoC" ?

spacecadet 3y ago
Not push back to me. As an "analog", I love SDR-SOC but what this is saying is that people may lack a fundamental knowledge when designing hardware devices that is contributing to these challenges. Id also push back against the article in the sense that I think the industry shot itself in the foot trying to provide too many options that all overlap. But, I do agree with the sense that minimizing the radio architecture to only what it needs to operate is smart.
- mwbajor 3y ago
  Not only that, but it also stems from "management" or "customers" unable to contemplate tradeoffs in the design. The reason FPGAs and RFSOCs are pushed so much in certain industries (we all know which one) is because the customer wants everything reconfigurable because they can't make decisions now, or in most cases, not competent enough to make decisions now. However, they are OK with buying a $25K+ FPGA or RFSOC which is why the market is there and also why few understand the actual art of RF design and radio architecture anymore. RF budgeting is almost non-existant. Most Jr. engineers have never seen a level diagram and the only places you will find them is in old watkins johnson app notes. SDR design is ultimately a race to the bottom in most cases in terms of good design but not necessarily cheap design.

cushychicken 3y ago

Man I love ADI’s tech notes. They’re some of the best in the business at technical marketing.

Scene_Cast2 3y ago

Here's a super crude ELI5, feel free to correct me. Signals crash course: Radio comms have a channel they need to fit into (let's say 1.1GHz - 1.2GHz as an example). This means that what you send over the air is basically a high frequency (~1.15GHz) sine wave. You encode data by slowly varying this sine wave (amplitude and/or frequency). The variations are going to be super slow, as fast changes in either amplitude or frequency will make your signal go outside the frequency limits. It's only the sine wave (aka the "carrier frequency") that's high frequency (1.15GHz in our case). There are several ways to digitize this signal. You can do "direct RF sampling" - you need a very high sampling rate (2.4GHz+ due to Nyquist in our case) and just sample the high frequency signal. You can multiply (in analog domain) the RF signal from the antenna by another frequency to keep all the slow changes but change the carrier frequency. This means that you don't need such a high lower sampling rate (you can get away with much less than GHz). If your new carrier frequency is just some low but non-zero frequency, that's super-heterodyne (IF). If it's zero, that's Zero-IF.

mwbajor 3y ago
Yes all correct, you didn't capture any of the tradeoffs but thats the basic idea.
gorkish 3y ago
Just have to say this is the first time I've seen someone introduce the concept of an RF mixer "backwards".
kurthr 3y ago
I was surprised they didn't use the word homodyne. One issue they didn't mention, but probably shows up in the noise figures is that homodyne receivers suffer from 1/f noise in the relatively wide band "direct conversion" amplifiers. Of course all of the heterodyne receivers suffer from 1/f up-conversion at the mixer.
- teleforce 3y ago
  It is rather a soup of terminologies namely homodyne, zero-IF, and direct conversion architecture are all referring to the same thing. It seems they are now settling on RF sampling for the newer transciever architecture technology but sometime direct RF is also being used for example this latter terminology is referred by Intel for their new line of FPGA and RF transceivers product integration.

spr-alex 3y ago

I am looking for PCB designers with RF experience. If this is what you're good at, please reach out

teleforce 3y ago

For both RF sampling and Zero-IF the two of the most popular analog manufacturers are TI and Analog Devices. It's rather intriguing that for RF sampling technology, Intel has recently join the bandwagon and called it a game changing technology when the RF sampling is integrated with high speed FPGA [1]. Intel Delivers a Game-Changing, Analog-Enabled Direct RF FPGA Portfolio: https://community.intel.com/t5/Blogs/Products-and-Solutions/FPGA/Intel-Delivers-a-Game-Changing-Analog-Enabled-Direct-RF-FPGA/post/1417363 https://community.intel.com/t5/Blogs/Products-and-Solutions/...

James2233 3y ago

[dead]