PROMAX MO-170 User Manual

DVBT MODULATOR

17

NEWSLETTER 15 / 2006

MO-170 T

esting DVB-T & DVBT-H

Signal level alignment and
in-band noise measure­ments (OP-170-E option)

Blanking a set of contiguous carriers
within the COFDM spectrum can
help in measuring the levels of
in-band noise (intermodulation pro­ducts, Gaussian noise). The MO-170
allows to vary the width of the spec­trum hole as well as its location
within the channel.

Test Transport Stream

When out-of-service tests are requi­red the MO-170 can be used standa­lone, internally generating a test TS

consisting of NULL packets filled up
with PRBS payload data. If the tests
do not involve displaying a picture on
a monitor, no external transport stre­am input is needed. The MO-170
automatically synthesises the bit rate
needed to operate the modulator
depending on the DVB-T parameters
in use.

In case a moving picture is required
the GV-998 can be used to supply a
test pattern through the ASI or SPI
input.

6, 7 and 8 MHz channel bandwidths
(user-selectable)

2k & 8k modes

Master and slave operation

Hierarchical modes

Frequency agility (1 Hz step)

High MER

COFDM signal power is measured by
taking the average of the power within
the channel. To simplify the process
of aligning signal levels across a
transmission or reception chain, the
MO-170 can generate a single central
carrier whose peak power is 3 dB
above the average power of the DTT
signal.

Getting your Bit Error Ratios
right

A unique feature of the MO-170 is
the insertion of bit errors in different
stages of the DVB-T modulation

In addition to the more common test
modes such as:

-

- Internally generated test TS.

-

- Carrier blanking.

-

- Single carrier generation.

-

- Controlled insertion of errored bits

to emulate a given BER before or
after the Viterbi decoder.

The MO-170 is a multi-purpose DTT modulator providing a complete test suite which can be
used to perform measurements at different points on the DVB-T signal chain. The wide selec­tion of test options available in the MO-170 makes it the perfect companion for anybody
interested in checking and validating a variety of critical aspects throughout the DVB-T system.

The MO-170 includes other novel
features as an option (OP-970-E):

-

- Addition of white Gaussian noise

with selectable C/N.

-

- Simulation of fixed and mobile

multipath channels with up to 6
taps of variable amplitude, delay,
phase and Doppler frequency.

DVB-H is also possible as an option

These and other features present in
the MO-170 simplify the set-up of
complex test systems and allows
measurements over real conditions
without having to spend a fortune.

Spectrum hole unveilling the presence of inter

-

moduation products within the channel

Sample of a test signal with PRBS payload data

Testing DVB-T & DVBT-H

with twice the bandwidth of the DVB­T signal is digitally added to the

COFDM signal. C/Ns between 3 and
40 dB in steps of 0.1dB can be
selected. In addition, the RF signal
level (COFDM and noise combined)
can be further attenuated from 0 to
60 dB in 1 dB steps. This provides
the means to either keep the signal
power constant whilst varying the
C/N (e.g. to plot the BER vs. C/N of
a demodulator), or to keep the C/N
constant whilst varying the signal
power (e.g. to find the sensitivity of a
receiver).

The DVB-T signal may be switched
off while the noise is still on, and vice
versa. This way, noise and signal
average powers can be measured
externally using the appropriate
equipment so as to verify the
selected C/N reading. The fact that
both noise and signal are digitally
synthesised has the added benefit of
generating C/Ns with a precision

that is difficult to achieve in a tra­ditional assorted test set-up.

QEF VBER insertion in the MO-170 and BER

measured by the TV EXPLORER

QEF VBER insertion in the MO-170 and BER

measured by the TV EXPLORER

Fixed/mobile multipath
channels, SFN/MFN simu­lation and more (OP-170-E option)

COFDM was the modulation chosen
for digital terrestrial TV broadcasting
because of its superior performance
in dynamic and static multipath
channels.

Among other applications, the chan­nel simulator can be successfully
used to simulate the following scena­rios:

-

- Pre-echoes in a SFN or, in general,
any power delay profile found in
practice in single (MFN) & multiple
transmitter (SFN) networks.

-

- Static channels corresponding to
roof-top fixed and portable recep­tion. In particular, good 6-ray
approximations can be generated
for the F1 and P1 channels defi­ned in Appendix B of document
ETSI EN 300 744. Other 6-path
profiles are those defined in
Appendix K.2 of document ETSI
TR 101 290.

-

- Mobile channels with pure Doppler
shift. An example of this is the 0
dB echo profile proposed in
Appendix K.3 of document ETSI
TR 101 290.

The channel simulator may be used
in conjunction with the C/N generator
to evaluate the performance of a
DVB-T system for a pre-defined mul­tipath channel as a function of the
amount of additive noise present in
the channel.

A novel feature in the MO-170 is the
possibility of simulating channels with
up to 5 echoes (plus the main path)
of variable attenuation (0 to 40 dBc in

0.1 dB steps), delay (0 to 445 ms),

phase (0º to 359.9º with resolution of

0.1º) and Doppler frequency (zero for

fixed channels and ranging between

-830 Hz and + 830 Hz in 0.1 Hz steps

for mobile channels).

Amplitude and phase of fixed Ricean channel F1

simulated with the MO-170

DVBT MODULATOR

18

NEWSLETTER 15 / 2006

chain. This can help to check the
accuracy of the BER estimation
algorithms implemented in high-end
professional receivers.

A Channel BER (CBER or BER
before the Viterbi decoder) ranging
between 7.6×10-6and 1.25×10-1is
generated by modifying the sequence
of bits at the input to the constellation
mapper. Analogously, the MO-170 is
able to generate a Viterbi BER (VBER
or BER after Viterbi) going from

3.7×10-9to 6.2×10-2by properly
processing the bits at the output of
the Reed-Solomon encoder. The
main advantage of this technique
when compared with varying the C/N
to get the desired CBER or VBER, is
its high resolution and unparalleled
accuracy.

Start making noise

(OP-170-E option)

A traditional set-up for measuring
DVB-T performance versus C/N
typically includes a source of wide­band white Gaussian noise, an RF
power meter plus selective channel
filter or a spectrum analyser, and a
varying number of high-precision
variable attenuators and directional
couplers. The C/N generation function
available in the MO-170 makes this
kit no longer required and allows a
much simpler configuration.

In the MO-170, white Gaussian noise