Tait TM8000 Series, Service Manual

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TM8100 Mobile Radio Service Manual General Description 55
May 2004 © Tait Electronics Limited
Automatic Gain Control
The AGC (automatic gain control) is used to limit the maximum signal
level applied to the image-reject mixer and ADCs in order to meet the
requirements for intermodulation and selectivity performance.
Hardware gain control is performed by a variable-gain amplifier within
the quadrature demodulator device driven by a 10-bit DAC (digital-to-
analogue converter). Information about the signal level is obtained from
the IQ (in-phase and quadrature) data output stream from the ADCs.
The control loop is completed within custom logic. The AGC will
begin to reduce gain when the combined signal power of the wanted
signal and first adjacent channels is greater than about –70 dBm. In the
presence of a strong adjacent-channel signal it is therefore possible that
the AGC may start acting when the wanted signal is well below
–70 dBm.
Noise Blanking
(B1 band only)
For the B1 band only, noise-blanking circuitry is included. The noise
blanker removes common sources of electrical interference such as
vehicle ignition noise. The programming application allows for disabling
the noise blanker if it is not required. The noise blanker functions by
sampling the RF input to the receiver for impulse noise and momentarily
disconnecting the first LO for the duration of the impulse. The response
time of the noise blanker is very fast (tens of nanoseconds) and is shorter
than the time taken for the RF signal to pass through the front-end
hardware, so that the LO is disabled before the impulse reaches the IF
stage where it could cause crystal filter ring.
Digital Base-band Processing
Custom Logic
The remainder of the receiver processing up to demodulation is
performed by custom logic. The digitised quadrature signal from the RF
hardware is digitally down-converted to a zero IF, and channel filtering
is performed at base-band. Different filter shapes are possible to
accommodate the various channel spacings and data requirements.
These filters provide the bulk of adjacent channel selectivity for narrow-
band operation. The filters have linear phase response so that good
group-delay performance for data is achieved. The filters also decimate
the sample rate down to 48 kHz. Custom logic also performs
demodulation, which is multiplexed along with AGC and amplitude
data, and fed via a single synchronous serial port to the DSP. The stream
is demultiplexed and the demodulation data used as an input for further
audio processing.
Noise Squelch
The noise squelch process resides in the DSP. The noise content above
and adjacent to the voice band is measured and compared with a preset
threshold. When a wanted signal is present, out-of-band noise content
is reduced and, if below the preset threshold, is indicated as a valid
wanted signal.