A Comparison of DDS and Analog FM Broadcast Transmitters

Transcription

1 A Comparison of DDS and Analog FM Broadcast Transmitters George Davies BW Broadcast October 3, 2012 Abstract A study investigating the noise performance of Direct Digital Synthesis (DDS) and analog FM broadcast transmitters, comparing performance when unmodulated, modulated with single frequencies and modulated with audio. The tests are carried out on three transmitters, two implementing DDS: a PTEK FM150ES and a Zhongchuan Digital ZHC618F- 100W, and one analog: a BW Broadcast TX150. Although the commonly held belief is that DDS provides considerably greater performance compared to analog transmitters, this paper demonstrates that this is not the case for the devices tested. The tests show that for the two DDS devices chosen, the RF spectral output of the transmitters does not conform to the ETSI EN harmonised standard for FM broadcast transmitters, making them unsuitable for use in many countries. In some cases they may also become dangerous to use as they cause interference in bands reserved for aircraft automatic landing and communications systems. Throughout all of the tests, the analog BW Broadcast TX150 conforms fully to the standard. The results are plotted as frequency spectra given with 1MHz bandwidth to see spurious signals close to the carrier, and at 50MHz to see signals further from the carrier. 1

2 Contents 1 Introduction FM150ES ZHC618F-100W TX Results Carrier kHz Tone kHz Tone Audio Signal Conclusion 14 4 Appendix Test Method Test Standard All Figures

3 1 Introduction In recent years, there has been a shift within the industry towards direct digital synthesis (or DDS) as the method of excitation within FM transmitters, as opposed to fully analog systems. As a result of advances in consumer and professional audio electronics the word digital is often associated with better performance. In this white paper we compare excitation methods, and investigate if high end DDS systems offer similar or superior performance to analog systems. We will compare the noise performance of three similarly priced models of FM transmitter, the PTEK FM150ES, the Zhongchuan Digital ZHC618F-100W and the BW Broadcast TX150. As the results show, only one of these fits the ETSI EN harmonised standard for FM broadcast transmitters. 1.1 FM150ES The PTEK FM150ES is a medium power FM transmitter with a stated maximum output power of 165W. It utilises the Analog Devices AD9910 as its DDS chip. 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 a stated maximum stated output power of 100W. As with the FM150ES, it also utilises the AD9910 DDS chip. As opposed to the PTEK it has only XLR analog L+R audio inputs. 1.3 TX150 The BW Broadcast TX150 is a medium power analog FM transmitter with a maximum output power of 150W. The TX150 is equipped with both XLR analog L+R and AES/EBU audio inputs and BNC composite MPX in and out. 3

4 2 Results 2.1 Carrier Figure 1: ZHC618F Analog Left and Right Inputs Carrier Only Figure 2: PTEK FM150ES (Wideband) MPX Input Carrier only 4

5 Figure 3: BW Broadcast TX150 (Wideband) MPX Input Carrier only Discussion As figures one through three show, even without modulation, clear differences can be seen between the three transmitters. Starting with the ZHC618F (Figure 1), we can see in both the 1MHz and 50MHz traces the spectrum sits below the mask, but in the outer 200kHz regions, the signals are only slightly below the mask, and some spurious signals breach it. The two most notable are at ±384 khz. The FM150ES (Figure 2) however already shows spurious signals breaching the mask. Although within the 1MHz region the spectrum passes, in the 50MHz range, clear spurious signals are rising to around 20dB above allowed limits, with at least ten distinct spikes visible on the trace. Already the FM150ES does not satisfy the harmonised standard limits. Inspection of the trace from the TX150 (Figure 3) shows a more familiar situation. The carrier is a clean spire, and by the outer 200kHz region the noise floor has dropped to -100dBc. 5

6 2.2 1kHz Tone Figure 4: ZHC618F Analog Left and Right Inputs 1kHz Tone Figure 5: PTEK FM150ES (Wideband) MPX Input 1kHz Tone 6

7 Figure 6: BW Broadcast TX150 (Wideband) MPX Input 1kHz Tone Discussion As expected, a similar pattern emerges when observing the RF output signal modulated with a 1kHz tone. Figure 4 shows the ZHC618F. Although the 50Mhz trace is mostly clean, the 1MHz shows the outer bands of the trace breaching the mask slightly. This would cause the device to breach the harmonized standard. Figure 5 shows the FM150ES when modulated with a 1kHz tone. As expected from extrapolating the carrier only trace, we see major breaches of the mask. In the 1MHz range, the addition of the 1kHz modulation causes the shoulders of the carrier to breach the mask, but upon inspection of the 50MHz trace, we see the spurious signals from before have been expanded and are now causing interference of 20dB with a bandwidth in the range of MHz. This interference is within the Aeronautical Radio-navigation (automated landing systems for aircraft) and Aviation Communication bands. Figure 6 shows the frequency response from the BW Broadcast TX150. As expected 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 200kHz region. 7

8 2.3 10kHz Tone Figure 7: ZHC618F Analog Left and Right Inputs 10kHz Tone Figure 8: PTEK FM150ES (Wideband) MPX Input 10kHz Tone 8

9 Figure 9: BW Broadcast TX150 (Wideband) MPX Input 10kHz Tone Discussion By increasing the modulation to 10kHz, we can see the two DDS units breach the frequency mask to a greater extent. At 10kHz modulation, the ZHC618F performs very poorly, with some spikes rising to a full 30dB above the specified mask, as shown in Figure 7. Once again, this breaches the mask and therefore fails the harmonized standard test. The PTEK FM150ES breaches the standard even further when modulated with a 10kHz tone. Figure 8 shows the poor noise rejection, with spikes rising as high as -52dBc. The noise breaches the mask considerably with the wideband noise dropping below the allowed -95dBc only at approximately 3MHz either side of the carrier. Once again, the TX150 passes the mask with 10kHz modulation. On inspection of Figure 9 we see the spectrum fits neatly inside the mask with no spurious signals rising above the limits specified by the standard. 9

10 2.4 Audio Signal Figure 10: ZHC618F Analog Left and Right Inputs Audio. Top: Single sweep, Bottom: Max Hold 10

11 Figure 11: PTEK FM150ES (Wideband) MPX Input Audio. Top: Single sweep, Bottom: Max Hold 11

12 Figure 12: BW Broadcast TX150 (Wideband) MPX Input Audio. Top: Single sweep, Bottom: Max Hold 12

13 Discussion By modulating the three transmitters with an audio signal, we arrive at a good approximation for real world use. As figures 10 and 11 show, in this test, we once again see that the ZHC618F and the FM150ES failed to meet the harmonized standard. Finally, we can see the BW TX150 fits easily within the mask and even when taking a max hold of over 2 minutes, no signals breach the mask even slightly, showing complete EM compatibility under the ETSI EN V1.2.1 harmonised standard for FM broadcast transmitters. 13

14 3 Conclusion As this white-paper has shown, throughout the testing procedure the Zhongchuan Digital ZHC618F and the PTEK FM150ES do not meet the ETSI EN harmonised standard. During the testing process it was shown that they produce unacceptable amounts of interference, both close in and out of band, which would cause high levels of interference to transmissions within the FM radio broadcast band and others. The fact that the TX150 does not suffer from these issues and other digitally induced spectra products allows it to fit neatly within the harmonized standard limits. Clearly we can see although the commonly held belief is that DDS provides considerably better noise and distortion performance, in this case at least, the analog transmitter outperforms the DDS units, and will produce less interference to other users of the radio spectrum. 14

15 4 Appendix 4.1 Test Method To test the three devices, a Prism Sound DScope Series III analog and digital audio analyser was used to produce 1 khz and 10 khz pure sinusoidal tones which were fed into the device under test (DUT). The RF output of the device was fed through a suitably rated RF attenuator before then being measured on a high specification Rhode and Schwartz FSEA 30 Digital Spectrum analyser. The blank carrier was also recorded for comparison. The input signals were fed into 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 audio processor. As before, the processor output was fed into the FM150ES and the TX150 via composite in and into the ZHC618F via analog L+R in. 75 µs preemphasis was applied using the DSPXtra-FM. For the max hold images, the images were taken over a time period of two minutes. For each of the tests that were performed, the transmitter was adjusted to achieve ±75kHz peak deviation. For testing, all the devices were attenuated so that the peak output power of the unmodulated carrier was 0dBm to allow for easy comparison between the figures included in this report. 4.2 Test Standard The ETSI EN harmonised standard defines a spectrum mask for which all out of band emissions must be below for radio transmitters in the FM broadcast band. The mask is defined as: Frequency relative to the centre of the channel (khz) Relative Level (dbc)

16 Figure 13: ETSI EN harmonised standard mask This mask is superimposed on each of the plots taken during testing for easy identification of any signals breaching it. 16

17 4.3 All Figures Here we have included all of the tests performed on the three transmitters. To save space, only thumbnails are provided, but full size versions will be gladly supplied upon request by contacting us by at MPX Input Analogue Left and Right Inputs 10kHz Modulation 1kHz Modulation Carrier Figure 14: BW Broadcast TX MHz 17