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

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

Not exactly blazing speed

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

My first RF Design

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?

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

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

Impedance matching ensures we don't introduce signal degradation

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?

• 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

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

http://tiny.cc/RigolSA

€€

[€€€....€€]

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"

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

Simple example:

Past 10 MHz, breadboard signal quality falls off

Image courtesy makerspaces.com

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

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

"Current follows the path of least resistance"

At DC, this is the path of least resistance.

http://tiny.cc/ECE_SE_highspeed

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

• GSG HackRF