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A Comparison of DDS and Analog FM Broadcast
Transmitters
George DaviesBW Broadcast
October 3, 2012
Abstract
A study investigating the noise performance of Direct Digital
Synthe-sis (DDS) and analog FM broadcast transmitters, comparing
performancewhen unmodulated, modulated with single frequencies and
modulatedwith audio. The tests are carried out on three
transmitters, two imple-menting DDS: a PTEK FM150ES and a
Zhongchuan Digital ZHC618F-100W, and one analog: a BW Broadcast
TX150. Although the commonlyheld belief is that DDS provides
considerably greater performance com-pared to analog transmitters,
this paper demonstrates that this is not thecase for the devices
tested.
The tests show that for the two DDS devices chosen, the RF
spectraloutput of the transmitters does not conform to the ETSI EN
302 018-2harmonised standard for FM broadcast transmitters, making
them un-suitable for use in many countries. In some cases they may
also becomedangerous to use as they cause interference in bands
reserved for aircraftautomatic landing and communications systems.
Throughout all of thetests, the analog BW Broadcast TX150 conforms
fully to the standard.
The results are plotted as frequency spectra given with 1MHz
band-width to see spurious signals close to the carrier, and at
50MHz to seesignals further from the carrier.
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Contents
1 Introduction 31.1 FM150ES . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 31.2 ZHC618F-100W . . . . . . . . . . . .
. . . . . . . . . . . . . . . 31.3 TX150 . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 3
2 Results 42.1 Carrier . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 42.2 1kHz Tone . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 62.3 10kHz Tone . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 82.4 Audio Signal . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Conclusion 14
4 Appendix 154.1 Test Method . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 154.2 Test Standard . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 154.3 All Figures . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 17
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1 Introduction
In recent years, there has been a shift within the industry
towards direct dig-ital synthesis (or DDS) as the method of
excitation within FM transmitters,as opposed to fully analog
systems. As a result of advances in consumer andprofessional audio
electronics the word digital is often associated with better
per-formance. In this white paper we compare excitation methods,
and investigate ifhigh end DDS systems offer similar or superior
performance to analog systems.We will compare the noise performance
of three similarly priced models of FMtransmitter, the PTEK
FM150ES, the Zhongchuan Digital ZHC618F-100W andthe BW Broadcast
TX150. As the results show, only one of these fits the ETSIEN 302
018-2 harmonised standard for FM broadcast transmitters.
1.1 FM150ES
The PTEK FM150ES is a medium power FM transmitter with a stated
maxi-mum output power of 165W. It utilises the Analog Devices
AD9910 as its DDSchip. It comes with BNC composite MPX and XLR
analog L+R audio inputs.
1.2 ZHC618F-100W
The Zhongchuan Digital ZHC618F is a medium power FM transmitter
with astated maximum stated output power of 100W. As with the
FM150ES, it alsoutilises the AD9910 DDS chip. As opposed to the
PTEK it has only XLR analogL+R audio inputs.
1.3 TX150
The BW Broadcast TX150 is a medium power analog FM transmitter
witha maximum output power of 150W. The TX150 is equipped with both
XLRanalog L+R and AES/EBU audio inputs and BNC composite MPX in and
out.
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2 Results
2.1 Carrier
Figure 1: ZHC618F Analog Left and Right Inputs Carrier Only
Figure 2: PTEK FM150ES (Wideband) MPX Input Carrier only
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Figure 3: BW Broadcast TX150 (Wideband) MPX Input Carrier
only
Discussion
As figures one through three show, even without modulation,
clear differencescan be seen between the three transmitters.
Starting with the ZHC618F (Figure1), we can see in both the 1MHz
and 50MHz traces the spectrum sits below themask, but in the outer
200kHz regions, the signals are only slightly below themask, and
some spurious signals breach it. The two most notable are at
384kHz.The FM150ES (Figure 2) however already shows spurious
signals breaching themask. Although within the 1MHz region the
spectrum passes, in the 50MHzrange, clear spurious signals are
rising to around 20dB above allowed limits,with at least ten
distinct spikes visible on the trace. Already the FM150ESdoes not
satisfy the harmonised standard limits.Inspection of the trace from
the TX150 (Figure 3) shows a more familiar sit-uation. The carrier
is a clean spire, and by the outer 200kHz region the noisefloor has
dropped to -100dBc.
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2.2 1kHz Tone
Figure 4: ZHC618F Analog Left and Right Inputs 1kHz Tone
Figure 5: PTEK FM150ES (Wideband) MPX Input 1kHz Tone
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Figure 6: BW Broadcast TX150 (Wideband) MPX Input 1kHz Tone
Discussion
As expected, a similar pattern emerges when observing the RF
output signalmodulated with a 1kHz tone.Figure 4 shows the ZHC618F.
Although the 50Mhz trace is mostly clean, the1MHz shows the outer
bands of the trace breaching the mask slightly. Thiswould cause the
device to breach the harmonized standard.Figure 5 shows the FM150ES
when modulated with a 1kHz tone. As expectedfrom extrapolating the
carrier only trace, we see major breaches of the mask.In the 1MHz
range, the addition of the 1kHz modulation causes the shouldersof
the carrier to breach the mask, but upon inspection of the 50MHz
trace, wesee the spurious signals from before have been expanded
and are now causinginterference of 20dB with a bandwidth in the
range of MHz. This interferenceis within the Aeronautical
Radio-navigation (automated landing systems foraircraft) and
Aviation Communication bands.Figure 6 shows the frequency response
from the BW Broadcast TX150. Asexpected from inspection of the
carrier only trace, we see a clean top hat with 75kHz deviation
with the noise again reducing to -100dBc in the outer
200kHzregion.
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2.3 10kHz Tone
Figure 7: ZHC618F Analog Left and Right Inputs 10kHz Tone
Figure 8: PTEK FM150ES (Wideband) MPX Input 10kHz Tone
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Figure 9: BW Broadcast TX150 (Wideband) MPX Input 10kHz Tone
Discussion
By increasing the modulation to 10kHz, we can see the two DDS
units breachthe frequency mask to a greater extent.At 10kHz
modulation, the ZHC618F performs very poorly, with some
spikesrising to a full 30dB above the specified mask, as shown in
Figure 7. Onceagain, this breaches the mask and therefore fails the
harmonized standard test.The PTEK FM150ES breaches the standard
even further when modulated witha 10kHz tone. Figure 8 shows the
poor noise rejection, with spikes rising as highas -52dBc. The
noise breaches the mask considerably with the wideband
noisedropping below the allowed -95dBc only at approximately 3MHz
either side ofthe carrier.Once again, the TX150 passes the mask
with 10kHz modulation. On inspectionof Figure 9 we see the spectrum
fits neatly inside the mask with no spurioussignals rising above
the limits specified by the standard.
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2.4 Audio Signal
Figure 10: ZHC618F Analog Left and Right Inputs Audio. Top:
Single sweep,Bottom: Max Hold
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Figure 11: PTEK FM150ES (Wideband) MPX Input Audio. Top: Single
sweep,Bottom: Max Hold
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Figure 12: BW Broadcast TX150 (Wideband) MPX Input Audio. Top:
Singlesweep, Bottom: Max Hold
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Discussion
By modulating the three transmitters with an audio signal, we
arrive at a goodapproximation for real world use. As figures 10 and
11 show, in this test, weonce again see that the ZHC618F and the
FM150ES failed to meet the harmo-nized standard.Finally, we can see
the BW TX150 fits easily within the mask and even whentaking a max
hold of over 2 minutes, no signals breach the mask even
slightly,showing complete EM compatibility under the ETSI EN 302
018-2 V1.2.1 har-monised standard for FM broadcast
transmitters.
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3 Conclusion
As this white-paper has shown, throughout the testing procedure
the ZhongchuanDigital ZHC618F and the PTEK FM150ES do not meet the
ETSI EN 302 018-2 harmonised standard. During the testing process
it was shown that theyproduce unacceptable amounts of interference,
both close in and out of band,which would cause high levels of
interference to transmissions within the FMradio broadcast band and
others. The fact that the TX150 does not suffer fromthese issues
and other digitally induced spectra products allows it to fit
neatlywithin the harmonized standard limits. Clearly we can see
although the com-monly held belief is that DDS provides
considerably better noise and distortionperformance, in this case
at least, the analog transmitter outperforms the DDSunits, and will
produce less interference to other users of the radio spectrum.
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4 Appendix
4.1 Test Method
To test the three devices, a Prism Sound DScope Series III
analog and digitalaudio analyser was used to produce 1 kHz and 10
kHz pure sinusoidal toneswhich were fed into the device under test
(DUT). The RF output of the devicewas fed through a suitably rated
RF attenuator before then being measured ona high specification
Rhode and Schwartz FSEA 30 Digital Spectrum analyser.The blank
carrier was also recorded for comparison. The input signals were
fedinto both the analog and composite inputs for the TX150 and the
FM150ES,and into the analog left and right inputs only for the
ZHC618F.For the final tests, audio was fed into a BW Broadcast
DSPXtra-FM audioprocessor. As before, the processor output was fed
into the FM150ES and theTX150 via composite in and into the ZHC618F
via analog L+R in. 75 s pre-emphasis was applied using the
DSPXtra-FM. For the max hold images, theimages were taken over a
time period of two minutes.For each of the tests that were
performed, the transmitter was adjusted toachieve 75kHz peak
deviation.For testing, all the devices were attenuated so that the
peak output power ofthe unmodulated carrier was 0dBm to allow for
easy comparison between thefigures included in this report.
4.2 Test Standard
The ETSI EN 302 018-2 harmonised standard defines a spectrum
mask forwhich all out of band emissions must be below for radio
transmitters in the FMbroadcast band. The mask is defined as:
Frequency relative to thecentre of the channel (kHz) Relative
Level (dBc)-500 -85-300 -85-200 -80-100 0100 0200 -80300 -85500
-85
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Figure 13: ETSI EN 302 018-2 harmonised standard mask
This mask is superimposed on each of the plots taken during
testing for easyidentification of any signals breaching it.
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4.3 All Figures
Here we have included all of the tests performed on the three
transmitters. Tosave space, only thumbnails are provided, but full
size versions will be gladlysupplied upon request by contacting us
by email at [email protected].
MPX Input Analogue Left and Right Inputs
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Figure 14: BW Broadcast TX150 87.9MHz
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MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 15: BW Broadcast TX150 97.9MHz
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MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 16: BW Broadcast TX150 107.9MHz
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MPX Input Analogue Left and Right Inputs
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Figure 17: PTEK FM150ES 87.9MHz
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MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 18: PTEK FM150ES 97.9MHz
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MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 19: PTEK FM150ES 107.9MHz
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N/A
N/A
N/A
MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 20: ZHC618F-100W 87.9MHz
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N/A
N/A
N/A
MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 21: ZHC618F-100W 97.9MHz
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N/A
N/A
N/A
MPX Input
Carr
ier
1kH
z M
odul
atio
n 10
kHz
Mod
ulat
ion
Analogue Left and Right Inputs
Figure 22: ZHC618F-100W 107.9MHz
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