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Old 24-Jan-2015, 2:14 PM   #87
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 173
Compare Several Preamplifiers OTA-DFW

Signal measurements from several preamplifiers were compared, while receiving selected UHF channels and one VHF channel in the Dallas Fort Worth area.

Preamps Compared (shown in image below):

- Channel Master 7777 (new version)
- Channel Master 7778 (new version)
- Channel Master 7777 (old version)
- Channel Master 7778 (old version)
- Winegard LNA-200
- Antennas Direct PA-18

All 7 preamps performed overall within ~0.5 dB of each other when comparing the measured MER* (modulation error ratio). The RCA TVPRAMP1R showed best UHF MER performance at 30.2 dB. MER is essentially the post-detection S/N ratio in dB. (MER performance graph below)

As expected, there was considerable difference in UHF output signal strength (approximately 9 dB variation). Both the CM7777 versions (old and new) had highest UHF output signal strength. (Output signal strength graph below)

Note: Previous measurements, made a day earlier, with these preamplifiers indicate that the results are somewhat repeatable. As the data in the graphs indicate, the precision of measurements can be made to within 0.1 dB, but mainly due to the propagation path, the accuracy is somewhat less. The rank order of the preamp’s measured performance was unchanged from one day to next.

*Modulation Error Ratio (MER)

Measured MER can be a good relative indicator of the overall system noise figure. Within limits, a system with a lower noise figure will have a higher MER. There is nearly 1:1 correspondence between noise figure and MER; a 1dB improvement in noise figure will provide about 1dB improvement in MER.

Other factors affect system MER, for example if two preamps had the same noise figure and one had higher gain, then likely the system MER would be highest for the preamp with highest gain. With typical preamps and system losses, there could be about 0.1 dB MER increase for about 1-2 dB of additional gain.

More to be posted, some tomorrow:

- Test Procedure & Configuration
- MER Performance per Channel
- Signal Output Level per Channel
- VHF Channel 8 Performance (MER & Signal Strength)
- Other Preamps
Attached Images
File Type: jpg 7 Preamps (Compare).jpg (46.1 KB, 9169 views)
File Type: gif Compare Preamps Output Sig. Strength_DFW.gif (9.3 KB, 6209 views)
File Type: gif Compare Preamps MER_DFW.gif (8.7 KB, 6469 views)

Last edited by tripelo; 24-Jan-2015 at 4:49 PM. Reason: # Measurements from 3 to 18
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