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Old 3-Nov-2017, 9:24 PM   #2
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,543
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.

Attached Images
File Type: jpg DisplayMaxImgCH31D.jpg (96.2 KB, 1376 views)
File Type: jpg Rob Bredin.jpg (129.3 KB, 1301 views)
File Type: jpg 75 ohm terminator2.jpg (16.4 KB, 1287 views)
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Last edited by rabbit73; 16-Nov-2017 at 2:24 PM.
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