WiNRADiO Universal FSK Decoder

The WiNRADiO Universal FSK Decoder is an extremely versatile signal decoding utility for WiNRADiO receivers (the list of supported receivers is given in the "System Requirements" section below).

The Decoder contains several powerful analysis tools, making it possible to determine transmission characteristics of an unknown signal automatically.

Looking at the decoder's main control panel on the right, you will see a drop-down list at the top, labelled Protocol. Here you can select the type of coding protocol used. Using the Add or Edit buttons, new protocols can be added, either created entirely from scratch (the number of variable parameters makes the number of usable protocols almost unlimited), or by deriving new protocols from existing ones by modifying some of their parameters.

A "protocol" consists of all the various settings and parameters which are needed to be able to decode a signal, including the type of coding, mark/space frequencies, baud rate, character format, translation table, etc.

The first stage of the signal decoding chain is the level detector, this is applied automatically when used with G3xxx and GxxDDC receivers.

The signal is then passed to the Spectrum Analyzer section for frequency domain analysis. The analyzer estimates the two "mark/space" frequencies of the FSK modulation process. These are displayed in the Estimated section of the Frequencies group on the control panel. The user has a choice either using these automatically estimated values or pre-set values which can be also manually entered.

The detected frequencies are indicated by two markers. The spectrum is simultaneously displayed as either a continuous line or bars in the top display and as a "waterfall" in the bottom one. The spectrum can be shown using either logarithmic or linear values.

If the mark/space frequencies are known, then manual entry is of course also possible. These values can be also preset as part of standard, or user-defined protocols.

There is also a facility to keep the shift between mark and space frequencies constant. The shift can be either 170Hz, 425Hz, 850Hz or a user-specified arbitrary value.

The result of the mark/space frequencies estimator, either in manual or fully automatic mode, is then passed on to the Signal Demodulator. The Signal Demodulator module utilizes the two specified mark/space frequencies and extracts the data signal. There are two modes available, optimal and high speed.

The Demodulator relies upon three input variables: the two FSK frequencies (mark/space) and the bit length. The mark/space frequencies are used always for the demodulation process, while the bit length is used only when the optimal mode is active.

In order to see the signal at different processing stages, a logic schematic of the demodulator is shown on the panel, with several test points (the cyan points).

Click on one of the test points, and the signal at that point will be displayed on the oscilloscope on top of the panel.

An eye pattern scope has been also implemented for the final output and the positive input of the data formatter.

The data signal extracted by the Demodulator is then sent to two separate processing modules.

Using the measured time intervals between two transitions, the Time Estimator creates a histogram, the upper one as shown on the adjacent panel.

There are two cursors, one corresponding to the length of single bits and the other to the length of two-bit pairs. If one of them is being dragged, the other one moves accordingly.

In addition to this first histogram, a second (bottom) one is generated using autocorrelation of the time intervals. This is used for final estimations of the bit length.

The estimated bit length parameter is used by the optimal demodulator, while the baud rate is required for the actual character decoding.

The automatic baud rate estimation is suitable only for continuous bit streams. Transmission modes such as Packet Radio, SITOR-A, ARQ6-70/90/98, SWED-ARQ require the baud rate to be set manually.

Each protocol is derived from five generic decoding modes: ITA2 (Baudot), IRA (ASCII), SITOR, Packet and Raw Bits. For each mode, a different version of the Decoder section is shown on the main control panel; the two examples shown here pertain to IRA and SITOR.