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IF switch integrates gain, control functions

Posted: 03 Jan 2007 ?? ?Print Version ?Bookmark and Share

Keywords:switches? IF switch? LNB? Zetex? satellite TV?

Zetex Semiconductors has launched a 4:2 IF switch said to be the first to feature integrated gain and control functions. Promising to improving IF signal reliability, eliminate false switching and reduce circuit complexity, the ZXHF5000 chip is expected to impact the design of twin and quad low noise blocks (LNBs) and satellite TV distribution networks.

The device achieves a 30dB channel isolation up to frequencies of 2GHz. By adding 19dB of gain, the ZXHF5000 avoids the 7dB insertion loss normally prevalent in traditional passive switch solutions.

Flat 19dB gain
While the traditional demand is for LNBs to handle a bandwidth of 950-2150MHz, an extended band of 200-2150MHz is now being advocated, to support more TV channels and the introduction of HDTV. The ZXHF5000 has been designed to provide a flat 19dB gain across the band with a slight increase in gain (2dB) between 1500MHz and 2500MHz to help with down feed cable losses.

Through each of the two down feed control pins, the ZXHF5000 detects the STB generated horizontal/vertical polarisation and high/low band selection signals, to enable any one of the four IF inputs to be connected directly to any one of the two down feed outputs of the LNB or satellite switch box.

To increase flexibility and to help minimise PCB area, the RF inputs can also be reprogrammed to be in reverse order, by applying a logic control signal. As a result of this unique mirroring function, two devices can be mounted back-to-back on a double-sided PCB such that the two sets of RF inputs are aligned. The 4-by4 IF switch created therefore helps avoid complex routing of RF tracks.

Low signal distortion
Impedance matching was another key requirement in the chip design. Integrated pre-amplifiers ensure that RF signal inputs are 50-ohm matched, enabling direct driving by the gain stage or mixers in standard LNB architectures. The two IF outputs are independent of each other and are matched to drive a 75-ohm cable load to the STB. The two outputs are highly linear and achieve an output third order intercept point of 16dBm across the band while keeping current consumption to a minimum, ensuring low signal distortion.

Current consumption is of crucial importance in LNB applications and the ZXHF5000 has intelligent power management features built-in. To save power the chip shuts down any unused output cable driver stages and any unused input stages while maintaining impedance matching with pre-amplifiers or mixers. The chip has been designed to operate at 3.3V in order to maximise efficiency when used with other Zetex direct broadcast by satellite (DBS) products.

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