TA0038 Datasheet, PDF (3/6 Pages) RF Micro Devices – DC ACCURATE FSK MODULATOR

The reference crystal which is supplied with the
RF2513 eval board is 7.15909MHz,. This normally pro-
duces a transmit frequency of 916.4MHz for a divide
by 128 setting. Other values could easily be substi-
tuted. Since the divider ratios are 128.454 and
128.444, the output frequencies are 919.6137MHz for
a logic “0” and 919.5421MHz for a logic “1” input.
Thus, peak to peak deviation is fixed at 71.6kHz. Since
the deviation is derived by numerical means as a frac-
tion of the crystal frequency, it is very accurate and
repeatable without the tuning. This deviation allows for
a wide range of data rates, from 1bps to 100kbps by
using different modulation indices. The upper limit is
actually limited by the loop bandwidth of the PLL, up to
the point where the modulation index becomes too
small. In Figure 3, a pseudo random data pattern
clocked at a 100bps bit rate is shown as an example of
the low data rate capability. The recovered data main-
tains its DC level through several consecutive data bits
providing evidence of the DC accuracy of this modula-
tor. In Figure 4, the same pseudo random data pattern
at 72kbps is shown. In each instance, a RF2917 evalu-
ation board tuned for 919.578MHz was used to
demodulate the data.
TA0038
Figure 4. Transmitted Data (top) and Recovered
Data (bottom) at 72Kbps
13
Figure 3. Transmitted Data (top) and Recovered
Data (bottom) at 100Bps
Figure 5. Kbps Eye Pattern
The PLL loop bandwidth is designed to be reasonably
wide and yet still provide reasonable spurious rejec-
tion. The loop bandwidth is one of the factors that
determine the rate at which the VCO changes from
one frequency to the other and, thus, the upper data
rates. As data rates exceed the loop bandwidth, the
modulation waveform becomes band-limited and takes
on the appearance of inter-symbol interference. Figure
5 shows the eye pattern of this modulator as seen at
the RF2917 receiver output. The flatness of the eye
pattern attests to immunity of this modulator to random
Copyright 1997-2000 RF Micro Devices, Inc.
13-189