Thread: 91XG or DB8e
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Old 13-Aug-2017, 8:33 PM   #25
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,569
Combining the Ouput of Two Preamps can Reduce Weak Signal Sensitivity

I assembled what equipment I had on hand to investigate what happens when the output of two preamps are combined with a splitter in reverse.

Equipment used:

Blonder Tongue HAVM-1UA Agile Modulator - 470-806 MHz, output ~ +30dBmV
Sadelco DisplayMax 800 Signal Level Meter (SLM)
Sadelco 719E Signal Level Meter (SLM)
2 Channel Master 7777HD Preamps
RCA TVPRAMP1R Preamp to increase sensitivity of SLMs
2-way splitter used as a combiner
3 20dB fixed attenuators
1 10dB fixed attenuator
1 6 dB fixed attenuator

For a test signal, I used the video carrier of analog channel 20 from the modulator.

The 6 dB attenuator was used to adjust the sensitivity of the SLM to keep the readings centered on one range of the SLM to avoid switching to another range. The noise floor of the measurement system was well below the noise from the 7777HD, so the NF of the RCA preamp could be ignored.

I made the first measurements with the DisplayMax 800 SLM, on the right in the photo. The modulator signal is quite stable within 0.1 dB, and the SLM has 0.1 dB resolution. However, I had a problem measuring the random noise from the CM7777HD because the SLM digital display would follow the rapid changes, making it difficult to read. I switched to the 719E SLM (on the left) and its slower response was able to average the noise reading for me. The noise was measured on a nearby channel not used by the modulator.

SNR of CM7777HD #1 

Modulator > atten > CM7777HD > RCA Amp > atten > SLM
  CH 20     70dB    #1, 16dB    23dB      6dB
                  |          |         |          |      
SIGNAL        -38.4dBmV  -22.4dBmV  +0.6dBmV   -5.4dBmV
              -87.2dBm   -71.2dBm  -48.2dBm   -54.2dBm

NOISE                    -41.4dBmV -18.4dBmV  -24.4dBmV
                         -90.2dBm  -67.2dBm   -73.2dBm

SNR = 24.4 - 5.4 = 19 dB
I then added a combiner and the second CM7777HD. I do have two modulators, but not enough attenuators to feed a weak signal to the second 7777HD. This shows the affect of the noise from the second CM7777HD on the SNR of the CH 20 signal:

SNR of CM7777HD #1 combined with CM7777HD #2

Modulator > atten > CM7777HD >
 CH 20      70dB    #1, 16dB  \
                             combiner > RCA Amp > atten > SLM
                              /          23dB      6dB     
                    CM7777HD >                             |
                    #2, 16dB              SIGNAL        -9.5dBmV 
                                          NOISE        -26.8dBmV

SNR = 26.8 - 9.5 = 17.3 dB

Loss of sensitivity from combining = 19.0 - 17.3 = 1.7 dB

My equipment isn't lab grade, but I have enough confidence in my measurements to conclude that combining the output of two preamps with a splitter in reverse causes a loss of sensitivity that can have a critical negative affect on marginal signals near the digital cliff.
Attached Images
File Type: jpg DisplayMaxImg1.jpg (92.7 KB, 925 views)
If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 13-Aug-2017 at 11:04 PM.
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