Weak Signal SNR Measurements, Test 2
A second test was made using the digital mode of the Sadelco DisplayMax Signal Level Meter (SLM). In this mode, the meter is sensitive down to -20 dBmV (-69 dBm). I added two preamps in series before the SLM:
Antenna > Variable attenuator > splitter > tuner and SLM with two preamps in series before the meter; no preamp for the tuner
splitter > CM7777HD 30 dB > CM7777HD 17 dB > SLM
The combined preamp gain, as measured, is 47 dB. This puts the measurement system noise floor at -67 dBmV (-116 dBm), but is limited to -106 dBm because the measurement bandwidth is ~ 6 MHz when measuring a signal in the digital single channel scan mode.
I measured the noise floor of the measurement system with the meter set to CH 33, but no signal connected to the input of the first preamp; without and with a 75 ohm terminator on the input:
Meter set to analog measurement, video carrier frequency of 585.250 MHz, 280 kHz bandwidth:
-18 dBmV - 47 dB preamp gains - 49 dB convert to dBm = -114 dBm
With 75 ohm terminator on input of first preamp:
-14.9 dBmV - 47 dB preamp gains - 49 dB convert to dBm = -110.9 dBm
Meter set to digital measurement, 6 MHz channel scan:
-12.6 dBmV - 47 dB preamp gains - 49 dB convert to dBm = -108.6 dBm
With 75 ohm terminator on input of first preamp:
-10.3 dBmV -47 dB preamp gains -49 dB convert to dBm =
-106.3 dBm
Meter set to digital measurement, single frequency center channel 587.000 MHz, 280 kHz bandwidth:
-18 dBmV - 47 dB preamp gains - 49 dB convert to dBm = -114 dBm (114 - 108.6 = 5.4 dB difference, digital to single frequency)
With 75 ohm terminator on input of first preamp:
-14.7 dBmV - 47 dB preamp gains - 49 dB convert to dBm = -110.7 dBm
CH33 was again used for the test signal. Vacant second adjacent CH35 was used for the noise floor measurement; there was too much spill-over from CH33 into CH34 with two preamps in series.
Code:
Weak Signal SNR Measurements, Test 2:
Attn Sony SLM Conv#1 Correct CH 35 Conv#2 C/N
dB SS SNR CH 33 > to dBm to DTV Noise > to dBm dB
dB dBmV dBm dBmV
0 72 26 +33.4 -62.6 same +6.9 -89.1 26.5 dB
10 50 24 +23.5 -72.5 no -9.4 -105.4 32.9 dB
20 30 18 +13.8 -82.2 change -10.6 -106.6 24.4 dB
21 30 17 +12.5 -83.5 -10.7 -106.7 23.7 dB
22 28 16/17 +11.7 -84.3 -10.7 -106.7 22.4 dB
23 26 15/16 +10.4 -85.6 -10.8 -106.8 21.2 dB
24 24 14/15 +9.2 -86.8 -10.7 -106.7 19.9 dB
25 22E 13/14 +8.8F -87.2 -10.8 -106.8 19.6 dB
26 MIN 0 NO PIC -10.7 -106.7
SS is Signal Strength
E is uncorrected errors
F is picture freeze
Conv#1 conversion to dBm factor is -96 dB
Correct to DTV dBm factor not needed
Conv#2 conversion to dBm factor is also -96 dB
This test confirms the conclusion of Test 1.
OTA 8VSB signals weaker than -91 dBm can not be received and decoded using additional antenna gain. The noise floor for a 6 MHz bandwidth signal is fixed at -106 dBm. This noise power exists in a 6 MHz channel even when there is no signal.
Signals weaker than an accurate NM -15 dB on a TVFool report will have an SNR less than 15 dB, and can not be decoded.
Additional antenna gain can improve the SNR of a weak signal, which is something a preamp can not do. But, if the signal is still weaker than -91 dBm, the additional antenna gain is of no benefit.
