A Wideband Synthesized Test Translator

A new wideband synthesized test translator has been developed to simulate the transmission paths for direct-broadcast satellite (DBS) systems and to automate testing of set-top boxes used in broadband wireless applications. The model UC-140/1485-003 test translator provides a convenient way to characterize DBS systems accurately and can be used as a test source in other wireless applications. The translator design is based on broadband high reliability military technology and is fully digitally compatible.

The test translator accepts a 100 to 180 MHz input signal at -20 to 0 dBm and converts the signal to a 920 to 2050 MHz output frequency range that is variable in 10 MHz steps. The translator uses double conversion to minimize spurious signals due to the relatively large bandwidth. The input signal is converted to the UHF range, and the image frequency and first fixed LO signals are attenuated easily from the output by the first IF bandpass filter. The second LO is synthesized and computer controlled in 10 MHz steps. This second LO translates the first IF to the 920 to 2050 MHz output frequency range. A proprietary technique is used in the synthesizer to eliminate the need for a VCO with a tuning bandwidth greater than one octave. For maximum stability, both the first and second LOs are locked coherently to a common 10 MHz crystal-controlled reference oscillator.

The output frequency range is divided into four overlapping frequency ranges using a switch-filter bank. The output power is adjustable from -60 to -20 dBm via an RS-232 control interface. An internal white noise source with an adjustable power level can be superimposed on the output signal to simulate added channel noise and to vary the system's signal-to-noise ratio.

In actual system measurements, the translator's spurious output performance far exceeded its specifications. The unit's image rejection was better than -70 dBc and, as shown in Figure 1 , the strongest spurious signal present in the 900 to 2100 MHz range is Ð40 dBc at 1248 MHz. Table 1 lists the test translator's key specifications. The test translator is self-contained; utilizes a 110/230 V AC, 50/60 Hz power supply; and is housed in a type 2U (three-inch high) 19-inch rack-compatible enclosure. Except for the filters all component parts are commercially available standard components, and most are available from stock.

Fig. 1: The test translator's output spectrum.

Table I: Key Specifications
Input frequency (MHz)	140±40
Output frequency (MHz)	920 to 2050
Gain flatness (dB)	±1
LO/carrier suppression (dB)	35
Output power (adjustable*) (dBm)	-70 to -10
Input/output impedance (ohm)	50/75
Input/output return loss (dB)	14
Image rejection (max) (dBc)	-50
Input power (dBm)	-20 to 0
Signal to noise (adjustable) (dB)	0 to 10
Frequency stability over operating temperature (ppm)	0.1
Phase noise at 10 kHz offset (max) (dBc/Hz)	-85
Operating temperature (°C)	15 to 35
*Optional