Rohde & Schwarz M3SR Series 4100, Руководство по обслуживанию

Страница: 43 / 138

Digital Selector - Function R&S M3SR Series 4100 HF Transceivers
1.7 6124.9007.82.01
1.3.2.1 Digital Selector - Function
The optional Digital Selector is available in two basic versions:
¾ R&S FK 4120 (20 dB bandpass)
¾ R&S FK 4140 (40 dB bandpass)
The Digital Selector is a bandpass filter that is automatically set to the operating frequency (receive and
transmit mode). It is used to improve intermodulation (receive mode) and phase noise (transmit mode).
The stop-band attenuation of one bandpass is 20 dB in 10 % frequency separation (R&S FK 4120).
Two bandpass modules can be connected in series to get 40 dB attenuation in 10 % frequency
separation (R&S FK 4140). The use of the Digital Selector is recommended in strongly disturbed RF
environments, i.e. for collocation problems, such as those occurring on board of ships. With a digitally
tuned filter, the Digital Selector improves the receiver input selectivity and the phase noise at the
transmitter (> 170 dB/Hz), and reduces harmonics and non-harmonics. Its bi-directional function
coupled with very short tuning times (< 10 ms) allows frequency-agile operation.
System Overview
The Digital Selector is composed of the Control Module and one or two bandpass filter boards, one in
case of R&S FK 4120 and two in case of R&S FK 4140 (see Fig. 1.3). On the circuit board of the
Control Module the interface for the Radio Control Bus (RCB) is located. All digital and protection
controls (overcurrent, overvoltage and temperature) are also located on this board. Basic filters
(lowpass filters of 1.5 MHz and 30 MHz, highpass filter 1.5 MHz) are introduced in the TX and RX path
to provide additional spurious and noise filtering outside the used frequency.
TX mode
When in transmission mode, the RF signal at X303 coming from the Radio Frontend Unit (RFU) is
switched by means of relays K110 and K100 to a digital 31 dB step attenuator with 1 dB steps and an
amplifier. The step attenuator is used to compensate the amplification deviation in the frequency and
temperature range. It will be adjusted in such a way that the amplification factor for the whole Digital
Selector stays in the range 0 ± 1 dB.
The relay K120 feeds the RF signal to a 30 MHz lowpass filter to avoid radiation of undesired spurious
and noise at frequencies above 30 MHz. After that, if the TX frequency is > 1.5 MHz, the relay K130
switches the RF signal to a 1.5 MHz highpass filter in order to avoid transmitting spurious and noise in
frequencies below 1.5 MHz. The RF signal leaves the Control Module at X310 and passes one or
successively two bandpasses 1 and 2 via X320 to X321. After that the RF signal is fed to K190. If the
TX frequency is < 1.5 MHz, K130 switches the RF signal to a 1.5 MHz lowpass filter. After that it is fed
to K190, bypassing the bandpass filters.
Coming from the bandpass filters (f > 1.5 MHz) or bypassing them (f < 1.5 MHz), the RF signal at K190
is switched via K160, K180, K1 and K170 to the output X302 RX TX to PA.