Receive several fm-radio stations at the same time (~10-20 MHz wide summary at 90MHz radiofrequency)

Question

What is a cheap way to receive and record on PC several (tens) of fm-radios (public, e.g. news, music, etc)? Such radios uses frequencies like 90.4 MHz or 102.7 MHz, sends a stereo (sometimes with digital text subchannel).

If I want to record tens of radios, I need to cover a spectrum of tens of MHz.

The first way is to buy two or three dozen usual radiorecievers and try to connect them all to PC (e.g. with half-dozen of multichannel external usb soundcards). I think, that this is not very cheap.

The second way is to build a single radio, which will downsample entire band 88-105 MHz into 3-20 MHz, then feed it into high-speed ADC, and do a software detection of each station.

Is the second way possible? cheap? How mush will it cost?

What hardware can be used, if I have no skills of making PCBs and using ADCs & FPGAs?

Answer 1

The second technique you mention is the way to do it, using what is called a "Software Defined Radio" or SDR. Many radio amateurs are using SDRs, and the simple ones are very cheap, about 30 dollars for a kit that down-converts the input into in-phase and quadrature baseband audio output which is fed into the stereo inputs of a PC sound card for digital signal processing. However, they are using relatively low-frequency signals on the HF amateur radio bands, and the hardware doesn't use any exotic components. Digitising VHF signals as you require and receiving several channels simultaneously is going to be rather expensive, the ADC alone is going to cost about 50 dollars and you will also need an FPGA and a DSP, unless you convert down to baseband and do the DSP on a PC. You will need a lot of high-frequency design experience, be able to develop code for the FPGA, write DSP code and be able to design a high-speed multilayer PCB, so you should start studying. :)

As for cost, I'd estimate 500 dollars for the hardware, including the PCB, assuming you designed it yourself.

Linear Technology makes suitable ADCs that can downsample at 750 MHz! They were good enough to give me a couple as free samples. I have suitable FPGA and DSP boards, so it's just a question of putting them together. :)

Answer 2

For your first solution you shouldnt need a sound card. They make usb fm receivers. Like this:
http://www.amazon.com/ADS-RDX-155-EF-Instant-FM-Music/dp/B000HNHA12/ref=pd_sim_sbs_e1
Buying tens of these for $200 will be cheaper (and far faster) than a custom solution using the aforementioned SDR (software defined radio).

Though if your goal is a fun project learning lots about FPGAs, PCBs, schematics, RF, and other miscellaneous electrical engineering the SDR options sounds perfect :).

Answer 3

Look into GNU radio. Here's their hardware page to start.

Answer 4

Actually, RTL-SDR can solve this in ... several 20 USD dongles, each covering up to 3 MHz of FM spectrum (I achieved clear reception of 4 or 5 FM stations with 0.4 MHz spacing per dongle).

As was found by osmocomSDR project, there are lot of Chinese DVB-T USB receivers, built on Realtek RTL2832U chip and some 50MHz - 1+ GHz I/Q radio frontend. This chip has hardware detector and decoder for DVB-T video signals and puts raw MPEG TS to the PC. But there is a feature when chip transfers all I/Q samples to the host. Using USB 2.0 it can transfer to 3.2 or 2.4 MS/s (millions of I/Q samples per second) and there are now several programs to do SDR processing of samples. There is even GNURadio integration.