Understanding & Measuring Video TV-RF Signals(III)

Understanding & Measuring Video TV-RF Signals

By Glen Kropuenske, CET, Sencore Application Engineer

Editor’s Note: This is the last of a three-part article.

 

Measuring TV-RF Signal Levels – dBmV

The signal levels received on televsion antennas or provided to TV sets by the cable system are typically in the millionths of a volt or microvolts (µV). Calculations with microvolts are difficult because they involve six or seven digit decimal numbers.

The decibel (dB)

A simpler unit of measurement, the decibel, was adopted for convenience. A decibel (dB) is a logarithmic ratio with levels from 0-100 that are simply added or subtracted instead of multiplied or divided. The decibel is adopted from early telephone engineers for television use. The formula for the decibel used for television work is:

Decibel (dB) = 20 log E1/E2.

The dB has no absolute value but merely tells how much greater or smaller a quantity is from a pre-established reference. The relationship between dB levels is non-linear. A value of 40dB is not twice as much as 20dB. It is a lot more. Here are some examples:

10dB

3.1 X reference level

20dB

10 X reference level

30dB

32 X reference level

40dB

100 X reference level

50dB

300 X reference level

60dB

1000X reference level

The reference voltage level or 0 dB in the television industry is the level to which any plus or minus number of decibels is referred. The reference level is 1,000 microvolts or 1 millivolt (1mV) measured across a 75 ohm impedance. Because the reference is 1mV, the unit of measurement is often shown as dBmV to distinguish between other reference dB levels.

This level was established in the early days of television as it was learned through experiments that this level was the minimum needed to produce a good noise-free video picture. Although most modern televisions are more sensitive and can produce good pictures down to 500 µV, the 1000 µV level provides a good margin of safety in design and is still used today.

dBmV

µV

dBmV

mV

dBmV

mV

-20

100

0

1

20

10

-19

110

1

1.1

21

11

-18

130

2

1.3

22

13

-17

140

3

1.4

23

14

-16

160

4

1.6

24

16

-15

180

5

1.8

25

18

-14

200

6

2

26

20

-13

220

7

2.2

27

22

-12

250

8

2.5

28

25

-11

280

9

2.8

29

28

-10

320

10

3.2

30

32

-9

360

11

3.6

31

36

-8

400

12

4

32

40

-7

450

13

4.5

33

45

-6

500

14

5

34

50

-5

560

15

5.6

35

56

-4

630

16

6.3

36

63

-3

700

17

7

37

70

-2

800

18

8

38

80

-1

900

19

9

39

90

0

1000

20

10

40

100

Chart 2: dBmV vs. voltage chart.

 

Calculating dB gain & losses

Signal losses and gains in the strength of a television signal can be easily calculated using the dBmV reference. Chart 2 shows the level in mV associated with dBmV values from -20dBmV to +20dBmV. The signal strength or level of a TV-RF channel increases and decreases as it passes through the cable distribution system. As the TV-RF signal passes through an amplifier, its dBmV level is increased. As the TV-RF signal passes though cable and a power splitter, the signal dBmV level decreases.

For example, the signal level of the video carrier from a wall cable jack for channel 5 may be specified as 3 dBmV. This indicates the video carrier for channel 5 (77.25MHz) has approximately 1.5 mV or 1500 µV of signal strength. If this signal was divided with a splitter and ran through a cable to another room, the signal strength may be reduced to –2dBmV or 800 µV.

The loss or gain of a device or cable in a cable distribution system may also be specified using dBmV. If an amplifier has an input level of 5 dBmV and the measured output is 20dBmV, the signal has increased from 1800 µV to 10,000 µV. The amplifier has provided the TV-RF signal with 15 dBmV of gain. Loss is also specified in dBmV.

 SL754D Video & Audio Carrier Level Measurements

The signal strength of the video and audio carriers of a TV-RF signal may be measured using a signal level meter. A signal level meter measures the peak amplitude of the RF video carrier of the TV channel providing an indication in dBmV. It also can measure the RF sound carrier level in dBmV.

Fig. 12 – A signal level meter measures both the audio and video carrier levels of the TV-RF channel in dBmV.

A signal level meter is an essential tool in antenna installation and TV-RF distribution system troubleshooting. A signal level meter will save much time and pay for itself quickly, no matter if you are troubleshooting a city cable system or a distribution system in a home or large building.

A signal level meter is basically a TV channel receiver with a level detector and readout meter. The basic blocks of a modern signal level meter are shown in figure 12. The tuner enables the meter to tune to individual TV-RF channels for measurements.

Modern signal level meters use electronic tuning with digital control of the tuner by a microprocessor. This enables fast tuning with little tuning error. It further enables the meter to be configured for tuning to off-air VHF/UHF channels and numerous cable channel plans including FCC, HRC, ICC and others around the world. The tuner can easily tune between the video and audio RF carriers for measurement readout of both the video and audio carrier levels simultaneously.

Fig. 13 – Basic blocks of a microprocessor controlled digital signal level meter.

The heart of the signal level meter is the level detection and measurement circuit. This circuitry detects the carrier level of the input video and/or audio RF level of a selected TV-RF channel. Automatic attenuators provide measurements though a signal range from –35 to 60dBmV. Microprocessor compensation makes possible level measurement accuracy of .75 dBmV.

The Sencore model SL754D is a signal level meter as described. The SL754D provides level measurements in dBmV for any TV-RF channel. It performs several special tests and provides automated inspection capabilities. The SL754D is fully microprocessor controlled providing great versatility in TV-RF signal analyzing.

Signal level measurements are easy with the SL754D. With a full battery charge, turn on the SL754D and connect the signal input to the RF Input receptacle on the top-side of the meter. Select the channel number of the cable channel you wish to measurement using the arrow keys or by pressing the numeric keys to select the channel number. Press the ENTER key to input the channel selection shown on the screen or press the EXIT key to start over.

Fig. 14 – The Sencore SL754D TV-RF signal analyzer.

The SL574D automatically tunes to the entered channel and displays the carrier levels of the video and audio carriers. An "Under" reading on the right side of the display indicates a signal level less than –35 dBmV.

 

 

 

 

 

 

Selecting an SL754 Channel Plan

The SL754D can be programmed to automatically tune to the carriers of channel numbers specified by many different cable channel plans. The built in channel plans that may be selected from the front panel are: CUSTOM 1, JAPAN, ISRAEL, GERMAN, BELGIUM, PAL B/G, PAL I, PAL A, VHF/UHF, IRC, HRC, FCC, CUSTOM 2.

To select a channel plan:

1. Press the CHANNEL PLAN Pushbutton
2. Use the up or down arrow pushbuttons to increment through channel plans.

(Selections are shown in the display)

3. Press the ENTER pushbutton to select a channel plan.

(You may exit the channel plan selections at any time by pressing the EXIT pushbutton)

The SL754D comes from the factory set to the FCC cable channel plan. In this mode, you can tune to the standard cable channels 2-134. Use this plan for cable and VHF channels 2-13. Do not use this plan to tune and measure off-air UHF antenna signals on channels 14-69. If you want to perform signal level measurements of off-air UHF channels, you must change the channel plan of the SL754D to the VHF/UHF plan.

 

Creating A Custom Channel Plan(s)

While the FCC cable or VHF/UHF Channel Plans fill most measurement applications, these plans increment through all the channel numbers. Extra time is sometimes needed to increment through inactive channel numbers, especially from the highest to lowest channel. Furthermore, many cable systems have specialty carriers that are not TV-RF signals. These may be at frequencies not specified by the FCC or VHF/UHF cable plans.

The Channel Plans of the SL754D include two channel plans that may be custom configured by the user. These channel plans are the CUSTOM 1 and CUSTOM 2 plans.

These plans enable you to configure the SL754 to tune and measure only the signals on a particular cable distribution system or available in a given off-air signal coverage area. This speeds-up measurement by eliminating the unused channels in the SL754D’s tuning sequence. It further permits adding any specialty channels or frequencies for easy access and measurement.

The CUSTOM 1 and CUSTOM 2 channel plans can be created and modified in the PROGRAM PLAN menu of the SL754D. To enter this menu, press the PROGRAM PLAN Pushbutton on the front panel of the meter. Two options are available to create a custom channel plan including: 1) Scanning and 2) Entering. A third option provides a means to edit the base plans or exiting CUSTOM 1 and CUSTOM 2 channel plans.

The CUSTOM 1 and CUSTOM 2 plans are created starting with one of the basic channel plans. The basic channel plans define the color system and audio carrier offset of the video and cable system for the SL754D. Customizing creates only the channels in these basic plans that you want to measure and permits adding specialty carriers to the tuning sequence.

The fastest way to create a custom channel plan is to use the scanning method. With the cable or antenna signal input to the meter, the SL754D scans all the channels of the basic plan and identifies all the TV-RF channels that have more than –5dBmV. These channels are then recorded as part of the customized channel plan. After the scan is complete you can delete unwanted channels, add wanted channels or add up to 9 specialty frequencies for measurement. Special carriers are noted on the SL754D as frequencies 1 – 9 designated on the display as F1 – F9. Once complete you exit the menu by pressing the EXIT Pushbutton and the meter momentarily reads "Custom 1 Saved".

Another option for customizing a channel plan is by "entering". This option requires that you enter the channel number and corresponding frequency of each TV-RF carrier. You must also enter the frequency of any specialty carrier you want in the channel plan. This customizing option is recommended if you only have a few TV-RF carriers, you need to enter a considerable number of channel frequency offsets or have a system with few RF channels and many specialty frequency carriers.

The modify option of the Program Plan menu gives you the versatility to delete channels, add channels, or modify specialty frequency entries in a defined custom channel plans. Push the PROGRAM PLAN Pushbutton, select CUSTOM 1 or CUSTOM 2 and select the MODIFY option. Next, select the defined custom channel plan (CUSTOM 1 or CUSTOM 2) as the base plan you wish to modify. You can then modify the plan as desired. When you exit the menu, changes you made to the base plan are saved in the CUSTOM 1 or CUSTOM 2 plan you selected at the start of the menu.

Once you have created or modified a custom channel plan, you’ll want to start using it. You can begin using either the CUSTOM 1 or CUSTOM 2 channel plans by pushing the CHANNEL PLAN pushbutton on the SL754D and selecting the CUSTOM 1 or CUSTOM 2 channel plan.

Fig. 15 - Menu path and display views for creating a custom program plan on the Sencore SL754D.