XPT/R2G
Pag.
12
between two amplifiers provide the right selectivity without introducing excessive distortions of the
modulated signal, which definitely has a bandwidth higher than the window imposed by the filters.
Accurately designed matching circuits ensure to keep an high quality of the signal, which will
subsequently be demodulated.
At the output of this process, the signal is demodulated in amplitude to drive the AGC and
the S-meter.
An amplitude limiter follows; at this point, the RF signal is ready for demodulation; in order
to achieve high performance, the traditional analogue frequency demodulator was not selected;
instead, a digital solution is adopted.
An additional conversion is made (10.7Mhz>700kHz) by means of a crystal oscillator and a
passive balanced mixer; the second intermediate frequency signal is fed to a fast trigger. The output
pulses of this stage have an average value that depends on the frequency modulation value; the
connection is perfectly linear, and, using this method, a demodulator that requires no alignments
and independent on selective circuits is obtained.
The RF signal is thus demodulated. The phase and amplitude distortion features of the audio
signal depend almost solely on the RF filters, particularly the IF ones. However, the symmetrical
distortions can be compensated.
An amplitude equalizer composed of an amplifier with a Zero restores the band flatness to
53kHz.
In addition to the MPX signal, the demodulated signal can also contain RDS or SCA signals
up to a base band frequency of 100kHz; beyond this limit, everything must be eliminated because it
would be a spurious signal in case of retransmission.
A elliptical filter with 100kHz cut frequency removes the spurious components of the
demodulated signal.
Both the intermediate frequency filters and the low-pass filter introduce a phase variation on
the demodulated signal; the stereophonic signal quality depends largely on the variation in the base
band phase. It is therefore necessary to introduce a phase equalizer to obtain stereophonic
separation greater than 50dB.
A peak-to-peak detector accurately measures the modulation; a piece of software can
provide the user with all the information in accordance to the current regulations regarding this
important parameter.
The MPX signal is fed to two output BNC connectors with programmable fixed levels of 0,
4. 1, 6 or 12dBm; the amplitude can be regulated by ±2dB around the fixed level.
The monophonic signal is obtained from the MPX signal by means of a 15kHz elliptical
filter having a slope of 40dB at 19kHz.
The receiver can be equipped with a stereophonic decoder to provide an output with good
level of separation (> 40 dB) of the LEFT and RIGHT channels.
The power supply is not provided with the traditional AC mains power transformer. A
switching-mode power supply generates 24V
dc
directly from the mains power, thus allowing the
receiver to be used both in Europe and in the Americas without any modification.
In this manner, yield and weight have been improved as compared to traditional solutions.
An auxiliary power supply generates all the voltages required by the receiver.
The system is internally controlled by a digital card that is interfaced with a keyboard and a
large 40x4 characters LCD display. Parameters can thus be easily set and read.
