Specifications

Frequency
Translating Group
Delay Receiver
The main group delay receiver starts with a 10.7 MHz bandpass filter,
which is the approximate carrier frequency for this measurement (output
of down converter should be a 10.7 MHz carrier with 453.125 kHz FM at
a modulation index of 0.1 to 0.3). The filter is quite broad because of the
large modulation bandwidth and the possibility of changing the carrier
frequency slightly based on calibration. A transformer provides imped
-
ance matching to the next stage.
The signal is then fed into a pair of wideband FM processing chips whose
job is to do the following:
q
Perform limiting amplification to reduce AM sensitivity
q
Provide an estimate of signal strength: Received Signal Strength
Indicator (RSSI)
q
Perform FM demodulation
Two chips are required to get enough IF gain to sufficiently limit the sig
-
nal. A coupling network is used to ensure stability and flatten the RSSI
characteristic.
The RSSI signal is combined from the two chips, shifted/scaled, and sent
to the ADC direct input. The base RSSI signal runs from 0 to -5V, which
needs to be inverted and scaled for the -2.75 to +2.75 Volt ADC range. A
unity gain follower is used to provide impedance matching from the RSSI
output of the FM processing chip. This signal is used to gauge the power
levels for calibration purposes.
The FM demodulation is via a quadrature technique using the tank of the
RLC circuitry. The Q is fairly low (4 to 5) to accommodate the large modu-
lation bandwidth and some carrier frequency flexibility. The balanced out-
put of the demodulation is converted to single-ended, then it is amplified
and filtered by a ceramic filter. The signal is amplified again before being
multiplexed with the RSSI output.
Wideband Noise
Figure Receiver
The front end mentioned above provides the initial gain ranging for the
25 MHz noise signal. The measurement is performed using the usual
Y-factor hot and cold measurements, so the function of the circuit is sim
-
ply filter-amplify-detect. In the field of Noise Figure Measurements, the
term Y Factor is defined as:
Power Measured with Noise Source on (hot)
Power Measured with Noise Source off (cold)
After being demultiplexed, the signal enters a sequence of three filter-am
-
plifier pairs. The filters are all centered at 25 MHz with about 6 MHz of
bandwidth. The amplifiers all provide approximately 20 dB of gain and
nominal 50 ohm impedance levels for the filters. Because of the high gain
present in this chain, relatively high levels of feedback are used to main
-
tain good stability.
THEORY OF OPERATION OPTION MODULES
MS462XX MM 2-23