I prepared a Preamp "Overload" Spread Sheet Calculator, based on limited info from those few manufacturers who provide specs. But "Overload" can also be a problem when the Tuner is subjected to very strong signals on the OUTPUT of a Preamp....and when close to broadcast towers, when NOT using a Preamp. When the Input of a Tuner is "Overloaded", it actually means that the non-linearities in the device result in Intermodulation Distortion (IMD) products being generated, which can reduce the sensitivity on particular channels when the IMD Noise levels rise up above the Thermal Noise Floor, which normally determines the Sensitivity level of about -85 dBm (+/- 5 dB).
FCC A/74 provides Guidelines for Tuner performance, including how strong the Undesired (U) signals levels SHOULD be tolerated above the Desired (D) signals level for Adjacent Channel, as well as a group of so-called "Taboo" Channels that are in Next-Adjacent (N=2) and further away channel locations. The CECB Coupon Converter Boxes made these Guidelines MANDATORY...but manufacturers are NOT required to follow them in any of their other products. It was NOT easy for CECB's to meet these and other specs, so they represent the State-Of-The-Art (in 2006) and it is unlikely newer equipment has significantly better tolerance to IMD:
FCC OET TR-05-1017 (Nov2005) reported on tests conducted on DTVs & STBs bought in 2005-2006, including Sensitivity, Noise Figure and Multipath Tolerance using RF Field Captures (cited in ATSC A/74)...but did NOT address any IMD issues:
FCC OET 07-TR-1003 (Mar2007) reported on additional tests conducted on DTVs & STBs bought in 2005-2006, including IMD desensitization due to ONE and TWO strong interfering channels. Figure 5-9 on (sequential) pg63 shows a susceptibility of a Desired Channel just above Sensitivity for a wide range of "Taboo" channels, not just Adjacent (N+/-1) and Next-Adjacent (N+/-2). Many have very high susceptibility as many as 7 and 14-15 channel positions (Ch44-45) higher than the desired channel (Ch30). For example with (non-EMI) Sensitivity= -85 dBm, D/U = -55 to -65 dB corresponds to Undesired Pwr(dBm)= -30 to -20 dBm.
Table 9-6 on pg102 summarized results for TWO interfering channels for a variety of separations between Desired (D) and Undesired (U) with channel separations N and K. Undesired signal levels that caused readily visible interference ("TOV") occurred over the range of U = -24 to -42 dBm when Desired signal was 1 dB above sensitivity threshold without interference. With a 10-12 dBd Gain Antenna (e.g. CM4221HD 4-Bay) and say 6 dB downlead Loss, Tuner could "Overload" with TVFool calculated (worst case) levels as low as Pwr(dBm) = -36 dBm (-42 dBm +12 - 6)....although most channel separations would be in the range of Pwr(dBm) = -25 dBm.
Charles Rhodes of TV Technology magazine had several articles on this subject and the related DTV/LTE EMI issues, including:
[EMI to/from nearby LTE Cell Towers & Phones]
FCC OET 9TR1003 (Oct2009) reported on tests conducted on numerous CECB Coupon Converter Boxes including IMD desensitization due to TWO strong interfering channels. Table 2-7 on pg31 summarized these results for a variety of separations between Desired (D) and Undesired (U) with channel separations N and K. Readily visible interference ("TOV") occurred over the range of D/U Ratio of -40 to -60 dB for a "weak" desired signal level of -68 dB (nearly 20 dB ABOVE sensitivity). Unfortunately, they did NOT test at Desired signal level just above sensitivity...but from the preceding test results, the D/U Ratio can be expected to be slightly higher. So worse case, multiple Undesired signal levels in the ballpark of -25 dBm (give or take) CAN result in desensitization on weak channel positions.
Unfortunately, the ONLY Tuner tested using Double-Conversion was the first ever, specially built and tweaked "Grand Alliance" ATSC Prototype, rather than the usual Single-Conversion Super-hetereodyne design. So none of these tests provide insight into how well CURRENT Double-Conversion Tuners perform, such as the Microtune Integrated Tuner Chip....could be better....or could be worse under some conditions (as I suspect).Edited by holl_ands - 2/7/13 at 4:46pm