In the NI-RFSA driver, the Configure Reference Level VI is used to set the reference level of the VSA. This must be set before calling the niRFSA Initiate VI.
Figure 4. Screenshot of niRFSA Configure Reference Level VI from NI-RFSA Help documentation
The reference level setting tells the analyzer the expected RMS power of all signals entering the analyzer. This is an important measurement setting for a couple of reasons. First, it adjusts gain and attenuation stages in the signal path to reduce compression in the amplification stages and prevent damage to the device. Second, the driver attempts to optimize its gain and attenuation settings in order to minimize noise in the measurement. Hence, it’s important to understand the nature of the input signal and to choose an appropriate reference level for the instrument.
Recall from the PvT graph in Figure 1 that the maximum RMS power was around -10 dBm. It’s clear by observing Figure 5 the reference level should be set at or above -10 dBm to ensure the analyzer is operating well within its linear range.
Figure 5. Power vs. Time of a GSM signal with the reference level
For comparison, look at the reference level as it relates to the corresponding spectrum shown in Figure 6. Note that the reference level is about 10 dB higher than the peak point along the spectrum trace. This is due to the fact that the spectrum shows data separated in frequency as opposed to the actual integrated power of the entire signal itself, as discussed in the previous section.
Figure 6. GSM Spectrum with reference level (10 kHz RBW, Ref. Level -10 dBm)
Reference Level Too High
Consider the implications in the time domain. See Figure 7 below, showing a GSM signal acquired with an appropriate reference level next to the same signal measured with a higher reference level. Notice that the measurement is noisier with a higher reference level. This is because the RF attenuation is too high and pushes the signal closer to the analyzer’s noise floor and decreases the signal to noise ratio of the measurement.
Figure 7. Left: GSM with an appropriate reference level. Right: GSM with a reference level too high.
Now consider the frequency domain shown in Figure 8. The increase in the noise floor is even more apparent, making it harder to see the full shape of the GSM channel. If the signal level were any lower, it would be overcome by the noise floor due to excessive RF attenuation.
Figure 8. Left: GSM signal with appropriate reference level. Right: GSM with reference level too high.
Reference Level Too Low
Consider Figure 9 below, which compares Figure 1 next to the same signal measured with a reference level that’s too low. A reference level set too low causes compression that can be seen in the time domain. Since there is less RF attenuation for the same power signal, the amplifiers are driven into compression.
Figure 9. Left: GSM signal Power vs. Time with a -10 dBm reference level. Right: GSM signal Power vs. Time with -40 dBm reference level.
Setting the reference level too low may result in damage to the device. For this reason, the driver will warn the user if it detects power too high above the expected reference level, or throw an error if it is too low.
Now consider Figure 8 showing the frequency domain. The spectrum is now distorted due to the compression observed in the PvT plots from Figure 7. The distortion creates additional frequency content that causes the noise floor to increase.
Figure 10. Left: GSM spectrum with an appropriate reference level. Right: GSM spectrum with a reference level set too low.
Above, we discussed how adjusting the reference level too high or too low can affect a power measurement. There are some other tradeoffs to consider. For instance, consider a signal whose power is very close to the noise floor. Averaging multiple acquisitions can be used to drop the displayed noise floor slightly to resolve the signal. While this allows the signal to be measured, it also increases the total measurement time due to the additional acquisitions required for averaging.