
An SDR-based FRS Transceiver Nuand, LLC
wider transition band to be used in the interpolating FIR filter that follows this stage, which
reduces the required number of filter taps
3
.
The Channel Map property is set to [13, 14, 15, 0, 1, 2, 3], in order to position
FRS channels 1-7 (on input indices 0 through 6) as shown in Figure 6. With respect to
channel mapping, the same approach is taken for FRS channels 8 through 14 in the second
Polyphase Synthesizer block.
To minimize the impact of aliases, the Polyphase Synthesizer applies a per-channel
filter at the output sample rate of 400 kHz. The filter used in this block passes signals within
the 12.5 kHz FRS bandwidth limit. Therefore, the pass band is located at
12.5
2
kHz. To reject
the image occurring at the next channel, the stop band is place before the “edge” of the
image, expected to occur at 18.75 kHz = 25 kHz −
12.5
2
kHz.
The parameters used with fdatool to generate these filter taps are shown in Table 4. As
shown by the magnitude response in Figure 7, spectral content outside of the desired region
should be significantly rejected. However, one must take note of this undesired spectral
content when evaluating the design and measuring out-of-band emissions.
Table 4: Filter parameters for polyphase synthesizer LPF
Design Method FIR: Equiripple
Density Factor 20
Order 191
Sampling Frequency (MHz) 400
Passband Frequency (kHz) 6.25
Stopband Frequency (kHz) 18.75
Passband Weight 1
Stopband Weight 1
Post-PS gain
A Multiply Const block follows each Polyphase Synthesizer. These apply gain
to compensate for the reduction of average power resulting from zero-stuffing during inter-
polation. However, the gain applied here is not a constant value. Instead, it is a function
of the number of enabled channels that are being supplied to the input of the Polyphase
Synthesizer. The goal here is to apply gain to the signal such that the output IQ values
remain within [-1.0, 1.0].
With only one input enabled, a gain that is just slightly less than the interpolation factor is
used to avoid clipping. Thus, a value of 16×0.95 is used. When a second input is enabled, the
system is combining two signal sources whose amplitudes may be within [0.0, 1.0], yielding
a signal with an amplitude within [0.0, 2.0]. Therefore, to keep our signal amplitude within
3
This reduces the number of convolution operations required.
9