How much is worth spending? Written by: Onno VK6FLAB
This week, I finally decided to buy my first software-defined radio with transmission capabilities. For me, this was not an easy choice, as I had to choose from products ranging from low prices to “beyond my budget,” and there are countless options among these choices.
Aside from price, one of the considerations is the so-called bit-depth. In the past, I have talked about how Analog-to-Digital Converters (ADC) use bits to represent radio signals. In short, the voltage from the antenna is represented as digital values inside the radio. No signal represents zero value, and maximum signal represents the maximum value suitable for the decoder. A concrete example might be an 8-bit ADC that can represent 256 different values.
If you look at the available options, you will see radios with 8-bit, 12-bit, 16-bit, 18-bit, and 24-bit. At first glance, you might say that more bits are better, but is more really better?
For example, the ANAN-10 and FLEX-3000 radios, which are priced about the same, have different ADCs. ANAN is a 16-bit device, while FLEX is a 24-bit device. On the other hand, HackRF One is an 8-bit device that costs twice as much as the 12-bit device ADALM Pluto.
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How do you choose, and what did you choose?
Essentially, you are choosing the dynamic range. You can think of it as the range of signal strength that can be represented using multiple bits.
There happens to be a formula. It is 20 times log10(2), which is the number of bits multiplied by the square root of 3 divided by the power of 2, representing decibels relative to full scale or dBFS.
In more recognizable terms, it boils down to a range worth 6 dB. A good approximation is the number of bits multiplied by six plus two.
For example, a 6-bit SDR has a dynamic range of 6 bits multiplied by 6 bits equals 36, plus 2 gives a range of 38 dB. An 8-bit SDR has 6 multiplied by 8 bits equals 48, plus 2 gives a dynamic range of 50 dB.
I am using rounded numbers here, but it gives you a reasonably accurate sense of proportion. The six times the bits plus 2 works until about 36 bits, then it drops by 1 dB until we reach 85 bits, and of course, we may not be able to buy such a device at our local ham shop for a while, and then it drops by 2 dB.
Another way to think about dynamic range is to view it as the difference between the weakest signal you can measure and the strongest signal. Given that SDR will use the entire radio spectrum, you may have to deal with local broadcast stations as well as the QRP signals you want to decode, so a larger dynamic range would be better.
Let’s provide some context. The Australian Broadcasting Corporation (ABC) has a local AM station at 720 kHz with an EIRP of less than 155 kW. My QRP station uses 5 watts. My signal is 45 dB weaker than the local transmitter.
This means that, compared to the broadcast station, for the SDR to detect my signal, the SDR needs to have at least 45 dB or a range of 45, which is 2 times 43 divided by 6 times 8 bits.
Now this is not precise or complete, but it should give you a sense of scale.
In this example, the amplitude range of the 5-watt signal is represented by a range of 1, while the broadcast transmitter is represented by a range of 255 values.
This means that if I am transmitting a Morse code signal, the best decoding signal is that the presence or absence of my signal would change the value representing my signal from 0 to 1.
You might think that this is not suitable for decoding more complex content, such as SSB signals. My Morse signal happens to be right at the background noise, so it may not even be detected at all.
Similarly, if there is not a 150 kW station but a 1500 kW station, then a range of less than 25 dB or 4 bits is needed.
Now, before you start pointing out that there are other issues, yes, there are sampling rates, clock stability, and two other issues. We will address those. I should also point out that you typically use positive and negative values to represent the voltage range, and I have not mentioned that the maximum value is calculated using RMS.
Meanwhile, I am excited to see my new toy arrive. I will get it sorted out.
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