From an Amateur Radio Perspective, this technology may — once again — fulfill the promise of a new frontier in research and innovation for the hobby, that may spread to commercial uses.
I see it as a pathway to renewed currency (to be 'current,' not as Specie) for our cherished hobby.
The thing is: SDR is still, largely, un-approachable by the common Ham.
For one thing, we seem to be confused by the question: “What is an SDR?” Does it always involve an external control device? Does a TRUE SDR process signals on-board, or allow something else to handle the translation of Modulated information to/from a human-readable format?
Another perceived barrier to adoption is outlined by the question: “Do I really have to pay that much for the term “‘Software Defined?’”
Lastly, even if one take the leap, what software will be available to make a piece of hardware work?
Towards the first question, I offer this:
“First, Software-Defined Radios — as a broad definiton — are not completely Analog in nature. Therefore, some element of Digital Hardware is integral to their operation.”
Yes, this would mean that there has been, for at least the last two decades, professionally manufactured radios, as well as hobbyist creations, which have relied on some level of digital facilitation, that could fall under this definition. After all, even the Kenwood TS-840 has a digital readout, right?
So, what differentiates these from the newest, latest, coolest-looking computer-hosted radios out there?
For one thing, there appears to be a threshold of implementation, where discrete logic is displaced by a more flexible, dynamic logic.
As an example, let’s take an original Heathkit HW-101. It is well-known to be a venerable, light-weight (for it’s time), analog, Direct-Conversion Transceiver. What if modified it to accept an external Digital VFO?
In the past, there have been numerous digital VFO kits, designed for the specific task of replacing unstable, noisy, or inaccurate analog resonators. These VFOs, for the most part, are designed as pure implementations of finite state machines (FSM), where a discrete (‘defined’) number of inputs generate defined changes in ‘state.’ Most notably: turning of the dial generates a pre-defined signal to the FSM, which causes it to change it’s frequency.
Notice that there is no mention of any other input to this FSM, besides the knob turn. It is up to the designer of the Digital VFO to ensure that just the right signal produces just the right result, using any technique she or he may choose. Moreover, only a physical change in the hardware of the Digital VFO will allow it to respond to more inputs, which requires (at least a partial) re-design.
Just about every radio on the market today shares the same ‘FSM,’ aspect of operation. That is, there are only pre-defined inputs and outcomes.
The Kenwood T M - D700 series of radios only respond to sets of pre-defined control inputs; the internal Packet (AX.25) Controller is able to be connected to any RS-232-enabled Terminal device in order to operate as a stand-alone TNC. But even in this case, there is no element of pure, dynamic control over the inner workings of the radio and controller deck. There is only the ability to change the Packet Controller from APRS to KISS TNC; a simple state change.
Going back to our example of the HW-101 with a Digital VFO:
What if, instead of discrete logic, we placed a sightly more advanced VFO that was designed to respond to inputs through a USB cable, connected to a stand-alone computer?
By relying on the USB standard (one that is defined in both physical and logical realms), we — necessarily — rely upon software, resident on the computer, to control the VFO frequency.
This, precisely, is what occurs in the case of the SoftRock series of radios: A USB cable connects a host machine (Mac, Unix, or Windows, in this case) to a small ‘USB Controller,’ (manufactured by FTDI, in this case) which is responsible for decoding and encoding information to and from the radio. The information, for the most part, alerts the on-board digital VFO (A Silicon Labs Si570, in most cases) to change to a specific frequency, as requested by the Host computer… which is running a piece of customized software… which is being controlled by the Ham Operator (auspiciously).
So, does the Example of a SoftRock USB control meet exceed the threshold for our definition of ‘Discrete Logic?’ It’s a grey line, to be sure. Yes, the radio is controlled by software, which can be seen — in itself — as a dynamic actor in the chain. However, the if VFO can only accept frequency change requests, or report it’s state through the FTDI USB controller, there is still a degree of re-engineering involved when it comes to doing anything besides changing from one frequency to the next.
‘Ah,’ you say, ‘… so that means the SoftRock — by definition — can’t be accurately described as a Software Defined Radio!’
Not so fast, I tell you! Here’s the rub: there are not one, but two wires (besides the power lead) emanating from the SoftRock. The first is the aforementioned USB cable. The second wire:
I/Q data.
Stay tuned, friends, for the next chapter in the Continuing Saga:
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