BFSK Generator on GNURadio – Telecommunications, Navigation & Electronics

Introduction

In previous posts I have looked at various aspects of GMDSS/DSC using Scicos. In this post I look at a BFSK generator for DSC VHF using GNURadio. The advantage of GNURadio is that it can be directly interfaced with the RTL-SDR and can decode in real time.

GNURC Schematic

Fig.1 BFSK Generator for DSC VHF in GNURadio Companion

Parameter	DSC VHF BFSK
Fc = VCO Center Frequency Phase Continuous BFSK	1700Hz=10681.415rad
F mark	2100Hz=13194.689rad
F space	1300Hz=8168.1409rad
Constants: ca &cm ((1xca)+cm)wc=w1 ((0xca+cm)wc=w2	cm=w2/wc=0.7647059 ca+cm=w1/wc=1.2352941 ca=.4705882

Fig.2 VCO Parameters for Sensitivity, Fmark, Fspace

Figure 1 shows the BFSK Generator GNURC schematic. The key component is the VCO. It’s main parameter is the sensitivity in rad/sec per volt. Using two constants (additive and multiplicative) the Fmark and Fspace frequencies can be generated. Fc the center frequency is set for 1volt = 1700Hz. Fmark will be generated by an input voltage of 1.2352941 and Fspace will be generated by an input voltage of 0.7647059. Figure 2 lists the parameters used to calculate ca & cm. This generator is completely analogous to the one developed in Scicos (Ref.3).

Figure 3 shows the VCO test circuit. The VCO is fed by either a hard “1” or hard “0”. Figure 4 shows the Fmark=2100 output for a “1”.

Random Data Generator

Figure 5 shows the schematic used to generate random data to drive the VCO. In Scicos I wrote a script file to create a data file, the same could be done in Python. In this case, I used the Random Source and applied S/H twice using a square wave for the data clock at 1200bps. The first S/H selects either 0 or 1 during the low portion of the square wave. The second S/H suppresses the multiple pulses during the first half of the square wave (the second square wave clock delayed by pi or half bit period). The Random Data Generator feeds a File Sink block that stores the data generated.