White Paper I BNC
Return loss measurements onconnectors for high-resolution
video signals

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Content
1 Abstract ............................................................................................................................................ 3THEORY.................................................................................................................................................. 32 Return Loss ...................................................................................................................................... 3
2.1 What Is Return Loss? ............................................................................................................... 3
2.2 About Return Loss ................................................................................................................... 3
3 How to measure Return Loss up to 18 GHz ................................................................................. 4
3.1 Challenges ................................................................................................................................ 4
3.2 Principle - Time Domain Gating (TDG) ................................................................................... 4
MEASUREMENT – TDG METHOD ........................................................................................................ 54 Return Loss Measurement up to 18 GHz ...................................................................................... 54.1 Measurement Setup & Condition ........................................................................................... 5
4.2 Measurement Results “non-gated” in frequency and time domain ................................... 5
4.3 Conclusion & Results................................................................................................................ 6White Paper – BNC
Title: NWP06© NEUTRIK AG. All rights reserved.
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NEUTRIK AG reserves the right to make changes at any time to thetechnology and / or configuration without announcement.

2

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ABSTRACT

Currently, data transport of high definition serial
digital interface (HD-SDI) signals at 3 Gb/s is stateof the art and standardized by the Society ofMotion Picture & Television Engineers (SMPTE).
For return loss and voltage standing wave ratio(VSWR) measurements, suitable equipment isavailable. With upcoming ultra high definition(UHD) signals and related high frequencies, thediscrepancies between available 50 Ω measuring
equipment and defined 75 Ω video standards
increase.
This paper explains the influence and importanceof return loss in a video system and shows asuitable test method which Neutrik believes isthe best for high frequencies. Further return loss
measurements compare Neutrik`s new rearTWISTUHD BNC connector, which is specifically designed
for high frequencies, with UHD optimized
connectors from common manufacturers up to18 GHz (ST 2082-1).

signals with high bandwidth (e.g. UHD) andresulting high frequencies, it can be a criticalfactor. In addition also the VSWR is a functionof the reflection coefficient, which describes thepower reflected from the end of the system and isanother representation of the return loss.

2.2 About return loss
With the transition to UHD signals, including
4K and 8K signals with their higher data rates
up to 24 Gb/s (ST 2083-1 pending) and related
in-creased clock frequencies, the impedance of
BNC connectors becomes more important thanever. Every impedance deviation has a negative
influence on the return loss and VSWR values.
In order to maintain the square waveform of digital
signals harmonics (table 1) up to the 5th ordershould be considered. Taking the 3rd harmonic
as a limit represents a reasonable compromise
between signal waveform and reliable test results.

DATA RATETHEORY
2

RETURN LOSS

3rd HARMONICS

APPLICATION

3.0 Gb/s

4.5 GHzHD acc. ST 424

6.0 Gb/s

9.0 GHz

4K acc. ST 2081-1

12.0 Gb/s

18.0 GHz

4K acc. ST 2082-1

table 1

2.1 What is return loss?
Return loss is signal attenuation caused byim-pedance variations in the structure of a cableor associated connection parts like Bayonet Neill–Concelman (BNC) connectors. These variations
cause some part of the signal to reflect (return)back to the source. At lower frequencies, returnloss is a minor effect; at frequencies above 50 MHz
it can have a significant impact on the signal
performance and the transmitted distance. ForWhite Paper – BNC

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HOW TO MEASURE RETURNLOSS UP TO 18 GHZ

3.1 Challenges
As mentioned, for high definition signals (HD) up
to 3 GHz, there are suitable return loss and VSWR
measuring devices available. For 4K and 8K signals
with related high frequencies (table 1), there is
only some dedicated 75 Ω return loss measurement equipment and related calibration kits available, but with limited frequency range (see references No 2.). 50 Ω based test instru-ment systems
have to be used to carry out measurements in therequired extended fre-quency range. The Time
Domain Gating (TDG) method can be used as analternative to the conventional S11 (ST2081-1 &ST2082-1) measurement procedure on frequency
domain in order to avoid calibration uncertainties
due to components (e.g. adapters, matching pad,etc.) which are not specified for higher frequencies. The major advantage of TDG is an easier visualization and localisation of impedance deviations.

3.2 Principle - Time Domain Gating (TDG)a) The device under test (DUT) is connected to
the network analyser via a short adapter cable.
This increases the distance between the test
port, any existing adapters and the DUT,
which enhances the distinct identification from
impedance deviations.
b) The measured signal is transformed from frequency to time domain (figure 1 & 2). Similar
to a conventional Time Domain Reflectometer(TDR), all impedance deviations from DUT
components (e.g. BNC connector, cable, etc.)
can be clearly identified.

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figure 1c) A gate (figure 2) with defined start and end
points is positioned on the DUT component(e.g. BNC connector, cable, etc.) which has tobe measured.

Gatefigure 2d) Current return loss values inside the gate aretransformed back from the time domain to thefrequency domain (figure 3), representing thereturn loss from a defined position within theDUT.

figure 3

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MEASUREMENT – TDG METHOD
4RETURN LOSS MEASUREMENTUP TO 18 GHZ

The following return loss measurement demonstrates the functionality of the TDG method byan ex-ample and compares the performance ofNeutrik`s new rearTWIST UHD BNC with wellknown connectors from US and French manufacturers, offering UHD-optimized BNC connectors,
in order to provide a verifiable reference of commonly used products in the professional broadcast
industry.

4.2 Measurement results “non-gated”
in frequency and time domain
The measured return loss values are transformed
from the frequency domain to the time domain.
The gate (figure 5) is manually positioned (table 3lists used gate parameters) to measure the BNC
con-nector and blend out the impedance of thecable, analyser, etc.
PARAMETER

VALUE

Spam

5 ns

Zero point

5,5 nsEnd point

8 ns

table 3

4.1 Measurement setup & condition
Figure 4 and table 2 show the measuring setup
including network analyzer, adapter cable, DUT
and a 75 Ω load for the return loss measurement
via TDG method.

Gatefigure 5

figure 4ITEM

TYPE

Network Analyser

HP 8722D

Adapter cable

SMA terminated with Belden 1694A(0.8 m length)
and a BNC female jack on the other side

COAX cable DUT

Belden 1694A (8 m)

BNC connectors I

Neutrik rearTWIST UHD BNC
NBNC75BTU11X

BNC connectors II

Common French manufacturer

BNC connectors III

Common US manufacturer

table 2White Paper – BNC

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4.3 Conclusion & Results

Figure 6 represents the optimum measuring
setup with minimum connection points toavoid additional adulteration from the returnloss values. In reality, real broadcast installations
contain multiple BNC connection points whichhave significant influence on the return loss
performance. Therefore sufficient distance(headroom) to the UHD limit is essential toguarantee appropriate signal transmission.

The goal was to receive verifiable return loss
characteristics of BNC connectors, measured witha suitable test method for high frequencies up to18 GHz (ST 2083-1). Due to limited calibration
kits for 75 Ω systems for high frequencies theTDG method (section 3.2) has been chosen tomeasure the return loss of the connector only,
suppressing the impedance deviations of the cableand adapters.

Neutrik’s new rearTWIST UHD BNC achieves
increased headroom compared to the connectors
from the US or French manufacturers andoffers additional return loss reserve for potential
impedance de-viations resulting from cablebending, incorrect connector assembly or faulty
connection interfaces without signal interruption.

To compare the new rearTWIST UHD BNC,
specifically UHD optimized connectors from
common manufacturers were used as reference.
All BNC connectors indicate low return loss values
and sufficient headroom to the required UHD
limit (figure 6) over the entire frequency range up
to 18 GHz (ST8083-1). Therefore all 3 connector
types are suitable for high resolution video signaltransmission.

0UHD-LIMIT

Return Loss [dB]-10

ST2594
ST292-1

ST424

ST2083-1

ST2082-1

ST2081-1

Headroom

-20

-30

-40

BNC Connector - Manufacturer 1
BNC Connector - Manufacturer 2
Neutrik rearTWIST UHD - BNC Connector-50-6005

10Frequency [GHz]figure 661520

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References / Literature
1 Neutrik BNC White Paper “Optimum Performance of Neutrik rearTWIST 75 Ω BNC-Connectors in HD 1080p applications”
2 Samtec “Characterizing Non-Standard Impedance Channels with 50 Ohm Instruments”
3 Belden “High-Definition Cabeling and Retrun Loss”
4 Axon “Adventures In Return Loss”

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USA
Neutrik USA Inc., 4115 Taggart Creek Road, Charlotte, North Carolina, 28208
T +1 704 972 30 50, info@neutrikusa.com
Japan
Neutrik Limited, Yusen-Higashinihonbashi-Ekimae Bldg., 3-7-19
Higashinihonbashi, Chuo-ku, Tokyo 103
T +81 3 3663 47 33, mail@neutrik.co.jp
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Fotan, Shatin
T +852 2687 6055, neutrik@neutrik.com.hk
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T +86 574 88250488 800, neutrik@neutrik.com.cn
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Neutrik India Pvt. Ltd., Level 3, Neo Vikram, New Link Road,
Above Audi Show Room, Andheri West, Mumbai, 400058
T +91 982 05 43 424, anklesaria@neutrik.com
Associated companies
Contrik AG
Steinackerstrasse 35, 8902 Urdorf, Switzerland
T +41 44 736 50 10, contrik@contrik.ch
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Felix-Wankel-Straße 1, 85221 Dachau, Germany
T +49 08131 28 08-0, info@adam-gmbh.de

www.neutrik.com®

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T +33 1 41 31 67 50, info@neutrik.fr®

Great Britain
Neutrik (UK) Ltd., Westridge Business Park, Cothey Way
Ryde, Isle of Wight PO33 1 QT
T +44 1983 811 441, sales@neutrik.co.uk

BNC White Paper 2015/11 V2 - NWP06 - Data subject to change without prior notice. © 2015 NEUTRIK . NEUTRIK are registered trademarks of NEUTRIK AG. ALL RIGHTS RESERVED.

Germany / Netherlands / Denmark / Austria
Neutrik Vertriebs GmbH, Felix-Wankel-Strasse 1, 85221 Dachau, Germany
T +49 8131 28 08 90, info@neutrik.de

BNC WHITE PAPER

Liechtenstein (Headquarters)
NEUTRIK AG, Im alten Riet 143, 9494 Schaan
T +423 237 24 24, F +423 232 53 93, neutrik@neutrik.com