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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 µs 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

Car

rier

1kH

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Figure 14: BW Broadcast TX150 87.9MHz

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MPX Input

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Analogue Left and Right Inputs


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MPX Input

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MPX Input Analogue Left and Right Inputs

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Figure 17: PTEK FM150ES 87.9MHz

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MPX Input

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MPX Input

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Figure 20: ZHC618F-100W 87.9MHz

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N/A

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a comparison of dds and analog fm broadcast · pdf filesis (dds) and analog fm broadcast...

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