Patent Application
20100007355
1. Field of Invention
The present invention relates to testing of radio frequency (RF) receivers, and in particular, to testing RF receivers to perform faster power measurements and calibrations.
2. Related Art
Most RF receivers, including wireless RF receivers, use an input filter, generally a band pass filter, to provide frequency band selectivity. This filter attenuates out-of-band signals that otherwise would be received and processed within the receiver, and thereby use receiver resources for undesired signals, and potentially prevent proper processing of the desired in-band signals. These filters typically have high quality factors (high Q) with relatively steep roll-off, i.e., frequency attenuation versus frequency, outside the desired frequency range. However, as is well known in the art, such high Q filters typically have attenuation ripple throughout the frequency pass band. Such ripple can often be as much as one decibel (dB) or more across the desired frequency band.
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Traditionally, this type of calibration involves providing a known signal to the receiver front end and measuring the received power at a given frequency. For example, a known power level will be transmitted from a signal source (e.g., a test instrument) in the form of a continuous wave (CW) signal or a packet-based signal. The received signal will be analyzed for power, and a gain offset factor will be applied and stored in the system so that the power at that frequency can be reported correctly in the future. Advantageously, particularly with modem digital receivers, such power calculations can be performed inside the device under test (DUT), thereby allowing the DUT to perform the desired calibration compensation without further interaction with the test instrument, rather than changing the input frequency. Hence, test time is generally limited by control of the test instrument so as to ensure that the input signal power (as provided by the test instrument) is stable and at the correct frequency, e.g., by allowing sufficient time for settling in terms of signal power and frequency.
Alternatively, in a time division duplexed (TDD) system, compensation for the filter ripple can be calibrated by transmitting power out of the transmitter of the DUT and measuring such transmitted power. In a frequency division duplexed (FDD) system, such as a cellular telephone system, it will generally be necessary to perform calibration for both transmit and receive functions.
In addition to calibration of the band select filter, which can generally be done at a single power level, it is often desirable to calibrate the received signal strength indication (RSSI) operation. This will also involve the band select filter calibration, since it will be necessary to compensate the loss variation introduced by the band select filter, while also calibrating the receiver gain linearity to ensure that the reported RSSI is correct over both frequency (due to the band select filter ripple) and the input power level. Generally, implementation is similar to that described above where a known power is provided by a reference source (e.g., a test instrument) and the DUT will generate a correction factor based on the known input power level. As before, since most receive measurements can be performed inside the DUT, the test time for this type of calibration is also generally limited by the speed of the test equipment.
In conventional test techniques, a signal generator is used to provide a signal at a known power level to the DUT, one frequency at a time. While this is generally done since it replicates normal system operation, it is also based on traditional RF test equipment architecture. For fast test times, such instrumentation must be able to change frequency quickly, which involves a trade-off between settling time and system phase noise performance. Generally, phase noise performance is improved at the expense of settling time. In modern digital communication systems, for example, with high modulation accuracy requirements, this can be problematic and require more costly test equipment.
In accordance with the presently claimed invention, a method is provided for testing a radio frequency (RF) receiver as a device under test (DUT) with one or more test instruments to provide a plurality of relative power correction factors, a plurality of received signal strength indication (RSSI) calibration factors, or both.
In accordance with one embodiment of the presently claimed invention, a method for testing a radio frequency (RF) receiver as a device under test (DUT) with one or more test instruments to provide a plurality of relative power correction factors includes:
transmitting, with the one or more test instruments, a broadband signal containing a plurality of sub-carrier signals each of which has a respective one of one or more predetermined power levels and is centered about a respective one of a plurality of frequencies;
receiving the broadband signal with the DUT;
selecting respective ones of the plurality of sub-carrier signals;
measuring a power level for each of the selected respective ones of the plurality of sub-carrier signals to provide a corresponding one of a plurality of power level measurements;
comparing each of the plurality of power level measurements with a corresponding one of the one or more predetermined power levels to provide a corresponding one of the plurality of relative power correction factors; and
storing the plurality of relative power correction factors for use by the DUT.
In accordance with another embodiment of the presently claimed invention, a method for testing a radio frequency (RF) receiver as a device under test (DUT) with one or more test instruments to provide a plurality of received signal strength indication (RSSI) calibration factors includes:
transmitting, with the one or more test instruments, a broadband signal containing a plurality of sub-carrier signals each of which has a respective one of a plurality of predetermined power levels and is centered about a respective one of a plurality of frequencies;
receiving the broadband signal with the DUT;
selecting respective ones of the plurality of sub-carrier signals;
measuring a power level for each of the selected respective ones of the plurality of sub-carrier signals to provide a corresponding one of a plurality of power level measurements;
comparing each of the plurality of power level measurements with a corresponding one of the plurality of predetermined power levels to provide a corresponding one of the plurality of RSSI calibration factors; and
storing the plurality of RSSI calibration factors for use by the DUT.
In accordance with another embodiment of the presently claimed invention, a method for testing a radio frequency (RF) receiver as a device under test (DUT) with one or more test instruments to provide a plurality of relative power correction factors and a plurality of received signal strength indication (RSSI) calibration factors includes:
transmitting, with the one or more test instruments, a broadband signal containing a plurality of sub-carrier signals each of which has a respective one of a plurality of predetermined power levels and is centered about a respective one of a plurality of frequencies;
receiving the broadband signal with the DUT;
selecting respective ones of the plurality of sub-carrier signals;
measuring a power level for each of the selected respective ones of the plurality of sub-carrier signals to provide a corresponding one of a plurality of power level measurements;
comparing each of the plurality of power level measurements with a corresponding one of the one or more predetermined power levels to provide a corresponding one of the plurality of relative power correction factors and a corresponding one of the plurality of RSSI calibration factors; and
storing the pluralities of relative power correction factors and RSSI calibration factors for use by the DUT.
The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention.
Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed.
In accordance with the presently claimed invention, the sophistication levels of modern digital-to-analog converters (DACs) and vector signal generators (VSGs) can be advantageously used to implement test signals with large baseband bandwidths, as well as wide RF modulation bandwidths. Accordingly, frequency changes can be implemented at baseband rather than at RF as in a traditional RF synthesizer. Since such bandwidth is wider than necessary for testing, it is possible to operate the test equipment at a single fixed RF frequency and change the generated frequency by changing the generated baseband frequency. This allows for faster frequency changes in terms of testing within the DUT.
Additionally, such wide baseband bandwidth allows multiple-tone baseband signals to be generated. Accordingly, a broadband signal with multiple tones at specific frequencies can be generated. Further, it is possible to control the individual power levels of these tones. As a result, it is possible to generate a broadband signal that offers power at all desired channels (tones) simultaneously within a given frequency band. If all tones have the same power level, it is no longer necessary to provide synchronization between the DUT and the test instrument, since it is possible to merely have the DUT change its selected input frequency to the desired channel, measure the received power and, following confirmation of a valid power measurement, continue with another desired frequency selection. Hence, test and calibration time is now limited only the by the DUT and no longer by the test instrument.
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If it is desired to calibrate power over multiple power levels, the DUT 12 communicates to the test controller 16 when it has completed its measurements at the current power level and is ready to measure at a new power level. The controller 16 instructs the test instrument 14 to change the power level of its test signal 15, following which the controller 16 will instruct the DUT 12 to begin measurements at the new power level. Alternatively, the test instrument 14 can change the power level of its test signal 15 after a predetermined time of transmitting at the current power level. The DUT 12, aware of this time interval, will complete its power measurements and wait for the end of the time interval before beginning measurements at the expected new power level, allowing time as appropriate for the power to settle.
With all test sub-carriers 100, 101, 102, . . . , 147 transmitted at equal power levels, channel selectivity as provided by the band select filter 22 might be of concern under some circumstances. Using the GSM specification for purposes of this discussion, reference interference levels are +9 dB for co-channel interference, −9 dB for immediately adjacent (200 kHz) channel interference, −41 dB for next adjacent (400 kHz) channel interference, and so on. Accordingly, the receiver will be capable of attenuating a signal one channel away (200 kHz) by 18 dB (+9 dB−(−9 dB)=18 dB). Hence, even with all tones 100, 101, 102, . . . , 147 at the same power level, the power of the immediately adjacent channel will contribute only minimal extra power (e.g., approximately 0.07 dB for one adjacent tone, or 0.14 dB when accounting for adjacent tones on both sides). Such additional power, therefore, can generally be disregarded as insignificant, particularly since the receiver will often provide channel selectivity in excess of the specified minimum. At channels further away, the suppression is even better (e.g., 50 dB and more), so virtually no change in power should be detectable. Further, if CW tones are used, additional attenuation should be expected (since power spreading across frequency which occurs with modulation is not present and, although relatively narrow band, cannot be easily filtered), thereby improving accuracy still further.
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While it may be possible to remove virtually all power at the odd sub-carriers 201, 203, 205, . . . , 247, it is possible that some power will nonetheless be introduced, e.g., from quantization errors as part of the digital signal processing, or potentially from intermodulation between sub-carriers and IQ mismatches in the IQ modulator. While disabling every other sub-carrier should prevent direct intermodulation products from being introduced from the attenuated sub-carriers, some higher order intermodulation products may still be introduced. Alternatively, the broadband signal 15 can be generated such that the sub-carriers are spaced further apart, e.g., by 400 kHz rather than the 200 kHz prescribed by the GSM standard.
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As noted above, intermodulation and IQ mismatches can limit the possible dynamic range of the test instrument 14, and, therefore, the dynamic range that a signal test signal 15 can produce. If a larger dynamic range is desired, changing power of the test signal 15 during a test may be necessary. While it is possible to have the test synchronized before and after such power change, an alternative approach is to use a predetermined time and power relationship.
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While CW sub-carriers are represented in the examples discussed above, (
Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.
