And now, after two detours, back to overload.
Earlier, I showed you that your signals were in overload territory as far as the Noise Margin figures were concerned. I now want you to concentrate on the Pwr (dBm) column of your tvfool report. WSOC has a signal strength (actually signal power because it's dB) of -15.9 dBm. WCCB isn't far behind with -16.5 dBm. If you add the average gain of 12 dBd for the DB8, that brings you up to -3.9 dBm. If you then add the gain of 8 dB for the 3414 and subtract 4 dB for cable loss, you are now at +0.1 dBm. Your tuners will not like that.
ATSC Recommended Practice:
Receiver Performance Guidelines
Document A/74:2010, 7 April 2010
RECEIVER PERFORMANCE GUIDELINES
5.1 Sensitivity
A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from
–83 dBm to –5 dBm for both the VHF and UHF bands.
And with two very strong signals it's even worse:
5.2 Multi-Signal Overload
The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV signal at its input, received from transmission facilities that are in close proximity to one another.For purposes of this guideline, it should be assumed that multiple signals, each approaching
–8 dBm, will exist at the input of the receiver.
You are now 8 dB stronger than the allowed signal power.
THREE TYPES OF OVERLOAD
There are three types of preamp or tuner overload, in order of increasing signal strength:
1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.
As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:
2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).
3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.
As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.
Forum member holl_ands (who is a communications engineer) has made a preamp chart that shows the maximum input signal for preamps. Many of the preamps are no longer available, but it gives you the general idea.
http://imageevent.com/holl_ands/file...=0&w=1&s=0&z=4
Intermodulation Distortion
The IMD creates new spurious signals within the preamp (or tuner) itself that can interfere with the reception of your weakest desired signals if the spurious signals are stronger than the noise floor of the weakest desired signals. The spurious signals are caused by the interaction between two or more of your strongest signals.
IMD is not the only distortion that can be created within the preamp; you can also have distortion caused by signals so strong that the top of the strong signals are clipped, which causes compression of the signals. This can be seen if you increase the input to the preamp by, for example, 10 dB and the output increases by less than 10 dB.
Spurious Free Dynamic Range
The Spurious Free Dynamic Range needed is the difference in strength between your strongest signal and your weakest desired signal, plus 16 dB for the SNR (Signal to Noise Ratio) of the weakest signal, using the dBm Pwr scale on your tvfool report. This difference is expressed in terms of dB, not dBm, because the original units are the same. The difference between the strongest signal and the weakest signal is the Signal Dynamic Range/Dynamic Range, which is 16 db less than the SFDR, because it doesn't include the SNR of the weakest signal.
Another way to think of SFDR is from the top of the strongest signal down to the bottom (noise floor) of the weakest desired signal. The top of the spurious signals must be at or below the noise floor of the weak signals if they are not to cause interference. The Signal Dynamic Range is from the top of the strongest signal down to the top of the weakest desired signal.
You can also use the NM scale for your calculations, but I prefer to use the Pwr scale because most tuners drop out around -85 dBm.
Continued on page 2 of this thread.