Page 41 - EngineerIT November 2021

RADIO COMMUNICATION

Table 2. PLL retuning time using fast PLL retuning mode
PLL Reference Clock (MHz) Fast PLL Retuning Time (μs)
30 91
38.4 77
50 56
100 27
150 21
200 20
250 17
300 15

Table 2 shows the fast PLL retuning time required based on a
different PLL reference clock rate. At a PLL reference clock rate of 300
MHz, the fast PLL retuning time is approximately 15 μs. With a hop Figure 6: Transmit output for Link 16 Tx frequency hopping.
frame length of 13 μs for Link 16, the 15 μs of PLL retuning time when
using PLL mux mode can satisfy the timing requirement if the transition output port on the ADRV9002 evaluation board for observation. The
time is greater than 2 μs, as shown in Table 1. upper plot shows the performance of power vs. time. It can be seen
As described in the thesis paper “Performance Analysis of a JTIDS/ that transmit hopping happens every 13 μs with a transition time about
Link 16 Type Waveform Transmitted over Slow, Flat Nakagami Fading 3 μs between consecutive transmit hop frames. The lower plot shows
Channels in the Presence of Narrowband Interference ,” Link 16 the performance of frequency vs. time. In this experiment, the transmit
3
message data can be sent as either a single pulse or a double pulse, carrier frequency cycles through four different frequencies in a 1 MHz
depending on the packing structure. The single-pulse structure consists step size. As expected, the lower plot proves that the transmit output is
of a 6.4 μs on-time and a 6.6 μs off-time with a total duration of 13 also cycling through four different frequencies in a 1 MHz step size with
μs. The double-pulse structure consists of two single pulses that carry good frequency accuracy throughout the entire dwell time.
the same data but use different carrier frequencies, as shown in Figure Further measurements are performed to study the frequency
5. Therefore, the transition time could be 6.6 μs long (>2 μs), which accuracy of the Link 16 FH, using more advanced test equipment such
makes Link 16 FH feasible with the ADRV9002. as Keysight E5052B and R&S FSWP. In the example measurement
Figure 6 shows the ADRV9002 transmit output (power vs. time and shown in Table 3, the transmit carrier frequency is hopping at 400
frequency vs. time) with Link 16-type hop frames (transmit-only FH is MHz, 400.1 MHz, 400.2 MHz, and 400.3 MHz. The transmit input is
used for simplicity). Note that in order to show the minimum transition constructed to produce 400 MHz output for all the hop frames. The
time achievable by the ADRV9002, the experiment does not follow the measurement duration is set at 100 μs, which includes seven complete
standard Link 16 pulse structure in Figure 5. The on-time is increased hopping frames. The frequency is measured at every 128 ns time
from 6.4 μs to 11 μs and the off-time is reduced from 6.6 μs to 2 μs. interval. It can be observed that the PLL is fully locked at the beginning
A Tektronix RSA306B spectrum analyser is connected to the transmit of the dwell time. The frequency error during the dwell time depends
on the phase noise performance.
Table 3 shows the average, maximum and minimum frequency
offset (the absolute difference between the output frequency and
400 MHz) performance for these consecutive seven hop frames. In
most frames, the average frequency error is less than 1 ppm. The
results are also found repeatable for tens of measurements. Note that
the measurements could vary depending on the equipment and test
configurations.
The ADRV9002 provides user capability to fine tune the PLL loop
filter bandwidth. The performance shown in Table 3 is achieved when
the PLL loop filter bandwidth is configured at 1200 kHz. Larger PLL
Figure 5: Standard Link 16 double-pulse structure. filter bandwidth improves the PLL retuning time, which guarantees the

Table 3. Frequency accuracy performance with Link 16 frequency hopping
Hop Frame Number Average Frequency Error (Hz) Max Frequency Error (Hz) Min Frequency Error (Hz)
1 348 730 46
2 424 997 4
3 267 563 20
4 327 892 7
5 253 569 2
6 394 903 12
7 253 677 17

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