OP25j

There are a number of alternatives for hardware to capture a P25 baseband signal. These include:

• USRP (using TVRX daughterboard and appropriate antenna).
• SSRP with an appropriate radio front-end and antenna.
• Custom interface to existing scanner using soundcard for quadrature sampling.
• Custom PSK to serial interface hardware.

For those without any hardware it is possible to investigate using signal samples which are available on the DemodulatorPage.

=== USRP ===
Initial work is proceeding using the USRP which is available from [[http://ettus.com Ettus Research LLC]]. This is a relatively low-cost software radio (< 1000USD) that works with the GNURadio framework. The example USRP shown below shows a fully populated USRP with TX and RX capabilities as well as identifying the major components:

Image(usrp.jpg)

The USRP is capable of wideband reception and requires no special-purpose hardware as everything is done in software. A USRP with the TVRX daughterboard (not illustrated) and an appropriate antenna can be used out-of-the-box or the BasicRX card combined with an appropriate RF interface.

=== SSRP ===
The SSRP is a promising alternative to the USRP from [[http://oscar.dcarr.org/ssrp/index.php David Carr]] and is significantly cheaper. There are presently no suitable radio front ends, the sampling rate is reduced in comparison to the USRP and it is not nearly so mature. Nevertheless it may be a good bet in future for mobile use or where the expense of the USRP cannot be justified.

=== Sound Card Sampling ===
The ADC built into the soundcard of many home computers is capable of sampling at rates of up to 200KHz. Using the soundcard to quadrature sample the discriminator output of a receiver is a cheap and cheerful solution. Not all scanners provide a discriminator output but there are scanner modifications that that can be made when this is not provided as standard. This approach may require some custom interface circuitry and the filtering within the receiver will probably limit reception to a single channel.

=== Custom hardware ===
Another approach for interfacing to existing scanners is to feed the discriminator output to a custom hardware circuit. Some are available built and tested from a variety of sources such as [[http://www.discriminator.nl/index-en.html discriminator.nl]]. This approach is low cost and pretty straightforward although limited to reception of a single-channel at a time.