Product Manual

© MiMOMax Wireless Ltd
Tornado Product Manual
16
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.