Broadcast Wind DTV RF Probe

One bit of technology coming out of our current USDA Phase II project was the development of automated probes to be used to capture DTV RF signal parameters over a long period of time in diverse locations, distances, weather conditions, in the vicinity of wind farms.

We developed an inexpensive, easily deployed field monitoring probe to record key television signal metrics, over a one year period, at remote, unattended measurement sites and transmit the data back to our central location for logging, processing and analysis. The metrics captured include date, time, 8VSB lock, RF frequency, signal strength, signal modulation error ratio (MER), receiver AGC gain, frequency offset, and transport stream (TS) data,  packet error rate (PER), reception margin over threshold of visibility (TOV) and detailed multipath (equalizer tap) data showing signal reflections (or echoes) from the wind turbines and other objects in the field.  The probe is able to capture all of these metrics at a rate of 2 times per second.  It is remotely programmable via broadband wireless modem and a Windows GUI. 

The electronically-controlled programmable attenuator enables determination of the signal margin at the probe’s receive site.  Determination of margin is accomplishedby increasing the attenuation to lower the signal to a level just abovethe threshold of visible errors (TOV) and audible errors (TOA).

We chose the Vaunix model LDA-102  (3.86” x 2.52” x 1.35”) 50-Ohm relay-switched step attenuator that covers a frequency band from DC to 1 GHz (i.e., the entire broadcast television band).  It can be easily varied in 1-dBsteps over a 63 dB range via a USB interface to the probe computer (it actually has 0.5 dB steps, but only 1-dB attenuation steps will beused for this field test). Attenuation accuracyis 0.3 dB or 5% of the selected attenuator value (whichever is greater),which is more than adequatefor determining margin in the field. The switchingspeed of this relay step attenuator is about70 nsec, which is significantly more than adequate in speed for this particular project.

The attenuator is powered (5 Volt) by on-board computer USB port so no separate, external power supply is needed.

The low noise RF amplifierprovides signal amplificationwithin RF probe not only to provide enough level to the reference DTV receiver (afterexperiencing insertion loss from the bandpass filterand programmable attenuator) but it also determines the sensitivity of the entire probe.The primary concernwith using any activedevice before the DTV receiveris overload from interfering signalssuch as other DTV signals, particularly from nearbyfull-power stations.The single RF channel bandpass filterin front of the pre- amplifier, and selection of test frequency without a high power station transmitting on first or second adjacent channels help avoid overload interference, but it is good practice to also use of a robust) amplifier with good overloadperformance; i.e., large third order intercept points, (IP3).

                                                    

A good performance commercial DTVreceiver that simulates at least fifth generation (5G) consumer receivers sold since 2006 was desired.The Silicon Dust model HD HomeRun Tech3 DTV receiver provides a set ofdiagnostic parameters that are retrievable from the 100Base-T Ethernetconnection (RJ-45) on theback of the receiver under computercontrol. While not neededfor this project,transport stream analysisis possible as well.

The computer controller, while not directlyhandling the RF signal, has control over the programmable attenuator (via a USB port) and the DTV receiver(via an Ethernet port), and gathers all the requiredfield test datafrom the receiverfor subsequent transmission over the Internet(via cellular modem)for final data processing at our centrallocation. It needed to be rugged and robust for various field environments and easily restart and pick up where it left off duringpower surges or failures.

A small form factor computer was found to meet all of these requirements.  The system featuresan Intel Atom D2500 processor with 4 GB DDR3 RAM, 120 GB solid state drive (SDD), dualGigabit LAN portswith Ethernet controller (to communicatewith the DTV receiver and an Internetmodem), 6 USB ports,dual external serial COM ports,and a single internal parallel port. While operating autonomously in the field, the computer needsno mouse, keyboard, or monitor. However,upon setup, there are ports for all three of these devices (mouseand keyboard via USB portsand monitor through eitherVGA or DVI-I ports).

The computer residesin a small well-shielded, fan-less enclosure that operatessilently, and includesboth an external 60 Watt 12V AC-DC adapter and an 80W internal DC-DCpower board that provide more than enough power for the computer.

The computer uses Windows 7 in 32-bit mode.  The probe’s control software was written in C.

The RF probeis contained in a PVC enclosure that easily allows all of the componentsto fit cleanly inside. There is easy accessto the components for trouble-shooting and calibration as needed.  It is weather-proof so it can be placed in inhospitable settings.

The RF probedesign contains several measurement features utilized in the DTV reception analysis and characterization of a receiver due to wind turbines located near DTV transmitters. Each of these measurement features is described below.

The probe designenables indirectly measuringsignal field strengthat its antenna input. The receiver captures signal power level atits RF input, and calculates the field strength at the antenna input knowing pre-measured gains and losses as well as using the well-known dipole factor for antenna conversion between field strength and power.

The overall systemgain, GS, is defined as the signal level increase between the antenna output (same as coaxial downlead input) and the DTV receiver input (i.e., accounting for the coaxial cableloss, the filterinsertion loss, the attenuator insertion loss plus extra programmed loss, the splitterloss, and the pre–amplifiergain). Once this system gain is known for a givenRF channel, the field strengthcan be calculated with the following formula:

F.S. (dBµV/m)  =  S – GS +A + K – GA   )

where              F.S. is the DTV field strength (indBµV/m) at the antennainput

S is the DTV signal power level at the DTV receiver input (in dBm)

GS is the overallsystem gain (indB) at the RF test channel centerfrequency

A is the selected attenuation level (indB)

GA is the forward antenna gain (in dBd) at the RF test channel center frequency

K is the dipole factor (in dBµV/m-dBm) at the RF test channel center frequency

The speedof the receiver’s signal level measurement is important (e.g., AGC speed) since dynamic signal levels are measured from the wind turbine’s rotatingblades.

The RF probeprovides a signalquality measurement at the output of the DTV receiver’s equalizer referred to as Modulation Error Ratio,MER (in dB). This measurement is similar to the conventional signal-to-noise ratio (SNR), except that it uses both the demodulated in-phase (I) and quadrature (Q) baseband channelsignals for the calculation. This parameter, when accurately determined in the DTV receiver’s 8-VSB demodulator chip, provides an indication of the signal qualityat the equalizer’s output (i.e., the error correction circuitry’s input). Typically, good modern-day receivershave a 15 dB value for MER at TOV.

It is important to note that the “noise” referred to in the signal qualitymetric is not only the traditional additive white Gaussian noise (AWGN)found at the inputto the receiver’s analog tuner component,but also any other undesired signals that might be present,  such as co-channel DTV interference signals, adjacent channel cross-modulation and/orinter-modulation signals due to circuitnon-linearities, and multipath signals(delayed replicasof the noise–like desired DTV signal)that are not completely canceled.All of these “noise”components are represented in the MER valueprovided by the RF probe.

Multipath is considered a lineardistortion, and can therefore be removed by a linear equalizer in a DTV receiver (assuming that the equalizer is robust enoughand the receiveloops are locked).However, if the echo delayis longer than the correction range of the internal equalizer or the speed of the equalizer is not fast enough to follow dynamic multipath(e.g., from a moving airplane or from moving turbine blades),then the MER value will reflect this lack of perfect correction in the form of a higher MER value. If the receiver becomes unlocked(e.g., carrier recovery or symbol clockrecovery or data frame recovery fails),then no valid MER valueis available from the receiver. It should be noted that if the variousrecovery loops remained locked, even though the unit may be below the error threshold (evendown to 4 or 5 dB SNR in some ATSC receivers), it is still possiblefor many 8-VSB demodulators to provide a reasonably accurate value of MER sincethey often use the periodically-transmittedbinary data frame sync as a known reference signal to calculate MER.

The RF probeprovides a data quality measurement in the form of transportstream (TS) packeterrors. While this particular ATSC receiverused in the RF probeproduces a signalquality number (valuebetween 0 and 100 that varies logarithmically with the number of packet errors),the actual numeric value of packet errors is a better parameter to use for this application. The actual implementation of the errorcounter in the DTV receiverhas been determined to be just a raw counter of packet errors that is never reset at a particulartime interval, but only during a channel change.To determine if new packet errors have occurredsince the last polling of this counter, subtraction of the two error counternumbers is required.

This error countervalue indicates the number of uncorrected transport stream MPEG data packets (as determined by the Reed-Solomon decoder) at the output of the 8-VSBdemodulator, regardless of whether the causeof the errors is low signal level, multipath, interference, or overload. This error value can be used to determine reception capability at a given site for variouspropagation conditionsat the time of testing.

The RF probeprovides a means to measure the actual reception margin (in dB) at a given locationat a particular time. Margin,in this context, refers to the amount of signal level reduction underactual site propagation conditions that can be toleratedbefore TOV is reached. It represents an indication(or amount) of “safety” in terms of signal level. For a situation where there is essentially no multipath or interference, the measuredsignal level can be used to directly compute the SNR value at the input to the receiver’s tuner and compare it to the knownSNR value at TOV (≈15 dB for a good receiver) to determine margin. In our field tests multipath is present from the wind turbines so the margin is measured utilizing the programmable RF attenuator for signal reduction and the ATSC receiverfor packet error detection, under the controlof the local probecomputer which implements an iterativealgorithm that methodically findsTOV and therefore the margin value.

Sample Probe TOV Data Output:

The presence of multipathor interference can cause the SNR valueat TOV to be something greater than 15 dB (i.e.,TOV degradation), and therefore this measurement can indicate a quantitative degradation amount that propagation effects have on the signalthreshold at a test site. From this quantitative margin value, statistical analysis can be performed on the data obtainedfrom varioustest sites in the regionwith multiple transmit antennaheights above groundlevel compared to the turbineheight above groundlevel.

Absent from the vanilla DTV receiver’s capabilities was the ability to measure and log signal multipath information.  The Broadcast Wind team worked together with the consumer receiver’s manufacturer and the holders of the 8VSB chip’s IP to develop a program to query the receiver’s equalizer and draw the multipath information out of its hardware’s registers.  We then wrote software giving the probe the capacity to display and to log equalizer multipath data, thus rounding out the probe’s data reporting and collection capabilities to a level exceeding most of what’s available on the market today.

The multipath data is also stored in its numeric form on the probe and uploaded to the Broadcast Wind database for evaluation.  A slice of the data collected is shown below:

Since the probes are operating in remote, unmanned environments, the operating software has been designed to monitor operational health and to take corrective action in the event of failure.  In the event of communication loss, the probe will log the event, and continue to record all of the RF data locally until the system automatically resets.  The probe emails health status and diagnostic metrics to the Broadcast Wind offices twice daily.

When a communication outage occurs, the system self heals, and notifies Broadcast Wind of the exception via email.  No data is lost during communication outages since all data is stored locally, at the probe, and copied to the cloud once communication is restored.

When grid power is available, the probe is powered by an external 60 Watt 12V AC-DC adapter.  For those locations where grid power is not available, we have developed a solar / battery solution using the lowest monthly average sun hours (December) in sizing the system so that it will continue to run 24/7/365.  The design calls for 5 days of backup power (for cloudy days) and a conservative 70% depth of discharge.   

Field data from the probes will continue to be collected and analyzed over the course of our project and will be used to gain a much better understanding of the dynamics between wind turbines and digital television signals.  These analyses are also being used to enhance and refine the predictive capabilities of our company’s RF interference modeling software: “WINTIP”.