User Manual

SGD-SB2025NT-TUM, Part 1
Jan 12 Page 32 TECHNICAL DESCRIPTION
the Micro Controller board on SKD-3. The RSSI from IC1 is buffered by IC5A and connected to
the Micro Controller board via SKD-4.
RF from the VCO on SKU-1 at a nominal level of +3 dBm is applied to the fractional-N synthesiser
(IC10) main divider input. This signal is compared with the reference oscillator frequency and the
correction voltage from the synthesiser’s charge pump output is filtered then amplified by the non-
inverting low noise operational amplifier (IC11A). This correction voltage is fed back to the VCO to
maintain loop lock as well as being fed to the Micro Controller via SKD-14. A lock detect signal
from IC10 is also fed to the Micro Controller via SKD-16. The operational amplifier uses a 25 V
power supply (generated on the Micro Controller) in order to provide a wide tuning range voltage to
the frequency control varicaps located on the VCO board. Frequency programming data for the Rx
is sent to the synthesiser chip from the Micro Controller via the a serial data line on SKD-18 under
the control of the Clock (SKD-15) and Strobe (SKD-17) lines
The Micro Controller, through an enable signal on SKD-8, controls the local oscillator signal to the
mixer. This signal switches the supply to the local oscillator amplifier and is used to enable or
disable the Rx.
Provision is made for the optional injection of an external reference frequency. If this option is
selected, CN3 is fitted.
5.3 P OWER A MPLIFIER M ODULE
The SB2025NT now supports Simoco’s new full switching bandwidth PA Module (Wide Band PA),
which requires no tuning, equals, exceeds or covers several of Simoco’s band allocations, (refer to
Appendix A for details of the band allocations). This new generation PA features much wider RF
bandwidth, higher efficiency, greater stability out of band and to zero power levels.
This PA is now being rolled out as the standard production PA.
5.3.1 Wide Band PA (50 Watt Model)
RF from the Exciter passes via a coaxial cable to the input of the PA Module and is first attenuated
by a 50 Ω pad, which is used to provide a good 50 Ω source impedance for the first LDMOS driver
amplifier. The RF is amplified to around 5 W at the driver output, and is band dependant. (Note.
This point does not have 50 Ω impedance and the drive power cannot be measured directly with a
50 Ω Wattmeter). The signal from the driver is then matched by a broadband network to drive the
low input impedance associated with the final transmit LDMOS PA transistor. The transistor’s low
drain impedance is then also matched back to 50 Ω by a broadband matching network covering a
very wide bandwidth. Prior to transmission, a low loss 13 element elliptical low pass filter, filters
out the unwanted harmonics to less than –90 dBc.
A dual directional coupler consists of coupled microstrip transmission lines fabricated on the PCB
artwork. The sampled RF energy is rectified to provide a proportional DC voltage output.
The PTT signal enables the amplifier circuit by providing bias to the transistors. A thermistor, TS1,
physically located on the PA heatsink, monitors the heatsink’s temperature and is monitored by the
Micro Controller.
5.3.2 Wide Band PA (100 Watt Model)
The 100 W models are base on the 50 W technology, using two LDMOS final PA transistors to
achieve a 100 W RF output power rating. The final transistor’s low drain impedance is then also
matched back to 50 Ω by a broadband matching network covering a very wide bandwidth. Prior to
transmission, a low loss 13 element elliptical low pass filter, filters out the unwanted harmonics to
less than –90 dBc.