Product Manual

Product Manual for Tornado Radio Unit
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oscillator located on the DPS PCB. The required local oscillator frequency (i.e. receive frequency minus 45.1MHz)
is programmed by the unit central processing system which controls the synthesiser via a 3-wire serial interface
bus. The frequency is settable in 6.25 kHz increments (5 kHz optional).
The synthesiser control loop incorporates a low noise op-amp active filter and level shifter, the output of which
feeds the voltage-controlled oscillator (VCO). The VCO uses a LC resonator tuned by high-Q varicap diodes to
minimise phase noise and jitter. The required local oscillator frequency ranges from 354.9 to 424.9MHz.
The output of the VCO passes through an RF cascade buffer IC, which amplifies the low-level signal from VCO
whilst providing high reverse isolation to minimise any variations in VCO loading. The output feeds the splitter
network and in turn feed the mixers of each receiver channel.
3.4 TRANSMITTER RF/IF SECTIONS
The transmitter has two channels, each with separate RF, up/down converter, and IF stages. The power supplies
and stepped attenuator settings can be independently controlled. A common local oscillator feeds both channels
simultaneously. RF output from each channel is by means of a PCB-mounted 50Ω SMB connector. With the
exception of the VCO/synthesiser sections the descriptions below apply equally to either transmit channel.
3.4.1 FORWARD SIGNAL PATH
The transmitter employs a fixed frequency ‘direct IF’ with single up conversion to the final RF. It includes a fixed
and manual tuned IF filters to attenuate DAC spurs. The mixer is a quadrature up converter and also provides an
image reject function due to 90deg phase splitting of the input signal. The adjustment of gain is provided by a 1.5-
33.5dB stepped attenuator programmable in 0.5dB steps. Power amplification follows consisting of devices biased
to provide a reasonably linear characteristic to support the required modulation types. A directional coupler on the
PA output provides a sample of the signal for the feedback path. The PA bias is controlled via DAC outputs. The
PA bias tracks temperature based on a predefined tracking curve. An ADC monitor measures PA final and driver
current, forward and reverse power. PA temperature is monitored for each channel by dedicated temperature
sensors.
3.4.2 FEEDBACK SIGNAL PATH
The RF signal from the directional coupler has adjustment of gain provided by a 1.5-33dB step attenuator
programmable in 0.5dB steps. An image reject mixer provides attenuation of any external signal on the down
converter image frequency. The RF signal is down converted to a 15.3835MHz IF feedback signal which is the
same as the forward path signal. This IF signal is amplified and summed with the forward path to close the loop.
3.4.3 LOCAL OSCILLATOR
The transmitter local oscillator consists of a programmable fractional-N phase-locked loop (PLL) frequency
synthesiser. This uses a stable reference frequency derived from the DPS 40MHz clock. The required local
oscillator frequency (i.e. transmit frequency minus TX IF) is programmed via a serial interface bus from the DPS.
The LO frequency can be set in 5 kHz increments.
The synthesiser control loop incorporates a low noise op-amp active filter and level shifter, the output of which
feeds the voltage-controlled oscillator (VCO). The VCO uses a LC resonator tuned by high-Q varicap diodes to
minimise phase noise and jitter. The required local oscillator frequency range is 384.6165MHz to 454.6165MHz
(70MHz total).
The output of the VCO passes through a resistive attenuator into a buffer amplifier which raises the power level.
This is followed by two Wilkinson splitter networks, resulting in four 50Ω outputs. These outputs feed the up
conversion and down conversion mixers for each of the two transmitter channels.
3.4.4 INTERNAL DUPLEXER
The duplexer takes one receiver and one transmitter and duplexes them onto a single antenna port. Two duplexers
are used in each radio unit. The antenna port connector is a waterproof N-type. Connections to the receiver and
transmitter printed circuit assemblies are made internally via two 50Ω SMB connectors and interconnecting semi-
flexible coax cables. Each duplexer has two band pass filters with notches and an LNA for the receive path. The
notch frequency of each element is tuned by a trimmer capacitor.
Electrically the two duplexers in each radio unit are identical. Physically they are different and present almost a
mirror image of the other. These are referred to as ‘Channel 1’ and ‘Channel 2’. The duplexers cannot be swapped
over.