TB8100 Installation and Operation Manual Circuit Description 17
© Tait Electronics Limited June 2005
in the Service Kit software. The signal is finally passed to the ADC
(analogue-to-digital converter) in the digital receiver via an anti-alias filter.
Receiver RF -
UHF Reciter
The incoming RF signal is fed through a band-pass filter, followed by a
simple low-pass network. It then passes through further stages of filtering,
amplification and AGC
1
(automatic gain control) before being fed to the
mixer where it is converted down to the 70.1MHz IF (intermediate
frequency). A VCO (voltage controlled oscillator) provides a +17dBm
input to the mixer, and a diplexer terminates the mixer IF port in 50Ω.. The
signal from the mixer is fed through a 4-pole crystal filter to the IF amplifier
which provides enough gain to drive the digital receiver. The signal is finally
passed to the ADC (analogue-to-digital converter) in the digital receiver via
an anti-alias filter.
Exciter RF Audio signals from the line or microphone input are fed to the exciter RF
circuitry via the DSP (digital signal processor) and CODECs (encoder/
decoder). These modulating signals are applied to the exciter at two points
(dual point modulation): low frequency modulation is via the FCL
(frequency control loop), which modulates the exciter synthesizer’s
frequency reference, and speech band modulation is supplied directly to the
VCO.
The VCO is phase-locked to the frequency reference via the synthesizer.
The output from the VCO passes through the VCO buffer to the exciter
amplifier, which increases the RF signal to +20dBm. This signal is then
attenuated through a pad to +11dBm. An 8VDC PA Key signal is mixed
in with the RF signal which is then fed to the PA.
The K-band and L-band reciters use two VCOs, with the appropriate VCO
stage being selected for operation according to the frequency of the channel
in use. Only one VCO can be operational at any one time.
Digital Circuitry The IF from the receiver RF circuitry is passed through an ADC and a DDC
(digital downconverter) to the DSP. The DSP provides demodulation, RSSI
calculation, SINAD calculations, muting, and decoding of subaudible
signals. Audio and RSSI from the DSP is passed via CODECs to the system
interface board.
Incoming audio from the system interface board or microphone is passed to
the exciter RF circuitry via the DSP and CODECs. The DSP provides the
audio characteristics, generates subaudible signals (e.g. DCS, CTCSS), and
controls the CODECs for line audio input.
1. AGC is available in H-band reciters only. It can be disabled using the Serv-
ice Kit software.
To Next Page
Loading...
Need help?
General
