Handheld testing

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Handheld DAB Radio Testing 2nd July 2014
Test report
© DTG Testing Limited
4511
5th Floor
89 Albert Embankment
London
SE1 7TP
United Kingdom
Handheld DAB radio testing / DRAFT report
Executive Summary
In order for handheld radios that use the headphones as antennas to be able to apply for the DAB
tick mark, an agreed method for testing minimum RF sensitivity of these products is required. Any
test method that is agreed needs to be able to provide results that allow objective testing to pass
certain criteria as well as being repeatable and cost effective to set up.
To investigate a range of potential test setup options for handheld radio testing, DTG organised a
test day using their GTEM cell. The test day looked at three test methods with the aim being to
compare the results of each and decide on the best way forward.
The three test methods investigated were:
-
using a test dummy with headphone inputs to replicate a human body wearing the
headphones;
using a wooden frame (stick man) to hang the headphones on to replicate typical
dimensions of a human wearing the headphones; and
using volunteers to test the radios inside the radiated chamber.
For each of the test methods, the minimum RF sensitivity of three handheld DAB radios was
measured over different positions replicating the user having the radio at waist height or at chest
height as well as having the entire test setup moved in different positions around the GTEM cell. The
DAB channel used during the measurements was 5A (174.928MHz) and the RF sensitivity was
assessed using the 1kHz tone onset of impairment method as used in the DRAP TEG minimum
specifications.
The conclusions from the testing were:
-
-
-
© 2014
the minimum RF sensitivity results when using the test dummy failed to meet the
requirements specified in the minimum spec for all of the radios tested. This would indicate
that the test dummy was either not representative of a human body or that the microphone
and speakers used to relay the headphone audio within the test dummy caused
interference;
testing using a volunteer in the GTEM cell produced consistent results between volunteers,
albeit over the small number of tests that were carried out;
testing using the stick man produced results that passed the requirements in the minimum
spec and that were generally consistent although with some outliers. The results were worse
than when a volunteers was inside the GTEM cell so may need to be given greater allowance
when being tested; and
testing using the stick man method was only consistent when the headphones were fully
extended. When the headphones were coiled the measurements were not repeatable and
differences of up to 18dB between measurements of the same radio were observed.
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The next steps are to define the criteria for the stick man setup so that it can be repeated amongst
test houses and to carry out further measurements over a wider range of DAB channels to build up a
set of results so that they can be considered statistical.
One proposal so that manufacturers of handheld radios can apply for the tick mark would be to use
the stick man method initially and any radios that are failing could be re-checked using volunteers in
the test chamber. This would avoid further delays for handheld radio manufacturers whilst allowing
a set of measurements to built-up as well as providing a cross reference between the stick man
method and using a test volunteer.
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Contents
Executive Summary................................................................................................................................. 2
1)
Introduction .................................................................................................................................... 5
2)
Test method .................................................................................................................................... 5
2.1
Test dummy ............................................................................................................................ 6
2.2
Test volunteer ....................................................................................................................... 12
2.3
Stick man ............................................................................................................................... 17
3)
Results ........................................................................................................................................... 20
4)
Conclusions ................................................................................................................................... 23
Version History
Report
Version
DRAFT
© 2014
Date of issue
Comments
Author
12/09/2014
Handheld DAB radio test report
[AB]
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1) Introduction
A test day was held at the DTG on the 2nd of July to trial a range of methods for testing handheld
digital radio receivers. The methods of testing, results and conclusions are presented in this
summary. The test day was attended by Gerry Thorn from Roberts, Yufei Ren from Pure, Rob Smith
from Frontier Silicon, Sam Bonham and Laurence Harrison from DRUK and Tony Murphy from
Phonak.
The DTG facilities include a GTEM 2000 which is used for radiated testing during DAB tick mark
conformance tests and which was used to trial the various test methods. The aim of the test day was
to provide results for minimum RF sensitivity measurements which can be assessed at the DRUK
approved product group meetings in order to decide on a test methodology for the tick mark.
2) Test method
The general test setup for DAB tick mark testing in the DTG GTEM cell is shown in Figure 2.1
Figure 2.1 Generalised digital radio receiver configuration in GTEM cell for DAB tick mark testing
For the handheld testing, the radios were place in the test area as indicated by the red square in
Figure 2.1. Three test setups were then assessed: one using a test dummy with in ear microphones
used to relay the audio from the DAB headphones, one using volunteers inside the GTEM cell who
could carry out the onset of impairment tests from inside the GTEM cell and one using a wooden
cross (stick man) with the headphones attached at a separation distance that could be expected
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when a portable radio is worn on a person. For the latter a small microphone attached to one of the
headphones was used to relay the audio outside the GTEM cell for assessment. For each setup the
minimum RF sensitivity of three handheld DAB receivers was measured by tuning the radios into a
1kHz test tone on DAB channel 5A (174.928MHz) and reducing the signal level until the tone started
to break up. The onset of impairment method as described in the DRAP TEG minimum specifications
was used as the criteria for minimum sensitivity1.
For the test dummy and volunteer setups, the DAB receivers were tested in in four different
orientations:
1) With the receiver at waist level with the dummy or volunteer facing the front of the GTEM
cell.
2) With the receiver at waist level with the dummy or volunteer facing the rear of the GTEM
cell.
3) With the receiver at chest level with the dummy or volunteer facing the front of the GTEM
cell.
4) With the receiver at chest level with the dummy or volunteer facing the rear of the GTEM
cell.
The idea behind this was to check the variation in the sensitivity caused by the position of the body
and also the position of the radio around the body.
For the stick man setup, the receiver was tested in a further six orientations:
1-3) With the headphones stretched to maximum length and the stick man moved in 3
different positions within the GTEM cell (position 1 was the recommended test area of the
GTEM cell, position 2 was 10cm to the right of this then position 3 was 10cm further forward
of this)
4-6) With the headphones coiled clockwise around a polystyrene cup to leave an identical
length of cable going vertically to the stick man for each receiver and then with the stickman
moved in the same positions as 1-3.
The aim of this was to ascertain the method of positioning the headphone and cable that produced
the most consistent results and also to measure how the results vary when the radio is not kept in
the same position in the GTEM cell.
2.1
Test dummy
The test dummy used was provided by Phonak and was a model called Kemar made by G.R.A.S
Sound & Vibrations. Phonak use the dummy to test hearing aids and as such it has built in ear
simulators that can detect audio and provide an output that can be accessed via a BNC connection at
the back of the dummy.
1
DAB Technical docs: http://www.getdigitalradio.com/industry/technical-documents/
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The dummy consisted of a head and torso so needed to be elevated in order to represent a similar
height to a human sitting down. As such it was setup inside the GTEM cell on a set of foam blocks
which were set at a fixed position and height. This ensured that the testing was repeatable between
measurements.
The testing was carried out with the dummy facing the front of the GTEM cell then repeated with
the dummy rotated 180° to face the rear of the GTEM cell. This was to assess whether the position
of the body in comparison to the direction of the generated DAB RF signal impacted the results i.e.
whether the test body impacted the results.
The testing was repeated for each dummy position with the radio attached to the chest of the
dummy to replicate a user having the radio in his or hers shirt pocket, then with the radio placed on
the foam blocks to replicate a user having the radio in his or hers trouser pocket.
Figures 2.1.1 to 2.1.4 show a picture of each test scenario.
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Figure 2.1.1 Test dummy facing front of GTEM cell with radio at waist height
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Figure 2.1.2 Test dummy facing front of GTEM cell with radio at chest height
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Figure 2.1.3 Test dummy facing rear of GTEM cell with radio at chest height
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Figure 2.1.4 Test dummy facing rear of GTEM cell with radio at waist height
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2.2
Test volunteer
For this set of testing, two volunteers were used The volunteer was sat in the GTEM cell on a
wooden stool which was setup so that the headphones were at the same height as when the dummy
was being used for the testing. The testing was done with the volunteer in the same orientations as
was tested for the dummy so direct comparisons could be made between the RF sensitivity results
for each.
Figures 2.2.1 to 2.2.4 show a picture of each test scenario.
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Figure 2.2.1 Test volunteer facing front of GTEM cell with radio at waist height
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Figure 2.2.2 Test volunteer facing front of GTEM cell with radio at chest height
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Figure 2.2.3 Test volunteer rear front of GTEM cell with radio at chest height
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Figure 2.2.4 Test volunteer facing rear of GTEM cell with radio at waist height
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2.3
Stick man
The stick man was made of a wooden tripod with adjustable legs to allow the height to be altered. A
wooden fixing was used to attach the headphones to and then a small battery operated microphone
was attached to the fixing next to one of the headphones in order to relay the 1kHz test tone from
the headphones to outside of the GTEM cell. Outside of the GTEM cell the microphone was
connected to the aux input of a receiver and the sound was then listened to using a set of noise
cancelling headphone connected to the receiver.
The receivers were tested with the headphones fully extended and with the headphones coiled up
and with the stick man moved in different positions within the GTEM cell.
Figures 2.3.1 to 2.3.2 show receivers being tested with the headphones extended and coiled.
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Figure 2.3.1 Stick man with headphone lead fully extended
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Figure 2.3.2 Stick man with headphone lead coiled
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3) Results
The minimum RF sensitivy results are compared for four of the test scenarios below, namely:
-
using a test dummy in the GTEM cell;
with volunteer1 in the GTEM cell;
with volunteer2 in the GTEM cell; and
using the stick man with the headphone cable fully extended
The test method using the stick man and the cable coiled has not been compared in this section as
the results varied by as much as 18dB between tests of the same radio on the same channel so it
was quickly discounted as a practical method of testing.
Note also, for volunteer 2, only tests were carried out on one of the test receivers due to time
limitations.
(NOTE for each graph the black line represents the RF sensitivity requirements from the minimum
specifications – approximately 36dBµV/m once the GTEM measurement uncertainty is taken into
consideration)
Results using the test dummy
Figure 3.1 Minimum RF sensitivity results using the test dummy
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Results with volunteer 1 in the GTEM cell
Figure 3.2 Minimum RF sensitivity results with volunteer 1 in the GTEM cell
Figure 3.3 Minimum RF sensitivity results with volunteer 2 in the GTEM cell
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Figure 3.4 Minimum RF sensitivity results with stick man in the GTEM cell
As can be seen the results for the test dummy are some distance above the minimum sensitivity line
and all of the test receivers would have failed to meet the requirements for all of the test
orientations. This shows that the test dummy can be discounted as a method of testing as it does
not provide a good replication of when a person is wearing the headphones.
When using the volunteers the results are consistent for each test orientation and between the
volunteers. When the volunteer was facing the rear of the GTEM cell the results worsened for each
receiver and when facing forward the results were generally within a dB whether the receiver was at
waist height or chest height across all three receivers. This shows that when a person is used to carry
out the tests then the orientation they sit in is key more so than the position of the radio on the
person.
When using the stick man, the results compared with the volunteers were worse by around 5dB for
receivers 1 and 2 and worse by between 2dB and 4dB for receiver 3. This shows that further testing
is required to assess whether an allowance could be allocated when testing with the stick man to
compensate for the poorer results compared to when a test volunteer is used. However, all three
radios passed the minimum sensitivity requirements when using the stick man and when it was
positioned in the uniform test area of the cell (listed as centre of GTEM on the graph key).
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4) Conclusions
The conclusions from the testing were:
-
-
-
© 2014
The minimum RF sensitivity results when using the test dummy failed to meet the
requirements specified in the minimum spec for all of the radios tested. This would indicate
that the test dummy was either not representative of a human body or that the microphone
and speakers used to relay the headphone audio within the test dummy caused
interference.
Testing using a volunteer in the GTEM cell produced consistent results between volunteers,
albeit over the small number of tests that were carried out.
Testing using the stick man produced results that passed the requirements in the minimum
spec and that were generally consistent although with some outliers. The results were worse
than when a volunteers was inside the GTEM cell so may need to be given greater allowance
when being tested.
Testing using the stick man method was only consistent when the headphones were fully
extended. When the headphones were coiled the measurements were not repeatable and
differences of up to 18dB between measurements of the same radio were observed.
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