In the first part of this blog series, I analyzed the local oscillator (LO) settling time of the transmitting (TX) path of the Radio420X FPGA mezzanine card (FMC). In this second part, I look at the settling time in the receiving (RX) path.
RX local oscillator reconfiguration
In order to test the settling time of the LO in the RX path of the radio transceiver chip (LMS6002D), like I did for the TX path, I started with the orthogonal frequency-division multiplexing (OFDM) reference design included in Nutaq’s ADP 6.5 software tools (see OFDM reference design: Moving into cognitive mode). The LO is reconfigured from a 433 MHz to 453 MHz frequency at the rising edge of a given custom register value. For these tests, the SPI clock is configured to 10 MHz and the SPI start signal is outputted at a GPIO of the system in order to monitor it with an oscilloscope.
The Radio420X contains many U.FL connectors that are useful for characterizing the RF-baseband conversion process done on the LMS6002D chip. These connectors can be used to inject an analog signal into the TX baseband path in order to validate the RF output stage. In the RX path, the baseband signal just before the analog-to-digital converter (ADC) can be monitored or, if the second RX variable-gain amplifier (RXVGA2) is disabled, a baseband analog signal can be directly connected to the ADC (see Figure 3.1 of the LMS6002D Programming and Calibration Guide).
To observe the settling time of the RX LO, the U.FL connector related to the RXOUT pin of the LMS6002D chip is used. This pin lets you access the analog baseband signal just before the LMS6002D ADC. In order to output the baseband signal to the RXOUT pin, the RXOUTSW bit (register 0x09, bit 7) needs to be set to 1.
By monitoring the SPI start and the RXOUT signals simultaneously, it is possible to see the RX LO settling time. A signal generator at 463 MHz is connected to the RX connector of the Radio420X. When the RX LO is reconfigured from 433 MHz to 453 MHz, a stable 10 MHz baseband signal should be visible when the LO is stable at 453 MHz. Figure 1 shows the oscilloscope acquisition triggered at the rising edge of the first SPI start pulse.
Figure 1: Oscilloscope acquisition of RX LO reconfiguration from 433 MHz to 453 MHz
The blue signal is the RX baseband signal of the Radio420. The twelve red SPI start pulses come from the six TX and the six RX registers that need to be written (see Radio420: Local oscillator reconfiguration latency when using SPI). The first marker shows that the RX SPI transactions start at 175 µs and the second marker shows that the TX transactions are completed at 196 µs.
In the time domain, it’s hard to determine when the RX signal become stable at a frequency of 10 MHz. Displaying it on a spectrogram plot makes the analysis easier.
Figure 2: Spectrogram of RX LO reconfiguration from 433 MHz to 453 MHz
The markers show the beginning and the end of the TX SPI transactions and the moment the RX baseband signal frequency becomes stable. It takes approximately 51 µs to become stable after the beginning of the SPI transaction. This time could certainly be reduced if the SPI transactions were performed faster, by increasing the SPI clock frequency for example. Furthermore, in this case since the initial and target frequency are relatively close to each other, the first SPI register (register 0x15) does not need to be updated since its value is unchanged.
Figures 3 and 4 show the reconfiguration of the RX LO from 400 MHz to 780 MHz. In this case, register 0x15 needs to be reconfigured since the target frequency is not in the same frequency range that the initial frequency. For this test, the signal generator is configured at 790 MHz.
Figure 3: Oscilloscope acquisition of RX LO reconfiguration from 400 MHz to 780 MHz
Figure 4: Spectrogram of RX LO reconfiguration from 400 MHz to 780 MHz
It took approximately 89 µs after the first RX SPI transaction before the RX LO frequency became stable.
The LMS6002D datasheet states that the settling time of the LO is typically 20 µs for both the TX and RX paths. After experimentation, the RX LO settling time varies depending on the initial and target frequencies. Also, the RX and TX settling times are not necessarily the same under the same test conditions. Like with the TX path, increasing the SPI transaction speed could reduce the overall reconfiguration time of the RX LO and, depending on the system requirement, all six RX register transactions may not be required.