GENERAL INFORMATION
1-5
Digital modulation
In addition to the wideband analog I and Q inputs a digital mode of operation is provided. This
allows the user to generate a vector modulated RF carrier from digital data inputs. The bandwidth
of the digital mode is sufficient to simulate radio systems which have been designed to work in the
frequency allocation of an analog voice channel. The digital modulation can be set to symbol rates
from 512 Hz to 34 kHz, and internal channel filters are applied with raised cosine, root raised
cosine or Gaussian characteristics.
Modulation formats can be defined as PSK, differential PSK, phase offset differential PSK,
QAM, FSK and GMSK with from one to eight bits per symbol (i.e. up to 256 QAM). The
programmable channel filter and data rate ensures that many of the different types of narrow band
digital modulation standards can be simulated by a single instrument.
The flexible digital interface gives the freedom to accept digital signals in bit or symbol format
using an internal or external data clock. The digital interface can be set to use positive- or
negative-edge triggering and normal or inverted data. An internal data source is also available
which can supply a PRBS (Pseudo Random Bit Sequence), all '1's or all '0's. A burst control pin
allows the generation of TDMA bursts with controlled rise and fall times.
The modulator can be requested to introduce IQ phase and gain errors and carrier leak to simulate
the performance of a receiver operating on non-ideal waveforms.
Advanced digital modulation
In advanced digital mode the instrument will produce TETRA modulation,
π
/4 DQPSK at 18
ksymbols/s through a root raised cosine filter with
α
of 0.35.
Modulation formats
Generic modulation types can be selected which include Phase Shift Keying (PSK), Differential
PSK (DPSK), Phase Offset DPSK (i.e.
π
/4 DQPSK), Time Offset PSK (OQPSK), Gaussian
Minimum Shift Keying (GMSK), Frequency Shift Keying (FSK) and Quadrature Amplitude
Modulation (QAM).
Specific modulation formats can be selected which provide the default data rates and channel filter
settings for numerous predefined systems which include NADC (D_AMPS), PDC (JDC), TETRA,
TFTS, APCO 25 (
π
/4 DQPSK), Inmarsat M, VDR, GSM, PCN (DCS1800), CDPD, DSRR, MC9,
MD24-192, Mobitex, Modacom, POCSAG, CITYRUF, ERMES, APCO 25 (4FSK). New user
settings can be created and stored to define other modulation standards.
Envelope control
For both vector and digital modes the front panel ENVELOPE IN socket may be used to simulate
the effect of varying the RF levels being received from mobiles in TDMA systems. It may also be
used to shape the rise and fall of an RF burst. Additionally, in digital modulation mode, the burst
control on the rear panel AUXILIARY IN/OUT socket allows RF bursts to be generated with
profiled rise and fall times synchronised with the data inputs.
Fading simulation
The built-in Rician and Rayleigh fading simulator with programmable path ratio and Doppler
speed allows the testing of receivers under the adverse propagation conditions, but note that this is
not available for GSM.
Incrementing
All major parameters can be incremented or decremented in step sizes entered via keyboard entry
or the GPIB. If no step size is entered for a parameter, the steps are preset to 1 kHz for carrier
frequency, 1 kHz for modulation oscillator and LF frequency, 1 kHz for FM deviation, 1% for AM
depth and 1 dB for output level.
In addition the rotary control can be used to vary the parameter with the sensitivity of the knob
being changed by means of the
×
10 and
÷
10 keys.