Minimum requirements for DVB-T receiving antennas for portable indoor and portable outdoor reception

Transcription

1 Deutsche TV Platform Minimum requirements for DVB-T receiving antennas for portable indoor and portable outdoor reception compiled by Working Group: DVB-T launch (a working group of the Deutsche TV Platform) Deutsche TV Platform Version 1.2 / as of: Document No.: DVBT_08/xx

2 Contents 1 General properties Properties of the receiving system Characteristic of the receiving antenna Appendix Measuring method 1: Determination of the figure of merit of a DVB-T receiving system Definition Assumptions / Requirements Schematic test setup Formulas Measuring the figure of merit m as a function of the field strength E Measuring method 2: Determination of the equivalent passive antenna gain Basics Formulas Document History Working Group: DVB-T launch Page 2 of 10

3 1. General properties Application: portable indoor / portable outdoor (stationary reception) Polarization: useable for horizontal and vertical polarization (linear and/or circular) Impedance: 75Ω Receiver interface: IEC connector cable, male as per IEC (plug) Construction types: passive / active Features: passive version: DC isolation (short-circuit resistance) active version: remote power supply by a receiver via HF cable (5V / max. 30mA) and 1 / or external power supply Frequency ranges Band III MHz Band IV / V MHz Return loss 2 active passive MHz > 5 db > 3 db MHz > 8 db > 5 db 1 Negative feeding to the receiver not permitted. 2 incl. antenna cable. Working Group: DVB-T launch Page 3 of 10

4 2 Properties of the receiving system For the entire receiving system consisting of an antenna, connector cable and receiver limiting values for the system figure of merit can be defined to ensure a sufficiently high signal-to-noise ratio C/N of the received DVB-T signal for standard modulation parameters. For the UHF range these are defined in the Technical Report ETSI TR (V1.2.1). For the VHF range, not ETSI standards but limiting values specified by the Institut für Rundfunktechnik IRT (Broadcast Technology Institute in Germany), are used. They consider a higher level of man-made noise in this frequency range. 3 The following limiting values result for the system figure of merit: Minimum figure of merit of the receiving system 170 MHz db(1/k) 180 MHz db(1/k) 190 MHz db(1/k) 200 MHz db(1/k) 210 MHz db(1/k) 220 MHz db(1/k) 230 MHz db(1/k) MHz db(1/k) Considering the receiver s noise figure of 8 db, the figure of merit can be converted to an equivalent passive antenna gain. An equivalent passive receiving system is assumed. 4 For the above mentioned parameters the following transformation factors can be used: Figure of merit [db(1/k)] equiv. passive gain [dbi] Figure of merit [db(1/k)] equiv. passive gain [dbd] Gain [dbi] Gain [dbd] db db -2.2 db 3 Schramm, Raul; Institut für Rundfunktechnik: DVB-T: Gewinn von Breitbandantennen für Indoor- Empfang im VHF-Frequenzbereich dated January 19, Calculation as per BBC R&D White Paper WHP066 (July 2003): Specifying UHF active antennas and calculating system performance. Working Group: DVB-T launch Page 4 of 10

5 Equivalent gain 170 MHz -5.0 dbi (-7.2 dbd) 180 MHz -4.5 dbi (-6.7 dbd) 190 MHz -4.0 dbi (-6.2 dbd) 200 MHz -3.5 dbi (-5.7 dbd) 210 MHz -3.0 dbi (-5.2 dbd) 220 MHz -2.6 dbi (-4.8 dbd) 230 MHz -2.2 dbi (-4.4 dbd) MHz +2.2 dbi (0 dbd) 3. Characteristic of the receiving antenna As most of the DVB-T receiving antennas have a simple design there is a preferential direction on at least one polarization plane. In order to avoid a channel-selective alignment of the antenna, the preferential direction should not change for the complete frequency range. The measurement of the figure of merit and/or the equivalent passive antenna gain is executed for all frequencies in the preferential direction which is visible or specified by the manufacturer and in the intended polarization plane. Antennas must satisfy the following minimum requirements: 1. The figure of merit and/or the equivalent passive antenna gain must not fall below the minimum required value for the complete frequency range. 2. The maximum permissible level fluctuations ( ripples ) caused by the receiving antenna within a channel bandwidth in the preferential direction are: MHz 4 db / 7 MHz MHz 4 db / 8 MHz 3. The maximum permissible level fluctuations ( ripples ) caused by the receiving antenna within a frequency band in the preferential direction are: active passive MHz 10 db 8 db MHz 10 db 8 db MHz 15 db 10 db Working Group: DVB-T launch Page 5 of 10

6 4. Appendix Two possible measuring methods for determining the figure of merit and/or the equivalent passive antenna gain are explained now. For both measuring methods the following instructions for the alignment of the cable must be observed: The HF cable is aligned horizontally backwards for one meter. If remote power supply via a receiver is possible for an active antenna, this method has to be chosen. Optimally the power supply is realized directly via the receiver in use. If this is not possible, the power supply is implemented via a power supply input connector at the end of the HF cable from the antenna. If the antenna s power supply can be realized only by means of an external power-supply unit or a 220V cable, then this cable has to be aligned horizontally backwards for one meter parallel to the HF cable. 4.1 Measuring method 1: Determination of the figure of merit of a DVB-T receiving system Definition The figure of merit is an evaluation criterion for the entire DVB-T receiving system. In this context, for all system components the environment is considered. For a minimum required C/N defined by modulation parameters, it is possible to deduce to the required figure of merit of a receiving system from the expected field strength E for any location (see ETSI TR ). For each receiving frequency the figure of merit is a function of the minimum required signal-to-noise ratio and of the received field strength: G/T = f (C/N min,.e). Antenna Transmitter C/N t E [dbµv/m] Received field strength n f V Cable loss C/N n f Receiver C/(N t +N e ) G/T ant G/T sys Working Group: DVB-T launch Page 6 of 10

7 4.1.2 Assumptions/Requirements Digital system TV test receiver (e.g. Rohde&Schwarz EFA DVB-T) with n f = 7dB typ. TV test transmitter (e.g. Rohde&Schwarz SFU) Cable length = 2m 75Ω impedance Signal bandwidth 7MHz (VHF) or 8MHz (UHF) Anechoic chamber suitable for the measuring frequencies (typical values: Shielding > 100dB; suppression of reflection > 20dB) Schematic test setup T k =290 K typ. 3 m transmitter DUT cable receiver The transmitting antenna is aligned towards the antenna under test (height, polarization) Formulas linear: C lin = S * G * A wi A wi = λ 2 / (4*π ) N lin = k * T * B (C/N) lin = G/T * S * A wi /(k*b) logarithmic: (1) C/N = m + PFD + 10*log(A wi ) 10*log(k*B) PFD = E 10*log(120*π) 120 (2) PFD = E in (1) Working Group: DVB-T launch Page 7 of 10

8 m = C/N E + 10*log (k*b) 10*log (A wi ) m = C/N E + K( f ) S = Power density [W/m 2 ] PFD = Power flux density [dbw/m 2 ] m = Figure of merit [db/k] E = Field strength at the point of reception [dbμv/m] B = Bandwidth [Hz] (7 MHz for VHF, 8 MHz for UHF) k = Boltzmann s constant = 1.38*10-23 W /(Hz*K) C/N = Receiver s signal-to-noise ratio [db] (C/N) lin = Receiver s signal-to-noise ratio [linear] A wi = Effective antenna aperture of the isotropic radiator [m 2 ] G = Gain of the antenna [dbi] T sys = System noise temperature [K] T ant = Equivalent antenna noise temperature [K] T k = Noise temperature in the anechoic chamber [K] N t = Transmitter noise N e = Noise of the environment f = Test frequency [Hz] K(f) = 10*log (k*b) 10*log (A wi ) = Constant for test frequency (Frequency dependence of the effective antenna aperture) Measuring the figure of merit m as a function of the field strength E For measuring purposes, a DVB-T test transmitter (e.g. Rohde&Schwarz SFU) is used. The test signal is generated with the following settings: Example: 16-QAM Non-hierarchical modulation Guard interval ¼ Code rate 3/4 (VHF) or 2/3 (UHF) 8K COFDM The received field strength at the site of the antenna can be determined as a function of the set transmission level by means of a reference measurement with a field probe. It must be kept in mind that the result for the figure of merit will be determined in the range of constant figure of merit. In this case an increase in the field strength E of 1dB results in an increase in the signal-to-noise ratio C/N of 1dB. Working Group: DVB-T launch Page 8 of 10

9 4.2 Measuring method 2: Determination of the equivalent passive antenna gain Basics There are several methods for determining the gain of passive antennas. A substitution method, for example, may be used by comparing the receiver input voltage of the antenna under test with the input voltage of a reference dipole antenna in an electromagnetic field. The antenna gain is determined by applying a height scan and averaging. Another possibility to determine the antenne gain is the Schwarzbeck method ( quasi-free-space calibration ). Reflections in the received signal are eliminated by a height scan and averaging and the antenna gain can be determined. Assessing active antenna systems is more difficult. Additional to the amplification of the system of receiving antenna and preamplifier, its noise must also be taken into account. There is a description in a publication by the BBC 1, how to evaluate an equivalent passive antenna gain for a receiving system with an active antenna. The following measurements are taken: The total gain (system power) from antenna + preamplifier referred to a passive halfwave dipole is measured (see first paragraph of item 4.2.1). The noise power of the configuration antenna + preamplifier is measured. For measuring the noise power, the active antenna is placed in a shielded room / anechoic chamber and the emitted power is measured with a test receiver. For this purpose the test receiver must be calibrated with a noise source. Then the noise power is substracted from the system power by using the formulas in item Comparing it to the theoretically calculated performance of a system with passive antenna (half-wave dipole) results in the equivalent passive antenna gain Formulas According to the BBC publication, the equivalent passive antenna gain is given by: G equ = G t + 10 *log (T p /T a ) G equ = G t + 10 *log ((T Rec + T pass.ant ) / (T Rec + T act.ant )) (1) with: G equ = equivalent passive antenna gain G t = Total gain of the active antenna T p = Noise temperature of the total passive system T a = Noise temperature of the total active system T Rec = Noise temperature of the receiver T pass.ant = Noise temperature of the passive antenna = Noise temperature of the active antenna T act.ant 1 BBC R&D White Paper WHP 066, J. Salter: Specifying UHF active antennas and calculating system performance Working Group: DVB-T launch Page 9 of 10

10 The noise temperature of the receiver is calculated as: T Rec = (F-1)*T 0 (2) In this context, the receiver s noise is assumed to be 8 db (or F = 6.3). For the passive antenna, the noise temperature is: T pass.ant = T 0 (3) The noise temperature of the active antenna is given by: T act.ant = P/(k*B) (4) with: P = Noise power of the active antenna k = Boltzmann s constant B = Bandwidth (7.61 MHz for 8 MHz channels; 6.66 MHz for 7 MHz channels) Using formulas (2) to (4) in equation (1), the equivalent passive antenna gain G equ can be determined by measuring the total gain of the active antenna G t and measuring the noise power of the antenna P. 5. Document History Date Modification Edition Final version December 2007 Publication of version Final version Final version November 2008 Publication of version * * * Working Group: DVB-T launch Page 10 of 10