Rohde & Schwarz M3SR Series 4100, Service Manual, Page: 44 / 138

R&S M3SR Series 4100 HF Transceivers Digital Selector - Function
6124.9007.82.01 1.8
RX mode
When in receiving mode, the RF signal coming in via connector X301 RX only or X302 RXTX is fed into
the protection circuit (overvoltage, overcurrent) and to K100. It is switched to the 30 MHz filter via K120.
If the frequency is > 1.5 MHz, K130 switches it to the bandpass filters and K190 recovers it after
filtering. If the frequency is < 1.5 MHz, K130 switches the RF signal directly to K190, bypassing the
bandpass filters. After that, K160 and K110 switch the RF signal to RFU at X303.
Bypass Mode
This mode can be manually set via the Main Menu of the radio. In this mode not only the bandpass
filters are bypassed, but also the 30 MHz lowpass and 1.5 MHz lowpass and highpass filters. The by-
pass is switched by K100. The Bypass mode is used only in RX mode. In TX mode the bandpass shall
be always activated.
Bandpass filters
In the bandpass filters 1 and 2, the RF signal is filtered (via a block filtering, coils and capacitors
switched according to the frequency) and amplified to achieve the desired gain. R&S FK 4140 will use
both bandpass filters, while R&S FK 4120 will use only one bandpass filter. In this case, the output of
bandpass filter1 is directly connected to the input (SEL-OUT) of the Control Module.
The circuit is basically composed of a block of switchable capacitors and coils, an amplifier and a 20 dB
attenuator. The coils and capacitors are switched ON and OFF by means of relays, depending on the
frequency. The 15 tables relating frequency, relay status and temperature are located in the FLASH
memory of the Control Module. More than 6000 tunings are stored in the memory.
The amplifier compensates the losses caused by the entire filtering chain on control board and band-
pass filter. The amplification factor is about 12.5 dB at 1.5 MHz and decreases to about 10.5 dB at
30 MHz. The RF amplifier can be bypassed by the 20 dB attenuator.
The 20 dB attenuator is activated in 2 modes:
¾ Manual activation by the operator via the Main Menu of the radio
This can be necessary if the received signal levels are very high. This mode is only used for ex-
treme co-location situations. Only the 20 dB attenuator in bandpass filter 1 can be activated.
¾ Automatic activation
In case of very high input levels in Rx Mode, the 20 dB attenuators in bandpass filters 1 and 2 are
automatically activated in order to prevent damage.
RF level detectors
Two RF level detector circuits are integrated on the Control Module. These circuits rectify and amplify
RF signals which are only available in transmit mode. So it is possible to detect a missing input RF sig-
nal coming from the Frontend or a fault inside of one of the bandpasses.