I am planning to build a software defined radio. For this purpose, I need to sample analog signals using an analog to digital converter. In clasical Nyquist sampling, we need to sample with a rate \$f_s>2f_{min}\$ as we all know. But what I want is not to Nyquist sample but to band-pass sample. For example, I need to sample a signal with 1 GHz center frequency and 10 kHz bandwidth. My question is can I use MCP3002 adc with following analog input model to undersample example case.
http://www.imageupload.co.uk/images/2014/03/03/analoginmcp3002.jpg
Use MCP3002 for undersampling
adc
Related Solutions
Please note the following:
- The device actually does not sample RF. It downconverts to 0 Hz (on frequency downconversion) and then samples it.
Internally it has various analog stages to control gain, bandwidth, as well as state machines to overcome limitations of direct conversion.
For details please check the following block diagram of the AD9361. https://github.com/analogdevicesinc/iio-oscilloscope/blob/master/block_diagrams/AD9361.svg
- The RF bandwidth can be 56 MHz and 64 MSPS will be sufficient. Why? Because the device is outputting 12 bits of I & 12 bits of Q samples at each MSPS rate specified. Therefore real sampling rate can be assumed to be 2X of specified MSPS.
- The device samples RF at much higher rate but the real reason for that is a little different than just satisfying Nyquist.
- the type of ADC inside the device requires higher sampling to generate desired 12-bit resolution. It is an advanced Sigma-delta-modulator (3-bit SDM + sign i.e. -4 to 4). With some math, it converts these 3-bit values and generates 12-bit values. For details please check the AD9361 Reference manual section related to ADC overload detector.
- Device needs to do some filtering so aliasing doesn't happen. In order to do that, it samples at a much higher rate, then digitally downsamples. During this process, it attenuates ADC images. There is also an additional FIR filter inside for correcting downconversion imperfections.
- Note that the device still needs external filters for good performance.
If it passes DC it's not a high pass filter.
What you probably want is a high pass filter (or a bandpass filter centred on 36MHz) and a separate DC source, which are combined at the ADC input.
Your filter is probably designed to deliver signal into a typical load impedance (50 ohms, 330 ohms) which is usually orders of magnitude lower than the ADC input impedance, and that load is typically modelled as a simple resistor from the output to ground. See the first circuit below (replace the capacitor with your actual HPF)
simulate this circuit – Schematic created using CircuitLab
In the second circuit, you simply return that resistance to your (clean, low noise) DC voltage of choice, that combines your HPF and DC signals correctly. And as your HPF blocks DC, this does not affect the DC voltages in earlier stages of the circuit. (C3 is a decoupling capacitor, to keep the DC supply clean and noise free)
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Best Answer
Look at the LTC2285 used for this sort of thing: -
It can run at 125MSps flat out but it has a 640MHz sample and hold - this tells you what the main problem is when undersampling - you need a really fast sample/hold circuit to ensure you "capture" the detail in the much-higher-frequency carrier wave.
Even the lowly LTC2245 with its 10MSps throughput has a sample and hold that is rated close the 2255: -