Background:
When giving weak signal reception advice, I have been saying:
Quote:
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Originally Posted by rabbit73
It is not possible to receive a signal with a NM below -15 dB. If you are able to receive a signal with a NM below -15 dB, either the TVFool report is wrong, or the signal has been enhanced by Tropospheric Propagation.
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A forum member questioned my statement, saying that it IS possible to receive signals with a NM less than -15 dB if you use an antenna with more gain. At that time, I had no way to prove it either way, other than to say a signal that weak would be buried below the thermal noise floor at -106 dBm for a 6 MHz bandwidth DTV signal.
Since that time, I have been able to simulate the problem, and prove to myself that it is NOT possible to receive and decode DTV signals below -15 dB NM.
Weak Signal SNR Measurements, Test 1
The purpose of this test is to determine if additional antenna gain can make it possible to receive and decode 8VSB OTA signals with a NM below -15 dB.
Additional antenna gain would be indicated by a signal from the antenna that was more than 15 dB above the noise floor from the antenna. I will call this ratio C/N (carrier to noise) to differentiate it from the SNR for the TV.
Equipment Used:
Sony KDL32BX320
Sadelco DisplayMax 800 Signal Level Meter (SLM), used in Analog and Digital modes
CM7777HD Preamp #1, High Gain 30 dB setting to increase sensitivity of SLM
RCA TVPRAMP1R for tuner in Part B
CM7777HD #2, Low Gain 17 dB setting for tuner in Part C
20 dB 1% Fixed Attenuator
3 dB 1% Fixed Attenuator
Wavetek 7510A 0-10 dB Variable Attenuator
Setup: for Test 1, Part A:
Antenna > variable attenuator > splitter > tuner and SLM, with preamp #1 before the SLM; no preamp before the tuner
A strong local signal on CH33 was used, because it was fairly stable and had a good SNR before attenuation to make it a weak test signal . CH32 was vacant and was used for the noise floor measurement. The signal level meter (SLM) was set to make a single frequency measurement near center channel in the analog mode. (Digital TV signals, 8VSB-AM, are actually analog signals that carry digital information.) In that mode, it is sensitive down to -35 dBmV (-84 dBm), and the measurement bandwidth is 280 kHz.
I added a 30 dB preamp to increase the meter sensitivity to below -106 dBm. The gain of the preamp was verified with the signal level meter and a stable signal from a Blonder Tongue modulator. The linearity of the SLM was checked with fixed attenuators. The SLM has an internal noise standard, traceable to NIST, which the meter uses to check calibration at fixed intervals.
The noise floor of the measurement system was then -112 dBm.
Code:
Weak Signal SNR Measurements, Test 1, Part A:
Attn Sony SLM Conv#1 Correct CH 32 Conv#2 C/N
dB SS SNR CH 33 > to dBm > to DTV Noise > to dBm dB
dB dBmV dBm dBmV
0 76 28 +14 -65 dBm -29 -108 43 dB
10 56 25 +4 -75 -33 -112 37 dB
20 32 19 -6 -85
21 32 19 -6 -85 -33 -112 27 dB
22 32 18 -7 -86 -33 -112 26 dB
23 30 18 -8 -87 -33 -112 25 dB
24 28 17 -9 -88 -33 -112 24 dB
25 28 16 -10 -89 -84 -33 -112 23 dB
26 26 15 -11 -90 -85 -33 -112 22 dB
27 24 15 -12 -91 -86 -33 -112 21 dB
28 22 14E -13 -92 -87 -33 -112 20 dB
29 20 13P -14 -93 -88 -33 -112 19 dB
30 18 0F -15 -94 -33 -112 18 dB
SS is Signal Strength
E is uncorrected errors
P is pixelation
F is picture freeze
Conv#1 conversion to dBm factor is -79 dB
Correct to DTV dBm factor is +5 dB
Conv#2 conversion to dBm factor is also -79 dB
Conv#1 factor is -79 dB; 30 dB to subtract the preamp gain, and 49 dB for dBmV to dBm conversion.
Correct to DTV factor is +5 dB. According to the designer of the meter, Rob Bredin, it adds a nominal 6.8 dB for the digital single channel scan, but not for the single frequency analog measurement used above. The actual value depends upon the shape of the signal. The analog measurement for channel 33 was -11 dBmV and -6 dBmV for digital, a difference of 5 dB.
Conv#2 factor is also -79 dB to convert from dBmV to dBm for the measuring system noise floor.
C/N is the ratio of the signal from the antenna to the measurement system noise floor to differentiate it from the TV SNR.
SLMs are calibrated in dBmV and used by antenna installers and cable techs. It was necessary to convert to dBm to match a TVFool signal report
With the attenuator at 26 dB, the SNR was 15 dB for a -85 dBm signal, just where you would expect it to be with a tuner NF of 6 dB. A signal power of -85 dBm is 6 dB above the minimum required signal strength of -91 dBm, which is 15 dB above the -106 dBm noise floor for a 6 MHz bandwidth DTV signal.
Part B: I added an RCA TVPRAMP1R preamp before the tuner.
Attenuator at 0 dB:
SS MAX
SNR 26 dB, 2 dB lower than without preamp because of added preamp NF. The tuner NF is traded for the lower preamp NF and the tuner NF is buried in the amplified noise floor, making it irrelevant.
Attenuator at 26 dB:
SS 66, to offset distribution losses
SNR 15 dB at -86 dBm, 5 dB above the -91 dBm minimum required signal; a 1 dB increase in weak signal sensitivity over no preamp
Part C: I substituted the CM7777#2 for the RCA preamp
Attenuator at 31 dB:
SNR 15 dB at -87 dBm, 4 dB above the -91 dBm minimum required signal; a 2 dB increase in weak signal sensitivity over no preamp. A preamp with a lower NF would increase the sensitivity even more.
CONCLUSION:
It is not possible to receive and decode 8VSB OTA signals with an accurate listed NM below -15 dB at -91 dBm. Even though a narrowband measurement shows sufficient C/N for a SNR of the signal to noise floor at the antenna output, the noise floor for a signal with a bandwidth of 6 MHZ is -106 dBm. Even without a signal present, the intrinsic noise power in a 6 MHz bandwidth is -106 dBm. Any digital TV signal that is weaker than -91 dBm, will have an SNR less than 15 dB, and can not be decoded.
As you can see, there was additional SNR available at the antenna that could not be utilized by the tuner because of the fixed noise floor of -106 dBm for a 6 MHz bandwidth signal.
Code:
Bandwidth Thermal Notes
Noise Power
1 Hz -174 dBm
280 kHz -119.5 dBm My SLM in analog mode*
6 MHz -106 dBm Analog or digital TV channel
* Does not include Noise Figure of preamp used
to increase sensitivity of meter
