Test technique for set-top boxes

A test system (200) for testing for missing or shorted parts within a tuner circuit includes a signal generator (202) for applying a harmonic-containing baseband time varying RF test signal to the tuner circuit. The tuner circuit is tuned to a harmonic of test signal. A detector 208 coupled to the baseband IF output of the tuner circuit detects the voltage generated in response to the applied RF test signal. A voltage measurement device (210) measures voltage detected by detector to provide an indication of the gain. Significant changes in the gain indicate missing part(s) or short circuits.

TECHNICAL FIELD

This invention relates to a technique for testing set-top boxes and the like.

BACKGROUND ART

Current day set-top boxes typically include one of more tuners for tuning RF signals for extracting signals transmitted by a satellite or cable television service provider. A typical set top box tuner will include various components, including various integrated circuits, as well as passive components, (e.g., resistors, inductors, and capacitors) mounted on a circuit board inside an enclosure. Due to the limitation of board space and the nature of the RF signal received by the set-top box tuner, detection of missing parts and short circuits (“shorts”) has proved a challenge because of the difficulty in adding one or Integrated Circuit Test points on the circuit board for this purpose. Thus, other mechanisms become necessary to detect missing parts or shorts in the tuner circuit during product assembly.

In the past, set-top box manufacturers have relied on the Media over Cable Association (MOCA) transmission (TX) signal for detecting missing part(s) or shorts in the tuner circuit during product assembly. However, if the set-top box does not have a MOCA transmitter, another test technique becomes necessary. Moreover, the MOCA transmission signal frequency has a limited frequency range (475-625 MHz), whereas the parts in a set-top box tuner typically operate in a frequency range of 950-2150 MHz. Thus, the coverage range of the MOCA frequency for checking for shorted or missing parts is limited.

Thus, a need exists for a technique for testing a set-top box capable of checking the working frequency range of components within the box.

BRIEF SUMMARY OF THE INVENTION

A method for testing for missing or shorted parts within a tuner circuit commences by applying a harmonic-containing baseband time varying RF test signal to the tuner circuit. The tuner circuit is tuned to a harmonic of test signal. Thereafter, a baseband IF output of the tuner circuit measured to detect gain. If any part is missing or shorted, the gain will exhibit significant change.

DETAILED DESCRIPTION

FIG. 1depicts an exemplary high-pass filter circuit10of the type commonly found in satellite set-top boxes. The circuit10includes an antenna jack12coupled to an antenna14. An inductor16passes a DC voltage from a DC supply (not shown) on a line17coupled to a line18from the antenna jack. A gas discharge tube19and capacitors20and22each shunt the line18to ground. The series combination of an inductor24, a capacitor26, and a resistor28shunt the line18to ground. A series combination of capacitors24,32,34, and36coupled the line18to an output37of the circuit10. A separate one of inductors38,40and42shunts a respective one of capacitors32,34and36, respectively. An inductor44shunts the junction between capacitors32and34to ground whereas the inductor46shunts the junction between capacitors34and36to ground. An inductor47shunts the output37of the tuner circuit10to ground.

In practice, the components of the circuit10described above reside on a circuit board (not shown). Due to constraints on the size of the circuit board, placement of test pads becomes very difficult so use of a functional test technique becomes necessary to detect missing tuner components during manufacture. If the part is missing in circuit10, the gain will typically show a significant change. Therefore, measurement of the gain of the circuit10during production can provide a mechanism for detecting one or more missing parts. A commercial Vector Network Analyzer (VNA) can easily measure the gain. However, such devices have a high cost, effectively prohibiting their use.

In accordance with the present principles, gain measurement can occur using a low-cost test system200depicted in block schematic form inFIG. 2. As discussed in detail below, the test system200includes a signal generator202for applying a time-varying RF signal test signal, for example, a square wave rich in harmonics so that the periodic spectrum of the test signal drives the test circuit10ofFIG. 1. The circuit10can tune to one of the harmonics of the test signal. Measuring the baseband output of the circuit10using a log detector enables calculation of the gain.

In practice, the signal generator202of the system200provides a 10 MHz square-wave signal to the antenna input of the circuit10ofFIG. 1through an impedance match and DC-blocker204. The test system200also includes a band pass filter206for band-pass filtering the output of the circuit10for receipt by a log detector208, typically, although not necessarily, an Analog Devices model AD8306 High Precision Limiting-Logarithmic Amplifier. A voltage-measuring device210, such as, but not limited to, a Hewlett Packard Model 34401A multi-meter, serves to determine the voltage produced at the output of the log detector208. Thus, the voltage-measuring device210provides a measure of the gain of the circuit10in response to the time-varying harmonic-rich signal received at the input of the circuit10from the signal generator202.

As previously discussed, the conventional approach to testing the circuit10ofFIG. 1would make use of a Vector Network Analyzer (not shown) having a tracking signal generator that would sweep the frequency of the signal applied to the input circuit10. The test system200ofFIG. 2does not need require the use of such a tracking generator since the harmonic-rich square wave output signal of the signal generator202has proven successful in practice to sweep the input of the circuit10during testing.

When testing a circuit10used in satellite set-top boxes, for example the circuit10, using a square wave instead of other waveforms remains preferable because the satellite path typically will have a High Pass Filter, with a 50 dB MOCA band rejecter. Under such circumstances, a stronger input signal in the frequency range of 475 to 625 MHz becomes necessary to test this band.

A typical tuner in a Set-top Box can tune down to 270 MHz, which covers what the frequency range needed for testing purposes. Most tuners have Automatic Gain Control (AGC), which participates in the gain measurement. For a Broadcom SOC BCM7313 chip used in typical satellite set-top boxes, the AGC range can be measured by the following steps:

2) Kill the ISR (optional)

3) Write AII and AIT to manually control the IF and RF gain.

Using this approach, the tuner AGC ranges are:

IF gain: 0-30 dB when All changes from 0x0000 0000 to 0xffff fff0

FIG. 6and Table 2 below lists depicts voltage values measured by the log detector208ofFIG. 2for different level output signals of the tuner circuit10under test. 2.

TABLE 2dBmV02.1511−201.7331−401.3238−600.914−800.5108
Where dBm=48.83409*volt−104.634 for −80<dBm<10.

Table 3 depicts exemplary test results for the circuit10undergoing testing via the test system200ofFIG. 2

FIG. 8depicts the change in AGC during gain measurement.FIG. 9depicts a graph of IF output versus frequency, showing peaks at approximately 500 MHz, 1000 MHz, and 1900 Mhz.

FIG. 10depicts a graph of gain versus frequency when no parts are missing and when individual parts are missing such as capacitors34or30.FIG. 11depicts a set of gain versus frequency curves obtained when of an individual one of the components of the circuit10under test was removed.

FIG. 12represents an average of a collection gain versus frequency measurements as depicted inFIG. 11. Each component part was removed one at a time and the gain was measured in the manner described using the system200to apply a harmonic rich time-varying RF signal to the circuit10. The process was repeated to collect four graphs. The graphs were then passed through a computer program, such as Matlab, from Mathworks, to read the data from each graph, produce an average of the data, and plots it to a common graph with all other averaged data. The graph ofFIG. 12provided the basis for picking six frequencies for missing part identification. The frequencies chosen were (in MHz)410,550,610,910,1510,2150. Using these points each part should have a unique signature.

Using the aforementioned Matlab computer program, the gain for the six points is processed using the “compare.m” and “compare_direct.m” user defined Matlab scripts. The compare.m function takes the original missing parts data and compares it to the averaged data to see if each part will be identified correctly when compared to the template. The compare_direct.m function of the Matlab program uses the executables “oem.exe” and “mtune.exe” to configure and tune the tuner directly in order to test circuit10. Only the 6 points of interest are measured to decrease time taken for testing. Once the measurements are made the data is compared to each part template.

The following constitutes the software for performing the following functions:Comparison of the gain;Measuring the Square Wave Spectrum;Measure the log detector gain;Plot the log detector gain; andChoose the 6 gain values to compute the average gain