Spurious Performance
- Updated2024-08-02
- 6 minute(s) read
Spurious Performance
In addition to the desired signal, the PXIe-5611 I/Q Modulator RF OUT front panel connector also contains signal-related spurs. This topic briefly describes the sources of the most common spurs and how to reduce their magnitude or effect when possible. Spurious performance is divided into two categories, depending on where the spurs are generated. The first category is RF-related and includes harmonics, subharmonics, intermodulation distortion (IMD), carrier suppression, and RF image. The second category is baseband-related and includes baseband feedthrough and baseband negative third harmonic.
RF-Related Spurious Performance
Harmonics
The PXIe-5673/5673E Vector Signal Generator filters the modulated and upconverted signal before passing it to the PXIe-5611 RF OUT front panel connector. This filtering process reduces the harmonic content of the RF OUT output, but some harmonics still appear at the output. The second and third harmonics are the most prominent. If the harmonic levels as described in the device specifications are higher than what your application allows, consider external filtering, and confirm that the external filter is designed for a 50 Ω environment.
Subharmonics
The PXI/PXIe-5650/5651/5652 RF Analog Signal Generator is used as the LO source for the PXIe-5673/5673E. The PXI/PXIe-5650/5651/5652 RF OUT front panel connector contains subharmonics when the generated frequency is greater than 3.3 GHz. The PXIe-5611 filters its LO signal before using it. Filtering significantly improves the subharmonic performance of the PXIe-5673/5673E, but because the filters are non-ideal, some subharmonic content is still expected at the PXI/PXIe-5650/5651/5652 RF OUT front panel connector for carriers over 3.3 GHz. External filtering can be useful in further lowering the subharmonic content in demanding applications. Make sure that the external filter is designed for a 50 Ω environment.
IMD
IMD products are due to nonlinearities, and IMD products appear when more than one tone is generated. Intermixing occurs among the generated tones to produce spurs. The most prevalent type of IMD seen at the output of the PXIe-5673/5673E is the third order IMD (IMD3). For example, in the case of two tones generating at frequencies f1 and f2, the IMD3 products are spurs at frequencies 2f1 - f2 or 2f2 - f1.
The adjacent channel power ratio (ACPR) performance is largely dependent on the IMD performance, but ACPR and IMD are not equal. The PXIe-5673/5673E sets the IMD performance at relatively higher output levels such around 0 dBm or greater. At lower power levels such as –30 dBm or lower, the IMD performance of the PXIe-5673/5673E is dominated by the onboard modulators. Reducing the power level on the onboard modulators (the baseband power) can also reduce the IMD products appearing at the PXIe-5611 RF OUT front panel connector.
Reducing IMD Products
You can reduce IMD products for relatively low output power levels by completing the following steps:
- Set the Power Level Type property to Peak Power or the NIRFSG_ATTR_POWER_LEVEL_TYPE attribute to NIRFSG_VAL_PEAK_POWER.
- Set the Software Scaling Factor property or NIRFSG_ATTR_ARB_WAVEFORM_SOFTWARE_SCALING_FACTOR attribute to less than 1.
Reducing the baseband power by 1 dB in peak power mode reduces the desired signal power at the PXIe-5611 RF OUT front panel connector by 1 dB and that of the IMD products by 3 dB (for a total of 2 dBc improvement, assuming the output power is low enough so that the modulators dominate the IMD performance of the PXIe-5673/5673E).
Carrier Suppression
The PXIe-5673/5673E achieves very good carrier suppression after impairments are applied. The amount of carrier suppression when generating at low power levels might not stay at the same ratio to the desired signal compared to when generating at high power level.
Generating a modulated signal with a relatively high crest factor causes the digital gain on the arbitrary waveform generator to be backed off when the Power Level Type property is set to Average Power or the NIRFSG_ATTR_POWER_LEVEL_TYPE attribute is set to NIRFSG_VAL_AVERAGE_POWER. Although the average power appearing at RF OUT lowers when a higher crest factor is being generated compared to when a lower crest factor signal is being generated, the carrier power is the same with lower and higher crest factors. Subsequently, the ratio of the carrier appearing with the higher crest factor signal to the signal itself is higher than that of a signal with a lower crest factor. If the amount of carrier suppression appearing in the output is too low for the application, complete the steps in the Impairment Calibration topic to lower the undesired signal.
A very similar effect to carrier suppression occurs when the Power Level Type property is set to Peak Power or the NIRFSG_ATTR_POWER_LEVEL_TYPE attribute is set to NIRFSG_VAL_PEAK_POWER and the Software Scaling Factor property or the NIRFSG_ATTR_ARB_WAVEFORM_SOFTWARE_SCALING_FACTOR attribute is lowered significantly. Software scaling should be used with caution.
The power in the signal is distributed across the signal bandwidth when generating a broadband signal. For example, assuming a similar crest factor as a sine wave, a 0 dBm signal with a bandwidth of 4 MHz plateaus at -66 dBm/Hz as compared to a single sine wave at the same power level appearing at 0 dBm. If the carrier suppression is -50 dBc as compared to the sine wave, then depending on the resolution bandwidth of the spectrum analyzer analyzing the signal, the carrier may deviate from the signal plateau at the carrier frequency. (-50 dBc is a value used as an example. Refer to your device specifications document for information about expected performance.) In this example, the integrated signal power is still 50 dB higher than the carrier. Perform impairment calibration to achieve better results.
The following plot shows a possible broadband modulated signal with digital gain reduced by 35 dB.
RF Image
RF image occurs mostly due to phase skew and/or gain imbalance in the I and Q paths at baseband frequency. RF image degrades over baseband frequency. Manually calibrating the PXIe-5673/5673E in addition to calibration performed at NI can reduce RF image.
Baseband-Related Spurious Performance
Baseband Feedthrough
The baseband signal applied at the I+, I–, Q+, and Q– PXIe-5611 front panel leaks in to the PXIe-5611 RF OUT front panel connector depending on the frequency and the carrier frequency. For example, a single-sideband +10 MHz signal generated at a carrier of 1 GHz produces a desired signal at the RF OUT connector at 1,010 MHz. A baseband feedthrough signal also appears at the RF OUT connector at 10 MHz. The following table summarizes the typically expected value of this feedthrough relative to the desired signal in dBc for different carrier frequencies (fc) and baseband frequencies.
| Baseband Frequency (MHz) | fc< 250 MHz | 250 ≤ fc < 1200 MHz | 1200 ≤ fc < 2500 MHz | fc ≥ 2500 MHz |
|---|---|---|---|---|
1 | < –75 dBc | < –75 dBc | < –75 dBc | < –75 dBc |
10 | –52 dBc | –58 dBc | –68 dBc | < –75 dBc |
20 | –44 dBc | –50 dBc | –52 dBc | < –75 dBc |
30 | –40 dBc | –46 dBc | –48 dBc | < –75 dBc |
40 | –38 dBc | –44 dBc | –44 dBc | < –75 dBc |
50 | –36 dBc | –42 dBc | –40 dBc | < –75 dBc |
Baseband Negative Third Harmonic
Before being upconverted in frequency to the carrier frequency, the baseband signal undergoes a cubic order distortion that is also 180° off-relative to the baseband. A 180° phase-shift in the baseband third harmonic makes the baseband third harmonic appear at a negative frequency relative to the baseband signal at the RF frequency. For example, if you are generating a single-sideband continuous waveform at 1 MHz, write the following equation to the I and Q channels:
where t represents time
If the carrier is at 1.000 GHz, then the desired signal is at 1.001 GHz, and the baseband negative third harmonic is at 0.997 GHz.
Reducing the Spur Level
You can reduce the level of the spur by completing the following steps:
- Set the Power Level Type property to Peak Power or the NIRFSG_ATTR_POWER_LEVEL_TYPE attribute to NIRFSG_VAL_PEAK_POWER.
- Set the Software Scaling Factor property or NIRFSG_ATTR_ARB_WAVEFORM_SOFTWARE_SCALING_FACTOR attribute to less than 1.
Reducing the baseband power by 1 dB in the peak power mode reduces the desired signal power at the PXIe-5611 RF OUT front panel connector by 1 dB and that of the baseband negative third harmonic by 3 dB for a total of 2 dBc improvement.
Related Topics
Impairment Calibration—Refer to this topic for more information about I/Q gain imbalance.
Modulation Bandwidth Performance—Refer to the this topic for more information about image degradation.