SDR Based Test Equipment Beric Dunn, K6BEZ

1

About Me

Beric Dunn K6BEZ

G7JUO in late 80s

K6BEZ since 2011

Interests are QRP & Building Stuff

RF Design Engineer for 19 years

22017 K6BEZ

Background

“Build your own Antenna Analyser for under $20”

DDS based HF Antenna Analyser

2013 Pacificon

32017 K6BEZ

Background

Ham Radio 360 Workbench build project

Group build project

Intro to using Arduino

www.360workbench.com/

42017 K6BEZ

Limitations

Limited Frequency Range

HF Only

Sensitive to strong nearby signals

Detector has no selectivity, has no tunability

52017 K6BEZ

Improve Frequency Range

Replace DDS with higher frequency module

Bridge/Detector circuit should work OK for 2m

Need Surface Mount for higher frequencies

62017 K6BEZ

That Sensitivity Issue

Filter strong signal

AM band-block

What if strong signal is in ham bands?

Make test signal stronger

Add amplifier

What other similar products like MFJ-259 do

72017 K6BEZ

A Different Approach - Tuned Receiver

The “$20 RTL Dongle” you might already have

Linear receiver with over 40 dB of dynamic range

25 MHz - 1.8 GHz frequency range

Lower with “Direct Sample” mode

Can be purchased for under $10

http://www.rtl-sdr.com/rtl-sdr-quick-start-guide/

82017 K6BEZ

GQRX

Cross Platform

Can show 2 MHz spectrum chunk

from 25 MHz-1.8 GHz (with RTL-

SDR)

Can decode AM/FM/SSB...

Can’t show 2m band in one window

92017 K6BEZ

More Bandwidth?

Enter RTL-Power

Step the RTL-SDR over freq range

Command line tool:

rtl_power -f 24M:1.7G:1M -g 50 -i 15m -1 noise.csv

2014-07-27, 23:15:29, 127000000, 129500000, 4882.81, 240, -30.74, -30.91, -29.60, -30.99, -30.80, -30.47, -31.04, -

30.57, -30.60, -30.65, -30.81, -30.02, -30.29, -30.59, -30.22, -29.96, -30.23, -29.96, -29.69, -29.79, -29.23, -29.92, -

29.46, -29.12, -29.54, -29.00, -29.04, -28.89, -27.95, -28.44, -28.18, -28.05, -28.38, -28.32, -28.16, -28.21, -27.59, -

27.80, -27.93, -27.50, -27.79, -27.43, -26.96, -27.34, -27.47, -27.52, -27.04, -27.15, -26.20, -26.05, -27.28, -27.15

102017 K6BEZ

GUI wrapper for rtl_power

QSpectrumanalyzer (Linux)

https://github.com/xmikos/qspectrumanalyzer

Spektrum (Linux, Windows)

http://www.rtl-sdr.com/spektrum-new-rtl-sdr-spectrum-analyzer-software/

GUI for RTL Power (Windows)

https://sourceforge.net/projects/guiforrtlpower/

112017 K6BEZ

What about the other half?

Most analysers have a signal source and a detector

Have the detector (RTL Dongle)

What about a signal source?

DetectorSignal

Source

Device

Under

Test

RTL-DongleFilter/Antenna?

122017 K6BEZ

Bring The Noise

Natural enemy of Amateurs everywhere

Other than HOAs

Wide-band unwanted signal

Can it be used for good?

If we tune the detector, noise can become a useful signal source

132017 K6BEZ

Noise Source

“BG7TBL”

12V in

Noise Out

$20 on Ebay

142017 K6BEZ

Noise Source

152017 K6BEZ

Filter Measurement

Filter passes noise in passband and rejects noise elsewhere

162017 K6BEZ

Filter Measurement

No Filter With Filter

172017 K6BEZ

Directional Coupler

The last piece of or puzzle

Taps of signal in 1 direction but not the other

182017 K6BEZ

How can we use it?

Antenna measurements

Tap off reflected power

Dip in the noise shows where antenna is tuned

192017 K6BEZ

Where do I get one?

ZEDC-10-2B / ZFDC-10-5

$50 from minicircuits, cheaper on ebay

Check frequency range

Check Impedance

50 Ohms NOT 75 Ohms

Can get surface-mount part for under $10

202017 K6BEZ

Calibration

Remove slope from noise source so we are only measuring DUT

Take reference ‘thru’ sweep

Subtract this from measured data

Can do this in excel from RTL_POWER data

Slow, and not real-time

Or….

212017 K6BEZ

Automate it!

Python - has libraries for everything - even RTL-SDR

Pyrtlsdr (https://github.com/roger-/pyrtlsdr)

Script it to Calibrate, measure and plot

python sdr_sweep.py <f_start> <f_stop> <number_of_steps>

222017 K6BEZ

How Well Does it Work?

Demonstrate with Filter and Antenna

ADS-B 1090 MHz Band Pass Filter

Dual Band Antenna

232017 K6BEZ

Filter Measurement

1090 MHz Band Pass Filter

“passes 980 MHz – 1150 MHz

a 40dB drop outside the pass band”

242017 K6BEZ

Filter Measurement - Results

“passes 980 MHz – 1150 MHz

a 40dB drop outside the pass band”

252017 K6BEZ

Results - Antenna Measurement

Ed Fong J-Pole

2m & 70cm

On roof of my house through about 25’ of RG-8X

262017 K6BEZ

Results - Wide Sweep

Dips at about

150 MHz

450 MHz

272017 K6BEZ

Results - Zoom in on 2m

$50 (SDR+Noise+Coupler) $15000 Anritsu SiteMaster

282017 K6BEZ

Results - Zoom in on 70cm

292017 K6BEZ

$50 (SDR+Noise+Coupler) $15000 Anritsu SiteMaster

Next Steps

Modify Derek AG6PO’s code for the Ham Radio 360 Analyzer to include this

Run on Raspberry Pi with touch-screen

Try to get working with Direct Sample mode (HF!)

Make PCB with noise source and coupler built in…..?

302017 K6BEZ

Thank You!

Any Questions?

Presentation Materials: https://k6bez.blogspot.com/

k6bez@arrl.net

312017 K6BEZ