Managing Output Level Transients in RF Signal Generators
By Pablo Perez-Lara, Aeroflex
In an RF signal generator, changing settings like the RF level, operation frequency or modulation waveform can produce either positive or negative level transients at the output. A proper instrument design must avoid positive level transients because they can damage the DUT (Device Under Test) or additional equipment connected to it. This article explains how to manage level transients in signal generators by sequencing the events involved in the reconfiguration of the instrument.
When parameters such as the configured RF level or frequency of a signal generator are modified by the user, a perfect instrument would switch instantaneously as shown in Figure 1.
However, in practice, the different components in the generator require some time to reach their new state and a transition will be observed at the output, possibly exhibiting positive or negative level transients. A properly designed instrument should not only switch as fast as possible, but also prevent the apparition of positive level transients that, depending on their magnitude, could damage the DUT (Device Under Test) or other instruments connected to it. For instance, this could happen during testing of an LNA (Low Noise Amplifier), which has a small tolerance to overdrive, or a PA (Power Amplifier) that could deliver enough power to damage downstream test gear, e.g. a spectrum analyzer. Although the DUT and setup could be protected by means of power limiters, this is an expensive solution that would affect linearity measurements.
In this work, first the causes of level transients in signal generators are analyzed in detail to then propose techniques to manage them while achieving fast switching speed.
Causes of Level Transients in Signal Generators
The causes of level transients in RF signal generators are closely connected to the structure and operation of the RF system of these instruments. In order to illustrate this fact, Figure 2 shows the simplified block diagram of a typical example of the RF system in a signal generator. A frequency synthesizer and a switchable IQ modulator produce a CW (continuous wave) or modulated RF signal that goes into a filter bank to attenuate the harmonic content. The first element in the output system is the VGA (Variable Gain Amplifier), which provides continuously variable gain control, therefore allowing for i) a very fine output level step and ii) the possibility to compensate for gain fluctuations caused by temperature changes and aging. With this purpose, the power at the output of the VGA is monitored by means of a directional coupler connected to a power detector. The ALC (Automatic Level Control) block adjusts the VGA gain continuously to ensure that the detector reading tracks a target RF level.
One of the key specifications in commercial RF sources is that the output level range can exceed 140 dB. Such a wide gain range cannot be implemented exclusively with a single VGA stage. Moreover, if all the controllable gain was placed before the directional coupler in Figure 2, the requirements in terms of the directivity of the coupler and dynamic range of the power detector would become impossible to meet. These obstacles are overcome by means of a multistage step attenuator located after the VGA. For applications needing high output power, the instrument can also include an optional switchable PA at the end of the chain.
During normal operation, the gain of the VGA varies slowly to maintain the desired output power accurately and no level transients are observed at the RF output of the generator. The situation is different when the RF system of the instrument needs to be reconfigured, which normally happens as a result of changes in the requested output level, operation frequency, or modulation applied. These three cases can produce level transients and are analyzed separately below.
Changes in the Requested Output Level
When the user sets a different output level, the software driver of the instrument must reconfigure the RF system of the generator. Although the technique used to do this depends noticeably on the architecture chosen by the manufacturer and specific design of the generator, in practice any method will utilize internal calibration data to implement the following three common phases:
Calculate the new settings of the multistage step attenuator and switchable PA
Find the target RF level for the ALC and an initial estimate of the required VGA gain
Update the hardware with the new settings
Level transients can happen during the third step, when the hardware settings are updated, due to the timing of the step attenuator and the time response of the ALC. This can be explained by means of a simple example. Assume in the block diagram of Figure 2 that i) the multistage step attenuator comprises two switchable attenuators L1=30 dB and L2=60 dB, thus allowing for a maximum attenuation L1+L2=90 dB and ii) the VGA is adjusted for +10 dBm output power when both L1 and L2 are bypassed. A level change from -20 dBm to -50 dBm requires switching L1 out and L2 in. Due to the switching time of the attenuator stages or delays in the control signals, both actions do not occur simultaneously. Therefore it is possible that both attenuators are on or off for a short time interval when the requested level is modified. Figure 3 shows how this could result in negative or positive level transients at harmless -80 dBm or dangerous +10 dBm, respectively. A similar problem would happen if the target RF level is updated and the ALC responds faster or slower than the step attenuator.
Changes in the Operation Frequency
In order to obtain the right output power after a frequency change, the hardware must be reconfigured to compensate for the effect of the frequency response of the RF strip and differences in the signal level coming from the frequency synthesizer. This can produce level transients in the same way as a change in the requested output level.
Another mechanism that may cause level transients during frequency changes has to do with the switching speeds of the filter bank and the frequency synthesizer in Figure 2. If the filter bank switches much faster than the frequency synthesizer, in some cases there will be no signal present at the VGA input for some time. As a consequence, the ALC will set a high VGA gain. When the signal provided by the frequency synthesizer approaches the passband of the selected filter, a positive level transient can happen until the ALC responds by bringing the VGA gain down. It must be noted that the effect is especially severe when the filter has a steep transition band, because the VGA output level will increase much faster than the response speed of the ALC. An example of this mechanism is given in Figure 4. Initially the operation frequency is 6 GHz. When the user sets a 2.8 GHz operation frequency, a 3 GHz cutoff frequency low-pass filter is activated that makes the VGA input level decay abruptly. As a result, the ALC forces a high VGA gain to compensate for the low output level. Due to this, a positive level transient occurs when the output frequency is close enough to the passband of the filter. The ALC finally responds by reducing the VGA gain to the correct value.
Changes in the Modulation Applied
A vector signal generator will include an AWG (Arbitrary Waveform Generator) to output the analog IQ signals that drive the modulator in Figure 2. These signals are played from a waveform file that contains the I and Q samples. When the user switches between waveforms with different RMS voltage (e.g. when evaluating the performance of an amplifier under several modulation schemes), the VGA initial gain, ALC target level and output attenuation need to be reconfigured to produce the right output level. It must be taken into account that the AWG is able to switch waveforms almost instantaneously. Therefore if the new waveform starts playing before or after the analog hardware is updated, either positive or negative level transients can happen. This is illustrated in Figure 5. The RF level is configured at 0 dBm. Initially waveform 1 is played with 0.1 Vrms. The system switches to waveform 2, with 0.2 Vrms, before the RF gain is reduced, causing a 6 dB positive level transient. When the hardware is updated to bring the RF gain 6 dB down, the output level goes back to the correct value of 0 dBm.
Avoiding Positive Level Transients in Signal Generators
In the previous section it was shown that output level transients occur as the result of synchronization and time response issues arising when the level, frequency or modulation settings of the signal generator are updated. It is of interest noting that perfect synchronization is not possible, in the same way that two different devices will not exhibit identical time responses. Therefore, it can be concluded that either positive or negative level transients will always happen. However, the dangerous positive transients mentioned in the introduction can be avoided by forcing negative level transients every time the instrument settings are updated. This is achieved by means of a proper sequence of the several steps comprised by the reconfiguration of the generator. A typical hardware update sequence
1. Deactivate the ALC to avoid large level overshoots at the output of the VGA during changes of the frequency or modulation settings
2. Disable the RF output (this can be done by means of a switch)
3. Configure the multistage step attenuator at maximum attenuation
4. Apply new operation frequency
5. Send command to the AWG to start playing the new waveform
6. Set the initial VGA gain and ALC target level which depend on the new level, frequency and modulation settings
7. Wait until the frequency is settled and the new waveform is playing to activate the ALC
Wait until the ALC is settled to enable the RF output. In this way, the instrument avoids level overshoots due to excessive initial VGA gain and positive transients caused by the ALC responding faster or slower than the multistage step attenuator
9. Switch out stages of the output attenuator to configure the required gain. Positive level transients due to timing issues (see the example given in section 3.1) are avoided because the multistage step attenuator was initially at maximum attenuation
An example of the efficacy of the update sequence technique is shown in Figure 6, where a transition from -30 dBm to -70 dBm is measured for a real signal generator. In this case the level change mainly involves reducing the attenuation introduced by a couple of DSAs (Digital Step Attenuator) from approx. 34 dB to nearly 14 dB and switching in a 60 dB pad. When sequencing is disabled, the two DSAs change before the pad is switched causing a positive level transient close to -6 dBm. When the hardware update is sequenced, the positive level transient is prevented but a negative transient appears instead. The duration of the negative transient extends the time needed to reach the -70 dBm output level.
As seen in Figure 6, a disadvantage of sequencing the reconfiguration of the RF signal generator is the lack of output signal until the frequency synthesizer, ALC and multistage step attenuator are settled. The response time of these three elements is therefore critical to minimize the duration of the negative level transient. In this regard, first it must be taken into account that increasing the switching speed of the frequency synthesizer will normally compromise phase noise performance and spurious. Moreover, the sensitivity of the VGA and power detector in the ALC will typically vary with the RF frequency and VGA operation point making it difficult to achieve fast level response without incurring potential instabilities and ringing. Finally, the requirements in terms of linearity and switching speed for the multistage step attenuator are especially demanding because initially maximum attenuation is configured (e.g., GaAs FET based switches will tend to exhibit inconveniently long switching tails).
Aeroflex S-Series SGA and SGD signal generators mitigate these issues by employing an innovative fast low-noise frequency synthesizer architecture, together with a high-speed switching multistage output attenuator and an ALC carefully designed to compensate for variations in the responses of the power detector and the VGA. Applying these three solutions avoids positive level transients while providing excellent settling time figures in list mode operation: under 100 μs for level only transitions and below 150 μs for combined frequency and level changes.
Level transients can happen at the output of a signal generator when settings such as the configured level, frequency, or waveform are changed. Positive transients can damage the DUT or the measurement setup and must be avoided. Although sequencing the reconfiguration of the components of the RF system of the generator prevents positive level transients, negative transients happen instead that have a negative impact on level settling time. Aeroflex S-Series signal generators incorporate several fast switching techniques to minimize the duration of negative level transients to achieve excellent settling time figures.
About the Author
Pablo Perez-Lara is a principal RF design engineer at Aeroflex International Ltd. with experience in the design of test solutions for Bluetooth®, WiMAX, and LTE devices. He holds a PhD in Electronics and Communications Technology from the University of Malaga, Spain. Pablo is the author of several papers in journals such as IEEE Transactions in Microwave, Theory and Techniques, and the IET Proceedings on Microwave, Antennas and Propagation.
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