Test, QC & Monitoring Global Viewpoint – December 2017

A Guide To Selecting Test & Measurement Gear

TV Test & Measurement spans from peaking the glamour of show business to testing a DC bus. Waveform monitors are dedicated systems and displays designed to examine, tweak, monitor and graphically display source waveform information. Behind the scenes, general purpose instruments are necessary to identify and correct foundation-level technical issues before they turn catastrophic .

When most people think of TV T&M they think of waveform monitors. A waveform monitor is an oscilloscope-based device unique to TV and ubiquitous in TV stations, used to visualize the electrical form of a TV signal and precisely measure its voltages. In the days of analog video and rack-mounted waveform monitors, reading a waveform monitor was straightforward and easy with some training, as was the vectorscope. They were the 20th Century, TV T&M equivalent of manual typewriters.

Today’s digital waveform monitors are computers filled with new tools to assess and measure the parameters and minutia of nearly every visual content variant that can be observed, tested and measured. They are available in the newest formats such as 4K, and many offer a multitude of visual and audio variants and options. SDI typically contains multiple audio channels embedded on the same coaxial cable, which is the opportunity for some waveform monitors to include extensive calibrated audio monitoring.

Needs check

What's the best waveform monitor for the job? A few questions will define its features. Where will it be used and for what purpose? This will determine whether portable, rack-mounted or a rasterizer is best. How many simultaneously displayed channels are needed? Will it be used to shade multiple cameras or to monitor a single source? These questions will help reduce a field of many products down to a few. Once the basic needs and features are identified, it is time to research available models from high-profile manufacturers such DekTec Leader, Hamlet, Phabrix, Rohde & Schwarz,  Sencore, and Tektronix.

Features vary by manufacturer and model. Many features often must be ordered as options, such as 3G, loudness monitoring, data monitoring, test signal generation and color bars. Will the waveform monitor be synced to burst or Tri-Level sync? Does it need HDR or WCG options? Some new features can often be added with a firmware or software update. UHD and ATSC 3.0 make higher bandwidth and 4K capabilities, even if not needed today, an investment in the future.

Basic O-scopes

Of course, broadcast T&M covers more than measuring, analyzing and adjusting video content signals. Transmitter set-up needs a good RF spectrum analyzer. Another mission- critical T&M need can be DC-bus simple. Let's use a real-world example. Imagine a DTV transmitter with a 50VDC bus that powers the PAs. After many years of reliable service, the individual, 50V, commodity-level, switched power supply modules each randomly begin to fail.

When one 50V module output drops off for some unknown reason, the other modules drop off one-by-one until all six stop outputting DC. In this real-world example, all this happens usually in under 10 minutes. When the bus current drops, bus voltage drops, PA currents fall to zero, and the transmitter essentially runs out of gas. The only way back on air is to manually reset the 50V modules. This requires a trip to the transmitter site and manually cycling the power supply breaker switches. This fix may last another 10 minutes or for a month or more. In this case, both have happened. New modules make no difference and the sequence of module failure is always random.

What is going on here? We don't know. During the latest major 50V module tantrum, I rotated in some new replacement power supply modules as the system cycled through 10-minute array failures. Over the next 6 hours, the transmitter was still experiencing power supply array failure spells. I was running out of ideas and energy when I noticed the transmitter had mysteriously fixed itself. After more than an hour on air with no observable discrepancies, I went home. Two weeks later, and it hasn't hiccuped yet.

I hate it when transmitters fix themselves, because they don't. Instead, chances are the next time it fails more people will be watching and it won't fix itself again. Why? Because its the way of TV. These frustrating type of intermittent problems are the most difficult to troubleshoot, especially at a cow pasture transmitter site 50 miles from nowhere. It will take more than a digital voltmeter to resolve this issue.

These 50 VDC supply modules can intermittently fail, as the #1 and #3 modules did in this photo.

These 50 VDC supply modules can intermittently fail, as the #1 and #3 modules did in this photo.

Troubleshooting an intermittent switching power supply problem requires a good, general purpose oscilloscope to look for hash and trash on the DC and ground busses in the transmitter, and to eventually identify and isolate its source. The problem could be coming in on the building AC mains service, but is more likely being generated within the transmitter cabinet. Could it be a defective PA that otherwise appears to work fine, bleeding something back on the 50V bus? I won't know anything more without a scope.

I needed to establish a baseline and wait for the intermittent gremlin to return, but my old general-purpose oscilloscope won’t power up. A friend loaned me his analog 5 MHz BK1405. In need of a new scope, I checked eBay and found that particular model sells for $50, which works out to $10/MHz. I began to wonder if I could afford a new scope better than 5MHz. A visit to the Tektronix website showed me a terrific new general-purpose digital scope for the same $10/MHz price. A 50 MHz scope lists for $550. I was surprised at the value but unsure what I needed. The website also happened to offer the presentation “10 Factors in Choosing a Basic Oscilloscope.” That presentation is the source of the following information I used to make my selection.

Choosing a scope

Tektronix says the #1 Factor to consider is bandwidth. It is defined as the frequency at which a sine-wave input signal is attenuated to 70.7% of its true amplitude (the -3 dB or ‘half-power’ point). Bandwidth, along with sample rate, determines the smallest rise-time that an oscilloscope can measure. When selecting bandwidth, use the ‘5 X rule’, where Bandwidth = 5 x Maximum frequency of interest.

The sample rate is Factor 2, and the ‘5 X rule’ applies to the highest frequency in the circuit. Nyquist is an absolute minimum, and it only applies to sine waves and assumes a continuous signal. Glitches are not continuous, and sampling at only twice the rate of the highest frequency component is not enough.

The third factor is the number of input channels. For example, two channels allow comparing a component’s input to its output. Some ‘scopes share the sampling system between channels to save money, which can reduce the sample rate.

Compatible probes are Factor 4. During measurements, probes become part of the circuit, and can introduce resistive, capacitive, and inductive loading that alters the measurement. Use probes designed for use with your scope, and they have sufficient bandwidth to match that of the scope.

Triggering is Factor 5. Triggering synchronizes the horizontal sweep at the correct point in the signal rather than randomly. Specialized triggers can respond to specific conditions in the incoming signal. For example, triggers can make it easy to detect a pulse that is wider than it should be.

The next factor is the record length, because an oscilloscope can only store a limited number of samples. The waveform duration will be inversely proportional to the oscilloscope’s sample rate. Time Interval = Record Length / Sample Rate. A good basic scope will store over 2,000 points, which is more than enough for a stable sine-wave signal, needing perhaps 500 points. But, to find the causes of timing anomalies in a complex digital data stream consider a million points or more.

Automated measurements and analysis are factor number 7. Automated measurements appear as on-screen alphanumeric readouts. They are more accurate than direct graticule interpretation. Basic oscilloscopes often include an FFT function that allows viewing the spectrum of the acquired waveform. This can be useful for trying to determine the source of noise, for example.

The final three factors are easy of operation, connectivity and serial bus decoding. Connectivity enables integration by USB, Ethernet, Wi-Fi, and/or a video port for an external display.

After matching the above information with my own needs and budget, I ordered a new 50 MHz Tektronix TBS1052B for my road kit. The story isn't over yet. When my new scope arrives, I'll make some baseline measurements and wait for those 50V power supplies to misbehave again. Watch out for my new fly-swatter, gremlins.

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