User Guide

PAMS
Technical Documentation
NME–3
Technical Information
Page 43
Issue 1 10/99
Receiver
The transceiver uses a dual conversion linear receiver. The received
signal from the antenna is fed via the duplex filter to a discrete LNA ( low
noise amplifier ) which uses a BFP420 transistor. The LNA can be
switched between two gain settings in order to increase the dynamic
range of the receiver. Switching of the LNA is controlled using the PDATA0
line from MAD using a discrete transistor network. The gain step in the
LNA is activated when the RF input level at the antenna is less than –50
dBm.
Following the LNA, the signal is fed to the SAW interstage bandpass filter.
This filter in combination with the helical duplex filter defines the spurious
signal blocking for the receiver, in particular rejection of the image for
mixing to the first RX IF frequency at 71 MHz.
The first mixer is a double balanced (quad diode hybrid) type. This is
necessary in order to fulfill the strict blocking requirements, in particular
the half IF rejection is critical. To provide the required gain an IF amplifier
follows the mixer. Since the passive mixer requires a high LO signal level,
additional local signal buffering is provided by a discrete buffer. The mixer
uses upper sideband injection for generating the IF signal. The LO signal
is generated with the UHF synthesizer.
The first IF signal is then channel filtered by a bandpass SAW filter with a
bandwidth of ±100 KHz. It also provides attenuation to the image
frequency for mixing to the second IF frequency (13MHz).
This filter also stops signals and mixing products at frequencies outside of
the channel bandwidth from saturating proceeding amplifier stages,
therefore reducing receiver sensitivity.
The next stage in the receiver chain is the AGC amplifier which is
integrated into the PLUSSA ASIC. The control voltage for the AGC is
generated by the DAC in the COBBA ASIC and provides a gain control
range of 57 dB. The second mixer stage is also integrated into PLUSSA.
The LO signal for the mixer stage is generated by dividing the VHF
synthesizer frequency by a factor of four to produce 58 MHz. The resulting
2nd IF signal is at 13 MHz.
The 2nd IF filter is a ceramic bandpass filter located outside of PLUSSA. It
is centered at 13 MHz with a bandwidth of ±100 KHz. The purpose of this
filter is to attenuate signals at the adjacent channel frequencies except for
the neighboring channels at ± 200 KHz which are filtered digitally by the
DSP. The dynamic range of the DACs within COBBA is wide enough to
cope with the effects of fading, particularly when the signal strength of the
interferer is high.
After the 13 MHz filter there is a buffer stage in PLUSSA which converts
the signal from single ended to differential to drive the DACs. The buffer
has a voltage gain of 36 dB and the buffer gain setting in COBBA is 0 dB
or 9.5 dB if required.