SignalScopePage » History » Revision 8
Revision 7 (max, 04/22/2017 04:04 PM) → Revision 8/30 (max, 04/22/2017 04:04 PM)
= Signal Scope =
== Installation ==
Before running the scope app, check that the following standard pre-requisite packages are also installed:
* GNU Radio
* The op25 blocks
* Frank's gr-fsk4 block
* The op25 repeater block
The Python Numeric package is now also required. If you're running Debian or Ubuntu:
{{{
apt-get install python-numeric
}}}
Other than these pre-reqs, no special setup or installation is needed.
There are three overall options or modes depending on your hardware; these are
* USRP
* External receiver with discriminator tap connected to sound card
* External receiver with IF in the audio sound card range (e.g., 24 KHz), referred to as "audio IF"
== Running with the disc-tap option ==
The signal scope does not require the USRP. If you have a discriminator-tapped receiver, use the "-a" option:
{{{
./scope.py -a -v 25 -g 50
}}}
== Running with the USRP ==
It's helpful to find out the current calibration error beforehand (I use kalibrate), for example, +1234 hertz:
{{{
./scope.py -f 412.34e6 -RA -g 65 -c 1234 -v 25
}}}
== Running in the Audio IF mode ==
Receivers equipped with an IF output in the sound card range can be used. This is known as "audio IF" mode.
A soundcard sampling rate of 96K is used and the IF frequency (typically 24 KHz) is given using the {{{--calibration}}} parameter:
{{{
./scope.py -A -c 24e3 -g 50 -v 25
}}}
== Feature overview ==
* Spectrum plot
* Baseband oscilloscope
* Eye Pattern Diagram (Datascope) display supporting several standard symbol rates
* Constellation Diagrams
* Demodulated Symbol Output
* Correlation (including Fast Auto-Correlation)
* Direct-frequency entry, signal gain and fine-tuning controls
* User-selectable demodulator (FSK4 or QPSK)
In the USRP and audio-IF modes, several additional program functions are enabled (spectrum FFT, constellation diagram, PSK demod, and iDEN correlation).
In all modes, the {{{--wireshark}}} option is used to write received P25 packet data to Wireshark.
== Spectrum Display ==
[[Image(a.png)]]
The controls arranged along the bottom of the page are:
* Frequency: to retune, type the new frequency here and press ENTER
* Signal Gain: adjusts the baseband (demodulated) signal level
* Fine Tune: adjusts tuning frequency over +/- 3000 Hz range
* Demod: Selects demodulator (currently used in Demodulated Symbols only)
Except for the signal gain control, these controls are only available in USRP RX mode.
== Eye Pattern Diagrams ==
The scope input source can be connected either before or after the symbol filter using the Viewpoint toggle.
Also the proper speed must be selected from the available options.
[[Image(b.png)]]
== Constellation Diagram ==
[[Image(f.png)]]
The signal scope also features an angular population graph (shown above) in addition to the traditional constellation display.
In this mode the symbol magnitude (distance from center) is discarded. Instead the circle is sliced into segments and a count
of symbols found in each segment is plotted. This is similar to a histogram except that a straight line is drawn between each
result, and that the results are arranged in polar form instead of rectangular form.
With this display, the zone at the exact center of the plot can be used precisely to measure the degree of separation separation, or margin
between the symbol decision points. The plots plot below illustrate the difference, with poorer shows good convergence showing on the left image:
[[Image(mhp7a.png)]][[Image(c.png)]]
The two-color mode is used in these images, providing natural relief (compared to highlight the distinctive feature of π/4 DQPSK in which
successive symbols are chosen from two distinct constellations (each containing four possible symbol values) separated by 45° one above) :
[[Image(c.png)]]
== Demodulated Symbols ==
[[Image(d.png)]]
== Correlation ==
[[Image(e.png)]]
Also included is Frank's Fast Auto Correlation (fac):
[[Image(g.png)]]
== BUGS ==
Possibly bugs exist, here are a few of the known ones as of this writing
* Symbol filters totally brain damaged (need separate filters for each speed)
* When switching modes using the notebook tabs, leftover data from before may appear momentarily
* Highest and lowest speeds are not well tuned resulting either in sluggish updates or CPU exhaustion
