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Old 25-Jan-2015, 4:04 PM   #91
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 173
Compare Preamplifiers - Test Procedure & Configuration

Referring to Preamp Comparison data posted earlier.

Comparison method; signals and path from antenna to measurement receiver:

1. Received on Radio Shack all-band antenna (similar to VU-90) at approximately 29 feet AGL, thru 50 feet commercial grade RG-6 cable

2. Attenuated by 40 dB; two JFW serial step-attenuators total 37 dB followed by 12” RG6, then one Holland 3 dB FAM attenuator.

3. Amplified by preamp-under-test with its power inserter and power supply

(If available, preamp FM trap set to ‘In’, If dual inputs, set to ‘Combined’)

4. Attenuated 3 dB; Output thru 4” RG-59 and Holland 3dB FAM attenuator

5. Received from preamp and 3dB attenuation, measured on Sencore SLM1456CM

Image depicting Test configuration shown below:

The Sencore 1456CM executed a channel scan program wherein Modulation Error Ratio (MER) and Signal Level were measured and recorded while receiving DFW stations. Each test consisted of SML1456CM channel scan that measured signal MER and Strength for each of the following channels: 8, 9, 14, 23, 29, 36, 41, and 48. The scan was repeated three times (total three scan test time ~ 5 minutes). Then, the next preamplifier was tested, and sequence repeated until all 7 preamps had been examined.

Channel 9 data not planned to be included in the posted data. This is mainly because channel 9 is weaker than channel 8, to reliably receive it with all preamps would have required input attenuator adjustment during the tests.

The time from beginning-to-end for the complete test operation was about 75 minutes. This total time included, as a data integrity check, some replication of the first two scans at the end. Also included some other tests not reported.

The test data was downloaded from the Sencore 1456CM to a desktop computer for analysis and formatting.

The distance to the stations is ~28-30 miles, the path line-of-sight. Prior and post measurements indicated the signals were relatively steady during the test time. A clear stable-weather day was chosen to perform the tests. The signals are strong at this location, thus the need for 40 dB attenuation prior to the preamps. The objective was to lower signal strength such that the stations had only moderate S/N ratios within measurement range. (High signal strength can cause maximum S/N indication (max MER=36 dB) on the SLM1456CM.

The RF environment in DFW consists of dense signals. Virtually every usable UHF channel is occupied, only 2 VHF stations (Channels 8 & 9). The FM band has nearly every available channel occupied (many with full-power). Other strong signals include NOAA weather radio ~ 4 miles away.

The Radio Shack antenna responds to most of the above listed signals.

The test setup somewhat emulates conventional TV reception. The 40 dB of attenuation to preamp input reduces all signals, including FM and other potential interference sources. This reduction in signal strength could correspond to fringe area reception. The receiver is represented by the SLM1456CM and a transmission line loss between preamp and receiver being represented by a 3dB attenuator (similar to ~50’ RG-6 at UHF).


Reference charts below: TVfool & FMfool charts for the preamp compare test location.

More preamp test data to be posted later.

Attached Images
File Type: jpg Preamp Test Configuration.jpg (100.0 KB, 5569 views)
File Type: jpg Preamp Compare Test Location TVfool.jpg (122.2 KB, 5280 views)
File Type: jpg Preamp Compare Test Location FMfool.jpg (136.6 KB, 5578 views)

Last edited by tripelo; 26-Jan-2015 at 1:17 AM. Reason: Edited TVfool Image
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