Zetex announces technology breakthrough in LNB design

The new device achieves a staggering 30dB channel isolation up to frequencies of 2GHz. In addition, by adding 19dB of gain, the ZXHF5000 avoids the 7dB insertion loss normally prevalent in traditional passive switch solutions. Until now, achieving the combination of high isolation and positive gain has proven too difficult to overcome.

Geoff Stokes, DBS Systems Engineer at Zetex commented, “Attaining this level of isolation is simply not an easy task, we’re not doing any frequency conversion within the IC and so the circuit layout and track routing of the silicon design was crucial to the chip performance, as was pin-out and package selection.”

“European satellite operators are typically recommending a minimum isolation figure of 26dB, and it has been noted that this key design metric has been gradually reducing in some LNBs. Poor isolation means that LNB output signals are closer together in frequency and in a weak signal area this has an adverse effect on signal reliability and STB performance.”

Achieving the gain characteristics presented a major design challenge. While the traditional demand is for LNBs to handle a bandwidth of 950 to 2150MHz, an extended band of 200 to 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 to 2500MHz to help with downfeed cable losses.

Through each of the two downfeed 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 downfeed outputs of the LNB or satellite switch box. Unwanted signals at the control input, such as DiSEqC and transients, are also rejected.
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 4x4 IF switch created therefore helps avoid complex routing of RF tracks.

Impedance matching was another key requirement in the chip design. Integrated pre-amplifiers ensure that RF signal inputs are 50Ω matched, enabling direct driving by the gain stage or mixers in standard LNB architectures. The two IF outputs are independent from each other and are matched to drive a 75Ω 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 DBS products.