What exactly is the SDR-14?
The SDR-14 hardware is primarily a signal capturing device. It samples the input data at a fixed 66.67MHz rate with a 14bit A/D converter. This sampled data can be sent directly to the PC or it can be converted to a complex baseband signal and downsampled to lower sample rates before being sent to the PC. The SDR-14 uses a USB 1.1 link back to the PC and can handle up to about 635Kbytes/sec continuous data rates. This means that the SDR-14 can support real time signal capture of up to about 160KHz wide spectrum. The SDR-14 uses an NCO(Numerically Controlled Oscillator) that is user controlled from 1 to 33MHz in 1Hz steps to convert a frequency band to a complex baseband signal. For example, setting the NCO to 15.0MHz will shift the signals centered at 15MHz into zero complex frequency samples that are then sent back to the PC as I/Q data.
For sample rates faster than the USB can support, a memory buffer will store up to 262144 samples at up to 66Msamples/sec and then the SDR-14 will send the buffer at the slower USB rates back to the PC. This allows large FFT processing to be done on a block of contiguous sampled data at the full A/D sample rate. Demodulation is not possible since the blocks of data are not continuous.
The SDR-14 does not do any other signal processing except for the complex baseband conversion and downsampling. Any spectral analysis or demodulation must be done in software by the PC. In other words there is no digital signal processor in the SDR-14 available for demodulation, FFT's, or other custom processing. It is merely a sampling and down conversion/down sampling device.
The SDR-14 has 2 inputs. One is a direct input to the A/D for custom applications. The other input has a preamp and 30MHz antialiasing filter so that the input can be used for measuring/receiving signals from 0 to 30MHz without external filtering.
What is unique about the SDR-14?
The SDR-14 is unique in that it can sample and buffer data at a high sample rates before sending it back to the PC. The digital down conversion provides a complex baseband signal that can be easily processed by the PC. The USB port provides a high speed connection back to the PC allowing demodulation or continuous processing of signals in up to about 160KHz of bandwidth. This all with relatively low cost hardware. The interface to the SDR-14 is also open so 3rd party developers can write their own custom application software.
Typical Panadapers have fairly high bandwidth resolutions on the order of 2Khz or so. They cannot distinguish narrow signals while with the SDR-14 and a decent sized FFT, 1 Hz resolution is possible and just about any signal can be isolated.
What kinds of things can I do with an SDR-14?
The primary application of the SDR-14 is for spectral analysis of signals. Once the data is in digital samples, very accurate measurements/analysis can be performed on it.
A secondary application of the SDR-14 is as a general coverage receiver. It can perform as a respectable receiver but since it has a "wide open" 30MHz wide front end, it can be overloaded by any strong signals within the 30MHz band. The SDR-14 has built in attenuators that can be controlled from the PC to help. Any demodulation functions must be done by the host PC since the SDR-14 does not have any DSP functionality to perform demodulation.
Where can I buy one?
The SDR-14 is in production and is available through Universal Radio.
Can I write my own application software?
The SDR-14 USB interface is fully open and specified for use by 3rd party developers. Although the Spectravue program is provided for general use, it is hoped that many custom applications will be written as the unit becomes available.
To make it easier for 3rd party developers to use the SDR-14, an ActiveX component is being developed that will make the interface to the SDR-14 a lot easier. It will wrap all the low level USB I/O into some easy to use interfaces as well as provide the data after being processed by an FFT.
This ActiveX control and an example C++ application using it will all be provided open source as a useable control or just as an example of how to interface to the SDR-14 USB port and do FFTs etc.
Will the SDR-14 be offered as a kit?
No. The SDR-14 has fine pitch surface mount components on both sides of the board. Although there are some who could probably assemble it by hand, the logistics of providing a parts kit, detailed instructions, and dealing with customers who bent or broke the pins on a $100 chip are just too much to deal with. We would end up having to charge more for a kit than an assembled unit.
Why are there two different web sites for the SDR-14?
The SDR-14 was developed by two individuals and we split the product by hardware and firmware/software. RFSpace is going to manufacturer actual SDR-14 hardware and MoeTronix is supplying SpectraVue and all the other software components. Since SpectraVue can also be used without an SDR-14 unit with a soundcard, a license for the soundcard option can be purchased separately.
How does the SDR-14 analyze signals greater than its nyquist rate??
The Nyquist theorem says you cannot sample a signal whose Bandwidth is greater than Fs/2 not the highest signal frequency. So as long as the bandwidth of the input signal is limited to one-half the sample rate you are ok. Thus the SDR-14 can sample signals beyond its 33.33MHz Fs/2 sample rate using this technique called Harmonic Sampling.
There are physical limitations to the hardware that limit the practical highest frequency around 200MHz. Also as shown in the figure below, every other harmonic band folds back on itself causing the frequency spectrum to be inverted.
In order to use the SDR-14's Direct input, one MUST supply a suitable bandpass filter to limit the input bandwidth to the desired Harmonic sampling band. As can be seen, without a filter on the direct input, a 5MHz, a 61.67MHz, a 71.67MHz, a 128.33MHz, and a 138.33Mhz signal would be received on top of one another without any filtering.
Are there any future products/add-ons being considered?
Yes. We have more ideas than time/money to develop. Right now we are focusing on getting the basic SDR-14 out the door before spending time on new products. However these are a few future ideas kicked around for both software and hardware accessories:
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