As the U.S. government pushes ahead with plans to switch off the analog broadcast system in favor of an all-digital transmission, manufacturers must navigate through a growing number of government standards for television performance and design new sets better supporting over-the-air DTV (digital television) and HD (high definition) viewing.
The ATSC (Advanced Television Systems Committee)(1), an international organization of broadcasters, manufacturers and suppliers, has developed a receiver specification to ensure DTV receiver equipment receives over-the-air signals as reliably as analog channels. Today, ATSC A/74(2) is only a “recommended practice”. However, as U.S. consumer demand for over-the-air DTV and HD viewing increase, coupled with an aggressive government drive to reclaim analog spectrum, many believe it will become the de facto standard for quality ATSC receiver equipment.
This article will take a detailed look at tuner design and performance challenges that must be met to ensure A/74 compliance.
ATSC Digital Modulation Format
ATSC employs a digital modulation format based on a multi-level coding, normally 8, uniformly distributed around a small DC offset. This composite spectrum is then amplitude modulated onto a carrier and further processed to produce a vestigial modulated carrier, where the level of vestigial sideband is proportional to the DC offset (typically 0.3 dB). This modulation is referred to as 8-VSB (8-level vestigial sideband). A good reference document about 8-VSB is David Sparano’s paper, “What Exactly is 8-VSB Anyway?” (3)
The ATSC standard also applies data randomization, which effectively codes the data so the transmission appears as a noise-like signal with uniform power spectral density, FEC (forward error correction) in the form of Reed Solomon coding and Trellis coding – both of which enable the demodulator to detect for and correct received data errors – and data interleaver which “spreads” packet data over time to increase packet immunity against burst noise generated errors.
All of these are described in the ATSC A/53Digital Television Standard and discussed in Sparano’s work.
Implications of A/74 on Tuner Performance
Before discussing the implications of A/74 on tuner performance, it is important to understand the implications of additive noise to the signal quality.
As described above, ATSC employs 8-VSB digital modulation with various coding means to enhance noise immunity. From this, a theoretical CN (carrier-to-noise) can be calculated above which the demodulated data BER (bit error rate) after error correction will be deemed as unobjectionable to the viewer. This is often referred to as the TOV (threshold of visibility) or QEF (quasi error free) level.
The ACATS (FCC Advisory Committee on Advanced Television Service) has determined the level to be a BER of 3E-6. At this level the error protection will correct the majority of data errors occurring to deliver a quality picture without macro-blocking (blocks of corrupt data within the video picture) These are also referred to as uncorrected blocks.
At BER levels above this limit there is no perceived improvement in quality of service. Conversely, as the BER increases, the error protection is no longer able to correct for data errors, leading to macro blocking.
To achieve TOV with 8-VSB modulation, the theoretical value for C/N is 15 dB. However, perfect performance cannot be achieved in any practical implementation, so a level of 15.5 dB is typically applied as the TOV value. A margin of 0.5 dB is allowed over the theoretical limit to account for receiver impairments. This value is further degraded when multipath echoes are considered. Per A/74 it is predicted that the presence of typical multipath will require a further 3 dB improvement in C/N ratio to 18 dB over the theoretical limit to achieve TOV.
To provide adequate reception of ATSC transmissions in a typical multipath environment, an 18 dB C/N ratio must be achieved where the noise can either be received noise, tuner additive noise, or a combination of both. This C/N requirement has to be carefully considered when implementing a front-end tuner function.
The A/74 specification describes the expected performance from a complete tuner and demodulator receiver front-end to deliver acceptable quality of service. This article focuses only on issues relating to the tuner.
The ATSC standard allows digital channels to be transmitted in taboo channel locations relative to existing analog services. These taboo channels should not be occupied by other analog services, as intermodulation effects and other impairments will lead to channel degradation. This is generally managed through system planning.
New digital services in these taboo channels are transmitted at a lower power level to avoid interference with the existing analog services. This is possible since a digital service requires a much lower C/N than analog for the same level of picture quality.
A/74 describes a minimum desired-to-undesired channel ratio (D/U ratio) for each taboo channel, under which the receiver should exceed TOV. Three scenarios are described to cover weak, moderate and strong desired channel powers. Linear interpolation between specified values is generally assumed for other channel powers.
The three scenarios for DTV and NTSC (analog)-adjacent interfering channels are illustrated in Figures 1 and 2, where a negative D/U ratio implies the desired channel is weaker than the undesired adjacent by the specified dB, decibel, ratio. An example curve demonstrating linear interpolation is shown in Figure 3.
Finally, A/74 does not currently define a specification for a multi-carrier overload condition, rather it draws the designer’s attention to the fact that the front-end must work in real environments with a mix of high and low amplitude carriers. As ATSC gains popularity, more transmitters will be located in close spectral proximity. As a consequence, ATSC receivers will be subject to more multi-carrier adjacent energy, placing much higher demands on the receiver. Designers today need to consider situations that will place higher demands on a receiver that will inevitably exist as the FCC repacks spectrum. An example of a congested spectrum is seen in Figure 4.
Receiver Dynamic Range
Per ATSC A/74 the minimum recommended operating range in an echo free environment with no applied incident noise from the receiver is -83 to -8 dBm. Some in the industry have argued that this range is limited on the top end and needs to be extended to emulate real-world situations.
Lower Sensitivity Limit
The receiver’s lower operating limit is defined by the tuner NF (noise figure). The tuner NF is a measure of the noise added to the received signal relative to the source impedance noise. This is sometimes also referred to as the tuner AWGN (additive white Gaussian noise).
The required NF can be determined by calculating the total noise power described by the NF and relating this to the minimum operating power by the required C/N using the following formula:
This is illustrated in Figure 5.
In field use, this NF should be substantially achieved with the A/74 taboo channel D/U ratios applied as described in Figures 1 and 2.
The relevant taboo channels for consideration will depend on the tuner architecture. In a single conversion tuner there will be protection afforded to channels at greater than typically two channel offsets due to the front-end tracking filters. In double conversion solutions, which do not employ tracking filters, there will be no protection afforded.
It is far more challenging to meet the NF requirement in the presence of taboo undesired channels with a double conversion architecture compared to a single conversion. For example, the relative levels of N+2 and N+6 taboo channels is 13 dB for DTV into DTV. For a double conversion architecture to meet A/74 at the sensitivity point, this demands a 13 dB greater signal handling, and hence 13 dB greater dynamic range is required.
The A/74 D/U ratio should be substantially achieved at the sensitivity point. This in turn defines the additional allowed impairment to the C/N from the tuner due to N+/-1 spectral splatter; third order regrowth from the adjacent channel onto the desired.
The additional impairment from spectral splatter can be calculated from the RMS (root mean square) addition of tuner thermal noise and spectral splatter noise, which can be calculated from the tuner IPIP3 and desired and undesired signal powers.
The allowed spectral splatter can be determined by calculating the additional noise arising from the spectral splatter, which when added in an RMS manner with the tuner thermal noise results in the above defined 1 dB increase in total composite noise and results in 15.5 dB C/N and hence TOV. This can be calculated from the following:
First, calculate the integrated thermal noise power from the NF:
Now derive the allowed spectral splatter (Ps) by performing an RMS addition of the thermal noise power (PN) and PS which leads to composite noise (Pc);
Considering the above example of 1 dB greater than the lower operating limit, which defines an allowed increase of composite noise of 1 dB, so maintaining a 15.5 dB C/N;
This is shown in Figure 6.
For this test condition the desired is at –82 dBm (1 dB above the A/74 minimum operating limit) and the adjacent undesired is -49 dBm. Therefore, the spectral splatter that results from the undesired at -49 dBm (per A/74 adjacent channel D/U ratio) must be a maximum of -104.5 dBm (see Equation 2) or circa -56 dBc (-104.5 + 49) relative to the undesired channel. This implies an IPIP3 of circa -21 dBm (-49 + 56/2) minimum is required at the defined operating limit.
The implication of this requirement on the tuner AGC (automatic gain control) attack must also be considered where the AGC attack is the level at which the tuner front end AGC begins to reduce protecting latter stages from strong signal conditions. It is standard practice to determine this value on the composite received signal (the desired plus interfering) as this will ensure that the optimum C/(N+IM) (carrier-to-noise plus intermodulation) is obtained under all received signal conditions, for example with and without taboo interferer present.
In theory, the AGC attack could be set at the sensitivity point. Assuming the NF increases at 1 dB per dB of gain back off, as might be the case with some AGC architectures, the C/N achieved at the sensitivity point will be maintained for all signal conditions, thereby maintaining TOV. However, this is not a desirable implementation since there will be no margin to accommodate any other tuner impairments. Instead, it is normal practice to define the AGC attack above the sensitivity limit.
A typical value for AGC attack may be 5 dB above sensitivity limit, meaning the above derived IPIP3 requirement of –21 dBm would be 5 dB higher, or -16 dBm. Similar analyses can be considered for other taboo channels, and since they are of greater amplitude the signal-handling requirement may be greater.
When operating at maximum gain, which corresponds to the sensitivity limit, the tuner has counter demands of low NF and high IP3. In a typical single conversion tuner which employs a dual gate FET (field effect transistor) LNA/AGC stage and a silicon MOPLL (mixer oscillator PLL) integrated circuit, the NF will be dominated by the dual gate FET and the intermodulation by the MOPLL. This is because the gain of the FET will reduce the MOPLL thermal noise contribution when referred to the tuner input, whereas the gain will increase the signal amplitudes seen at the MOPLL and hence render this stage more susceptible to intermodulation generation.
Consider the above -16 dBm IPIP3 requirement in the case where the FET has 20 dB of gain. Assuming the IPIP3 is limited by the MOPLL this must achieve a minimum of +4 dBm IPIP3.
As can be seen when considering the tuner implementation, the demands on the designer are eased by deploying a FET stage and MOPLL with a high IPIP3 and P1dB.
Higher Overload Limit
The receiver higher operating limit is defined by the tuner P1 dB, which relates to the signal handling performance, and the IPIP3 which defines the level of spectral splatter from both the desired and adjacent and the FET AGC range.
When considering the above described application, where the AGC attack is set 5 dB above sensitivity point with an N+1 adjacent present, then the input power which defines AGC attack is -83+5+33 dBm = -45 dBm. Given the A/74 maximum operating limit of -8 dBm the RFAGC range requirement is circa 40 dB.
Maximum Signal Handling
When considering a desired only or desired at strong condition with maximum amplitude undesired per A/74, the maximum input level is -8 dBm.
This level can be higher when considering a multi-carrier environment, as seen in Figure 4.
Distribution experts have proposed a representative figure for the composite signal power of 8 channels at -8 dBm. This effectively determines a maximum mean input amplitude of +1 dBm, which the input stage should be capable of supporting. This is a demanding requirement is easier to achieve in a single conversion tuner implementation compared to a double conversion. This is due to the protection afforded by the tracking filters, which will suppress some of the 8 carriers. The designer’s task is also eased by deploying an FET stage and MOPLL with a high IPIP3 and P1dB.
Third Order Non Linearity, IPIP3
As previously discussed, third order non-linearity will lead to spectral splatter from N+1 DTV onto the desired DTV channel. The worst-case condition for this will be for a strong desired signal with N+1 taboo per A/74.
By extrapolation from the value for IPIP3 determined at the sensitivity operating point, a value of +13.5 dBm can be deduced when operating at A/74 specified strong condition. This figure is derived from an increase in desired power from -82 dBm to -28 dBm and an increase in D/U ratio from -33 to -20 dB per A/74.
This places further demands on the tuner. The designer’s task is again eased by implementing a single conversion design, employing a FET stage and MOPLL with a high IPIP3 and P1dB.
Image Cancellation Requirement
The image cancellation requirement is perhaps one of the least understood specifications in that the A/74 specifies a requirement for -50 dBm. However, there is a caveat within A/74, “This value should not be interpreted as just a tuner image rejection value in that it applies to the entire receiver.”
In simple terms, the D/U requirement for the image channels does not specify the minimum image cancellation required from the tuner. Instead it defines the applied D/U ratio under which the front end shall deliver TOV performance.
In practice the image channel will fold on top of the desired channel and appear as a noise like degradation to the carrier-to-noise. Considering no other noise sources, then the tuner image cancellation requirement can be calculated from the following formula:
With no other impairments a minimum image cancellation of 65.5 dB must be achieved. However, in real-world situations other impairments will exist. Multipath, for example, will increase the image cancellation requirement by a further 2.5 dB to 68 dB.
As discussed earlier, it is also a stated objective to achieve the A/74 requirement substantially at the operating sensitivity.
Using the same definition as “substantially at sensitivity” being 1 dB above minimum operating limit, the allowed level of image signal from equation 2 will be -104.5 dBm, which was the signal level calculated for a second noise source which raises the tuner additive noise power by 1 dB.
The required image cancellation in this situation is the difference between the A/74 defined image channel level and this derived allowed level of image channel converted to the IF output leading to a 1 dB rise in composite noise. This can be calculated from the following formula:
This is shown in Figure 7.
Ideally the tuner should achieve a minimum image cancellation across its operation range of 72.5 dB and a minimum of 68 dB. This is extremely challenging using standard tracking filter techniques. To meet the requirement will either compromise the desired channel filter flatness or require additional tracking filter stages to achieve additional image suppression. Neither of these are desirable since the former may render the tuner performance unacceptable for other distribution means such as analog terrestrial or cable and the latter will involve additional design complexity coupled with increased manufacturing cost. These issues are potentially compounded when aging and temperature variation effects are considered.
A solution overcoming these disadvantages is to apply active image cancellation within the tuner by using image reject mixer technology in the MOPLL section. With this approach it is feasible to achieve typically 30 dB of image suppression, which will ease the tuner tracking filter requirement to 42.5 dB. This helps lower manufacturing costs and is also far less susceptible to environmental aging.
Performance Requirements in PNP
PNP (Plug and Play) describes a receiver that is capable of receiving both cable and terrestrial services. A digital terrestrial receiver that also meets cable requirements is described as DCR (digital cable ready).
PNP applications are typically addressed today by deploying a single conversion tuner for ATSC and a double conversion tuner for DCR. This approach leads to increased component and design costs as well as board space requirements.
Neither of these solutions is optimum for both applications. For example, the single conversion tuner with traditional MOPLL technology will have neither sufficient signal handling nor channel flatness to meet the requirements of DCR. The channel flatness issue is exacerbated when the tracking filters are modified to try and meet the full image rejection requirements of A/74 due to the higher Q factor required. Conversely, a tuner optimized for cable will have neither sufficient narrow band signal handling nor NF to meet the requirements of A/74.
As previously described, a single conversion tuner based on an image rejection MOPLL will require 30 dB less image cancellation. The net consequence of this is a significantly flatter passband compatible with DCR requirements can be achieved. In addition the higher signal handling which also enhances A/74 D/U performance is of benefit in meeting the DCR intermodulation requirements.
It is now technically feasible to develop a single hybrid PNP tuner that is fully compatible with both A/74 and DCR requirements, while also reducing overall size and system cost.
Designing a tuner that achieves A/74 performance is challenging. Using MOPLL technology that incorporates both image rejection and high signal-handling capability will ease design, and help speed time-to-market for superior performance products targeting a high-volume consumer market
ATSC broadcast is still in the introductory phase, and not yet broadly deployed in the U.S. As the U.S. converts entirely to digital ATSC, to ensure satisfactory quality of the service for consumers the receiver should be designed to meet a minimum performance as described in the ATSC A/74 specification. To this end, a designer will find that traditional single conversion receiver architectures can be utilized with an MOPLL that employs image rejection with the enhanced signal handling capability.
Figure 8 illustrates performance achieved from a tuner(5) incorporating such a product with conventional tracking filter technology. This design exceeds the A/74 minimum recommended D/U performance under weak desired signal conditions by between 6 to 10 dB (depending on channel offset) to enable quality reception for the consumer.
This new MOPLL technology replaces the two tuners currently required in PNP receivers with a single conversion architecture that meets the demands of both ATSC and DCR.
Thanks to Thomson, especially Veit Armbruster and Alfred Selz for the cooperative effort in designing an A/74 compliant front-end based on the described technology.
- ATSC recommended practise; Receiver performance guidelines, published by advanced television standards committee see www.atsc.org
- What exactly is 8VSB anyway, by David Sparano
- Figure D.1, ATSC Recommended Practice: Receiver Performance Guidelines A/74, June 18, 2004" courtesy of the Advanced Television Systems Committee, Inc.
About the authors
Nick Cowley is employed at Zarlink Semiconductor. He can be reached at firstname.lastname@example.org.
Robert Hanrahan is currently employed at National Semiconductor. He can be reached at email@example.com. Both authors have been involved in the development of digital receiver products at Zarlink Semiconductor, and have been active members of the ATSC RF subcommittee.