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I spent a bit of time researching SDRs, mostly looking into the possibility of building an all-FPGA-based TRX. There are a lot of commercial ready solutions; some are cheap, some not, but they all tend to be operating from 50/70 MHz frequency range.

I am a complete newbie in RF processing and what escapes me is: if the circuitry is able to process a 50 MHz signal, why can't it do the same with lower frequencies? Is it some physics limitation? I guess it must be — otherwise everyone would advertise an operatiing range from DC to X MHz. Or is operating on lower frequencies uninteresting to everybody except hams?

My question is TRX specific — not RX only.

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Largely it has to do with filtering and bandwidth. An ADC requires an anti-aliasing filter to remove all input frequency components that are more than half the sample rate. Otherwise, these higher frequency components get aliased onto lower harmonics.

As an example, if I have a 40 kHz ADC, it should be able to handle at its input anything from 0 to 20 kHz. If I should feed it 25 kHz, it will appear as 15 kHz...not what you want in a radio. The anti-aliasing filter removes everything above 20 kHz so this does not happen.

If I have that same 40 kHz ADC, but I want to handle radio frequencies above 20 kHz, I need to introduce a mixer. This is a device that shifts one band of frequencies to another. So, if I want to work in the 40 meter band, I need a mixer that converts frequencies around 7 MHz down to 0 - 20 kHz for my ADC. However, my mixer will again require filters to avoid intermodulation and aliasing.

All of this is equally applicable to transmitting -- the process is identical, except in the other direction. However, some stages of the transmitter must handle high power, which makes the filters larger and more costly. This is why it can seem easier to find a receiver with coverage of more bands.

So greatly simplified, the coverage of an SDR, or in fact, any radio, is determined by what filters it includes. As an example, take a look at the SoftRock RX Ensemble, which includes filters for four bands, and by changing the component values, can be built to cover LF or HF. With a sufficient selection of filters it becomes possible to cover the entire RF spectrum, but this would be expensive.

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    $\begingroup$ If I should feed it 25 kHz, it will appear as 5 kHz Are you quite sure about that? Shouldn't it appear as 15 kHz? $\endgroup$ – AndrejaKo Oct 21 '14 at 21:37
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Another difference is FCC approval.

Approval involves a rigorous set of requirements that are applied to the device's frequency range, transmit capability, spurious emissions, and more. Its not easy and is very strict. Anything not FCC approved falls under the category of experimental / test equipment.

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What I've found is: a lot of the SDR designs are sending data. A fair amount of it, too.
The bandwidth of a signal increases when it sends more / faster information, and the bandwidth of HF signals is rather limited.
Higher bands have more space, and the added incentive of "License free" bands at 433 / 900MHz seems to be taking the focus off shortwave.

Edit Example: Analog have a page on SDR's. Their overview lists transceivers with a bandwidth of up to 56MHz covering "most licensed and unlicensed bands". That's a huge bandwidth, and incredibly convenient - but it comes at a price. The minimum useable frequency is 70 MHz.
While I'm happy nobody is trying to send video from their toy drone helicopter at 20MHz, I would like to see more SDR designs that cater to HF. For receivers AND transmitters. With so many people using the unlicensed ISM bands, however, I suspect shortwave is becoming a niche market.

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    $\begingroup$ Hi Alan. This reads to me more like a rant than an actual answer to the OP's question. Perhaps you can edit your answer to clarify how what you say answers the OP's question? $\endgroup$ – a CVn Oct 20 '14 at 8:28
  • $\begingroup$ @MichaelKjörling it's actually a tricky question. While there are some design restraints, technically it's possible to have a HF SDR. Companies make them, and sell them. The question of "why" then needs to move from physical to commercial restraints. Anyway, I took a stab at editing the answer, adding example & link... $\endgroup$ – Alan Campbell Oct 21 '14 at 20:51
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There are commercial amateur transceivers that offer "direct digital conversion" (DDC) using fast samplers (>100 MHz) and FPGA processing. You might check Flex Radio Systems 6000 series.

There is also the ANAN series of radios (eg ANAN-100D) from Apache Labs. These products are fully open sourced, so you can get schematics and listings to see what is involved in making a full featured SDR system. Hint: It's not simple!

Filters are an issue. On the Rx side, you want to reject possible out of band interference. (Anti-aliasing, too, but that's relatively simple to do.) On the Tx side you need to be sure that harmonics are well filtered. That means low pass filters for every octave of frequency (or so).

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