DigiPan

A PSK-31 Terminal used for Spectral Analysis!

 

EDIT: Feb 2009.

Although great for making PSK31 QSOs, I no longer recommend Digiapn as an Audio Spectrum Analyzer.

By expecting two audio notes 32Hz apart, it was found to introduce errors when trying to measure a single audio note.

I am now using DL4YHF's Spectrum Lab. This is a full-featured Audio Spec An that is superior in every way. http://freenet-homepage.de/dl4yhf/spectra1.html

 

A new way of measuring TV Carriers
Since late 2000 I have been experimenting with a new way of measuring TV carriers with a high(er) degree of accuracy by using the PC SoundBlaster card.
The sound card is fed with audio from my IC-756 and measurements are made by using a software package called "DigiPan" (available free on the WEB - see links below).
DigiPan is primarily intended as a PSK-31 Terminal Program but has another interesting application - measuring carriers (actually the beat notes produced) with a high degree of accuracy. Perfect for IDing TV carriers!
DigiPan features a user-friendly “waterfall” display, uses Digital Signal Processing, and gives a very accurate reading of the audio frequency to 0.1Hz. It displays carriers that one can barely hear!
Typically one would tune the receiver - must be in USB mode - to 48.238.000, 48.248.000, 48.258.000, 49.748.000 etc.
(This keeps the math easy)
Looking left at the screen-shot of DDQ-0 (46.172MHz nominal), the main carrier is the yellow & red line.
A stronger signal is represented as a red trace.
This can be fascinating to watch, particularly with multiple carriers around 49.750MHz!
A receiver that can tune 1Hz steps is best.
Screen-shot of DDQ-0 on 46.172MHz (nominal)
Park the diamond shaped cursor on the strongest trace.  Click on the carrier trace with the mouse and you can determine the audio frequency.
diamond cursor
PAL or NTSC?
Sidebands from TV video can be seen as long as the RX is not driven into compression. It is actually easy to see, because the fundamental carrier is shown in Yellow (or Red if strong enough) and weaker sidebands are in blue.
To determine whether you are receiving a 50Hz PAL/SECAM signal or a 60Hz NTSC signal, ie; measure the relative frequency difference by clicking on each sideband and by using simple math you can determine whether the difference is 50Hz or 60Hz.
You may be able to see even weak carriers from other stations in the “waterfall” due to their non-relationship to the symmetrical sidebands each-side of the main carrier.
50Hz sidebands from this TV carrier can clearly be seen
Audio Levels
Set the audio level into the sound card so that some background noise is seen, but do not overdrive the audio level (use the Windows audio mixer settings) and use the “line input” to the sound card, NOT the “mic” input.
Screen colours and audio threshold levels can be adjusted to suit your own preferences.
It is also interesting to play with your receiver controls such as filter selection, PBT and IF Shift controls whilst watching what is actually happening to the audio passband.
How does this all add up?
Say the VFO was set to 49.749.000MHz USB and DigiPan was reporting the beat note to be 996.1Hz.
The carrier frequency is 49.749.000MHz + 996.1Hz = 49.749.996MHz. (round the last digit up or down)
See why we tune the VFO to a nice even xx.xxx.000 !
Note that the receiver must be in USB mode for the math to work.
Audio frequency is 996.1Hz
Calibration - the key to success.
As with any measuring system you have to calibrate against a known good source before you can make any meaningful measurements.
Check the calibration of your receiver first - I use WWVH at 15MHz - and, if necessary, apply any offsets to your calculation.
Once calibrated you can then make your measurement.
If your receiver has a High Stability Xtal option then I thoroughly recommended you fit one to your rig, otherwise you will need to regularly check calibration to monitor (and correct for) the drift of your receiver.
IC-756 Calibration
The Icom IC-756 has a tweak hole on the right hand side where you can adjust the 30MHz Master Oscillator.
756 Master Oscillator tweak hole
Although fiddley you can get it to better than 1Hz.
Tune WWVH to 14.999.000MHz USB and tweak for exactly a 1,000Hz audio beat note trace.
Error Compensation:
Apportion any error to the frequency being measured, eg:
For R1; 49.750 / Cal Freq (MHz) x error Hz at Cal Freq.
Put simply, if your WWVH reading at 15.0MHz was, say, 2Hz low at 998Hz then a reading at 45.0MHz would be 6Hz low, ie;
45 / 15 = 3 then multiplied by the error of 2Hz which equals 6Hz.
Of course, in this example this error would be slightly higher if measuring a 49.750MHz carrier.
Give it a go!
If more keen 6M operators used software such as this we could triangulate signals and document the findings and build up a very accurate and usable database of early warning TV signals. The old early warning lists with 100Hz resolution are simply inadequate this day and age.
If you can accurately ID a carrier near you can and I'll add it to the master list kept here.
Download the Television Frequencies (MS Excel xls file 54kB)
DigiPan related Links
Anthony (Tony) Mann's TVDX page: http://www.physics.uwa.edu.au/~agm/tvdx.html
UKW/TV-Arbeitskreis der AGDX e.V. TV List: http://www.ukwtv.de/uk/publikationen/tvlist/tvlist.html
TV DX WebList. For more information about this list, check the FM&TV DX Plaza at: http://www.fmdx.com
Acknowledgement: Thanks to Ian Roberts, ZS6BTE, for alerting me to DigiPan.