Friday, May 17, 2013

Broadcast Wind DTV RF Probe


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. 




Probe Components

Programmable Attenuator

The electronically-controlled programmable attenuator enables determination of the signal margin at the probe’s receive site.  Determination of margin is accomplished by increasing the attenuation to lower the signal to a level just above the 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-dB steps 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 be used for this field test). Attenuation accuracy is 0.3 dB or 5% of the selected attenuator value (whichever is greater), which is more than adequate for determining margin in the field. The switching speed of this relay step attenuator is about 70 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.


Low Noise Amplifier

The low noise RF amplifier provides signal amplification within RF probe not only to provide enough level to the reference DTV receiver (after experiencing insertion loss from the bandpass filter and programmable attenuator) but it also determines the sensitivity of the entire probe. The primary concern with using any active device before the DTV receiver is overload from interfering signals such as other DTV signals, particularly from nearby full-power stations. The single RF channel bandpass filter in 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 overload performance; i.e., large third order intercept points, (IP3).
                                                    




DTV Receiver

A good performance commercial DTV receiver 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 of diagnostic parameters that are retrievable from the 100Base-T Ethernet connection (RJ-45) on the back of the receiver under computer control. While not needed for this project, transport stream analysis is possible as well.



Computer Controller

The computer controller, while not directly handling 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 required field test data from the receiver for subsequent transmission over the Internet (via cellular modem) for final data processing at our central location. It needed to be rugged and robust for various field environments and easily restart and pick up where it left off during power surges or failures.

A small form factor computer was found to meet all of these requirements.  The system features an Intel Atom D2500 processor with 4 GB DDR3 RAM, 120 GB solid state drive (SDD), dual Gigabit LAN ports with Ethernet controller (to communicate with the DTV receiver and an Internet modem), 6 USB ports, dual external serial COM ports, and a single internal parallel port. While operating autonomously in the field, the computer needs no mouse, keyboard, or monitor. However, upon setup, there are ports for all three of these devices (mouse and keyboard via USB ports and monitor through either VGA or DVI-I ports).

The computer resides in a small well-shielded, fan-less enclosure that operates silently, and includes both an external 60 Watt 12V AC-DC adapter and an 80W internal DC-DC power 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.


Enclosure

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

Probe enclosure with internal components

Probe System Output
The RF probe design 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.


Signal Level Measurement:  (dBm)

The probe design enables indirectly measuring signal field strength at its antenna input. The receiver captures signal power level at its 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 system gain, 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 cable loss, the filter insertion loss, the attenuator insertion loss plus extra programmed loss, the splitter loss, and the pre-amplifier gain). Once this system gain is known for a given RF channel, the field strength can be calculated with the following formula:

F.S. (dBµV/m)  =  S GS + A + K G )

where              F.S. is the DTV field strength (in dBµV/m) at the antenna input

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

GS is the overall system gain (in dB) at the RF test channel center frequency

A is the selected attenuation level (in dB)

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


Signal Quality Measurement:  MER (dB)

The RF probe provides a signal quality 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 channel signals for the calculation. This parameter, when accurately determined in the DTV receiver’s 8-VSB demodulator chip, provides an indication of the signal quality at the equalizer’s output (i.e., the error correction circuitrys input). Typically, good modern-day receivers have a 15 dB value for MER at TOV.

It is important to note that the “noise referred to in the signal quality metric is not only the traditional additive white Gaussian noise (AWGN) found at the input to 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/or inter-modulation signals due to circuit non-linearities, and multipath signals (delayed replicas of the noise-like desired DTV signal) that are not completely canceled. All of these “noise” components are represented in the MER value provided by the RF probe.

Multipath is considered a linear distortion, and can therefore be removed by a linear equalizer in a DTV receiver (assuming that the equalizer is robust enough and the receive loops are locked). However, if the echo delay is 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 clock recovery or data frame recovery fails), then no valid MER value is available from the receiver. It should be noted that if the various recovery loops remained locked, even though the unit may be below the error threshold (even down to 4 or 5 dB SNR in some ATSC receivers), it is still possible for many 8-VSB demodulators to provide a reasonably accurate value of MER since they often use the periodically-transmitted binary data frame sync as a known reference signal to calculate MER.


Data Quality Measurement:  Accumulated TS Packet Errors

The RF probe provides a data quality measurement in the form of transport stream (TS) packet errors. While this particular ATSC receiver used in the RF probe produces a signal quality number (value between 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 error counter in the DTV receiver has been determined to be just a raw counter of packet errors that is never reset at a particular time interval, but only during a channel change. To determine if new packet errors have occurred since the last polling of this counter, subtraction of the two error counter numbers is required.

This error counter value indicates the number of uncorrected transport stream MPEG data packets (as determined by the Reed-Solomon decoder) at the output of the 8-VSB demodulator, regardless of whether the cause of 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 various propagation conditions at the time of testing.
Reception Margin:  M (dB)

The RF probe provides a means to measure the actual reception margin (in dB) at a given location at a particular time. Margin, in this context, refers to the amount of signal level reduction under actual site propagation conditions that can be tolerated before 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 measured signal level can be used to directly compute the SNR value at the input to the receiver’s tuner and compare it to the known SNR 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 receiver for packet error detection, under the control of the local probe computer which implements an iterative algorithm that methodically finds TOV and therefore the margin value.


Sample Probe TOV Data Output:


Sample TOV data from the probe. Four measurements separated by 13 seconds each indicate TOV between 19 and 20 dB.

The presence of multipath or interference can cause the SNR value at 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 signal threshold at a test site. From this quantitative margin value, statistical analysis can be performed on the data obtained from various test sites in the region with multiple transmit antenna heights above ground level compared to the turbine height above ground level.



Multipath Measurement
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.

A screen shot from a remote probe’s GUI showing on-site multipath measurements.  The probe displays the full measure of pre and post echoes being received at the location in microseconds. The direct signal from the transmitter is in the center at 0 µs.  Small reflected pre and post multipath echoes can be seen to the left (pre) and right (post) of the main signal.

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:

A screen shot of the probe’s raw tap weight data stored on the cloud by the probe.  The tap weight in hardware registers nos. 500 – 509 are shown for four consecutive measurements separated by 5 seconds each. The direct signal from the transmitter is at tap no. 505 with pre and post multipath echoes represented by their relative weights in the registers to the left and right.



Self Check / Self Heal Capabilities
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.

Sample of diagnostic metrics emailed from the probe 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.

Screen shot of notification of communication outage


Solar Capability
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”.