DC to RF...starting where?

Chris Gammell

Analog Life, LLC

Presented at CCCamp 2019

Goals for this talk

  • Explain how I entered the world of working with RF
  • Explain it without burying you in math
  • Give you some resources to make your own way

Who are you?

Electronics experience?

RF experience?

Ham radio experience?

You may have more experience I do!

I wanted this talk to help shortcut some of the struggles I had (and have) when I was getting started with the field of RF.

Who am I?

Chris Gammell

  • 15 years of electronics design. 
  • Have worked in a couple industries
    • Semiconductor
    • Test and measurement
    • Industrial controls
    • Component sourcing
  • Co-host of The Amp Hour electronics podcast (https://theamphour.com)
  • Teaching people electronic design online for the past 5 years as part of Contextual Electronics

Started by working on these

10 PLC = 6 readings per second

Not exactly blazing speed

Higher speed stuff like digital was always there as well, but usually nothing very high speed either (<100 MHz)

So what changed?

RF is everywhere

Every product, that is

RF is everywhere AND in every product

And it's cheap

I assume we have similar goals

You’re interested in making things

You're excited about cheap connected (RF) hardware

And you wonder about how to actually integrate it.

How I got started

I copied the app note

This is actually pretty common

I'd guess at least 80% of most PCB designs are re-implemented app notes

Followed other guidelines in Mike Ossmann's "5 Rules of RF design"

(delivered at Supercon 2015)

https://www.youtube.com/watch?v=TnRn3Kn_aXg

My first RF Design

It worked!

Why?

Why?

  • The modem and the antenna were pre-matched
  • I minimized my interaction with it (kept the RF trace short)
  • Because I didn't need to take it to production

Can we do this all the time?

Maybe!

It depends on your design goals

If you have a completely uncharacterized antenna and source, you are going to have to work a lot harder.

The complexity of your circuit will also have a large impact on the likelihood of success.

Reasons I wanted to learn more about RF

Troubleshooting

What happens when things go wrong on the bench?

FCC / CE compliance testing

What happens when things go wrong at the $10K/day test lab?

Being a good RF citizen

Your signals might interfere with someone else's signals and that's not nice.

RF concepts that are tricky

Especially for beginners

We're dealing with things in the frequency domain.

When someone talks about "the spectrum" of a signal

They are asking about the frequency content contained within a signal that exists in the real world.

http://tiny.cc/RigolFFTplot

http://tiny.cc/

TekSE_FFTAnimation

Components act differently based on frequency

Capacitors

  • Block frequencies from "passing through" at low frequencies.
  • Allow frequencies to "pass through" at high frequencies
  • Where this transition happens depends on the capacitance of the component.

Inductors

  • Allow frequencies to "pass through" at low frequencies.
  •  Block frequencies from "passing through" at high frequencies
  • Where this transition happens depends on the inductance of the component.

Logarithmic scales

 

You're going to see units like "dB", or "dBm" which an easy way to refer to things that change in value by orders of magnitude

Logarithmic scales

 

As an added bonus: 

Gains add together

 

It's all about the power

 

Most RF circuits deal in power, not in just voltage or current

Analyzing RF subsystems is often about minimizing the degredation of the signal through the system

 

This is referred to as the "link budget"

What does the RF path look like?

Image courtesy of osmocom.org

We need to ensure all other parts of the system introduce minimal degradation and noise

If a link budget is 100 dB, that means the received signal is 1/100000th as much power as the transmitted signal

Impedance matching ensures we don't introduce signal degradation

Maximum power transfer happens when a source and a load are perfectly matched

A counter example:

 

What happens when there is a source (like an ESP32) and an antenna (like a PCB antenna) that are not perfectly matched?

Unmatched source and load

  • It may not work at all
  • You won't transmit as far as you thought
  • Your system will be less efficient
    • Energy is "bounced back" and is usually lost
      to heat or radiated emissions (bad)

What is impedance though?

 

Impedance refers to how a device passes or blocks electromagnetic energy at various frequencies. 

Why do we need to do matching at all?

Isn't the antenna delivered to work at 50 ohms?

  • The world is imperfect!
  • Environmental conditions can affect it, including things like the enclosure or thing surrounding the antenna

  • Antenna manufacturing variations means you might have different specs than stated

     

Pi network

http://tiny.cc/mo0mbz

"Match a 1000-Ω source to a 100-Ω load at frequency (f) of 50 MHz. You desire a bandwidth (BW) of 6 MHz."

Pi network

http://tiny.cc/mo0mbz

Measurement Tools

Spectrum Analyzer

Spectrum Analyzer

http://tiny.cc/RigolSA

VNA

€€

[€€€....€€]

Cal Kit

omlinc.com

Measurements

Smith chart

This is actually a measurement tool, which plots various measurements

Smith Chart

Image courtesy digikey.com

S-parameters

  • S11 - Return loss
  • S21 - Insertion loss
  • S12 - Power transfer from P1 -> P2
  • S22 - Reflected power P2 back toP1

Return loss (S11)

 

Also known as "reflection coefficient"

Return Loss

VSWR

VSWR

  • This stands for the "Voltage standing wave ratio"
  • Measures how well the antenna is impedance matched to the source that is radiating RF energy.
  • It is measuring the matched characteristics, not that it necessarily does.
  • VSWR defined by the equation below, including gamma (Γ), which is the S11 parameter shown earlier

Image courtesy antenna-theory.net

What "DC" assumptions fall apart at higher frequencies?

  • A wire is just a wire

  • PCB material isn't as important as the components on board

  • A capacitor is there for charge storage

  • Current can be isolated by ground cuts

  • Power planes are always a good idea

"A wire is just a wire"

Actually every wire is an inductor

http://tiny.cc/Inductor_QR

What does this mean for our circuit?

Simple example:

Past 10 MHz, breadboard signal quality falls off

Image courtesy makerspaces.com

"PCBs construction isn't as important as the components placed on that PCB"

4 Layer Stackup

The PCB stackup impacts every piece of your signal pathways, including the impedance/inductance/capacitance of each stripline.

More complex PCB example:

In the GHz range, the small inductance of a PCB trace can have outsized effects on your signal

  • Track width: 0.1 mm (3.94 mil)
  • Track length: 50 mm (1.97 in)
  • Height above GND: 0.2 mm (7.87 mil)
     
  • Track inductance:  63.2 nH
  • Impedance @ 2.4GHz: 953 ohm

http://tiny.cc/AAC_impedance

Ground plane should always be the layer below where your signal is running

Understanding your stackup is critical for treating your PCB as part of your circuit

https://tiny.cc/Stackup_JLC

"A capacitor is there for charge storage"

Image courtesy of Wikipedia

Image courtesy of iFuture Technology

http://tiny.cc/KemetCapPDF

http://tiny.cc/KemetCapPDF

"Current can be isolated by ground cuts"

Noise reduction techniques in low level analog involves cutting the ground plane to stop noise from "leaking" over

https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html

At low frequencies, the cutout means that the signals won't "get around" the cut, effectively shielding the isolated areas

High speed signals will treat this area as a capacitor and effectively "travel through" to the other side.

They often radiate energy while doing so. This is how we make PCB antennas (!)

"Current follows the path of least resistance"

The signal actually cares about the path of least impedance.

At DC, this is the path of least resistance.

http://tiny.cc/ECE_SE_highspeed

In higher speed signals, the impedance takes on more complex terms and the inductance starts to matter more.

The magnetic fields between two signals flowing in opposite directions cancel out

This means the inductance will be lowest directly below the signal path and the signal will preferentially flow black on the ground plane

http://tiny.cc/EIU_edu_wires

What about Bluetooth, Cellular, Wifi, LoRa, _______, etc, etc?

All of these communication methods are different versions of the same fundamentals

(and most are really brand names)

  • Bluetooth - 2.4 GHz
  • WiFi - 2.4 GHz and 5 GHz
  • LoRa, SigFox
    • 433 MHz (Global)
    • 915 MHz (US)
    • 868 MHz (Europe)
  • GSM Cellular
    • 900, 1800 MHz (Europe, Asia)
    • 850, 1900 MHz (US)
  • 3G, LTE, 5G
    • Various frequencies (see:  http://tiny.cc/cellular_freq)

Resources

Books

  • "A Practical Guide To RF And Mixed Signal Printed Circuit Board Layout" -  Brendon Parise and Scott Nance[1] 

  • "RF Circuit Design" - Christopher Bowick [2]
  • "Planar Microwave Engineering" - Thomas Lee [3]

[1]: https://amzn.to/2ZdnUzm

[2]: https://amzn.to/2HmQwey

[3]: https://amzn.to/2HnSWcY

YouTube

Development kits/programs

  • RTL-SDR + GNU Radio
  • GSG HackRF
  • ADI Pluto

Many thanks to Jeff Keyzer (@mightyohm) for helping create these slides!

 

Thanks to Derek Kozel (@derekkozel) for reviewing this presentation

Thank you!

 

Twitter: @Chris_Gammell

E-mail: chris@analoglife.co