In a message dated 05/10/2008 16:53:33 GMT Standard Time, "Bob Witte K0NR" [EMAIL PROTECTED] k0nr writes:
Just an update on my quest for D-STAR test equipment. The Utah VHF Society info below turned out to be extremely helpful. They point out that GMSK modulation actually has a lot in common with good ol' analog FM (more than I expected anyway). The short story is that I hosed the audio monitor output of my HP 8920A RF Comms Test Box (acting as FM receiver) into the sound card on my PC and recorded 25 seconds worth of D-STAR signal. Then I played this back into the modulation port of the 8920's signal generator with FM turned on, to create a d-star signal. My IC-91AD decodes the resulting D-STAR signal and I can control the signal level down to the sub-microvolt level. As expected, the IC-91AD loses the signal somewhere around a few tenths of a microvolt. The only tricky part was getting the audio signal levels right, including disabling the AGC in the 8920. Hi Bob, To break down this down I understand you are taking the raw GMSK output in audio form from the HP8920A RF comms monitor as decoded by the FM detector when presented with an external D-Star signal and recording this with your PC. When needed to use the same comms monitor as a GMSK signal generator are using this recorded audio as a GMSK FM modulation signal. Had wondered myself how this could be done and your explanation sheds light on a relatively simple way this could be achieved with existing analog radio test equipment. Very familiar with the HP8920A as I carried one in my comms maintenance truck for a number of years whilst working in the Middle East. Retired now back in the UK and unable to afford anything like this, but will give a try with the Marconi 2955 RF comms monitor I have to see if the same use can be realized. Just starting the assembly at present of the Funk Amateur version of Satoshi, 7M3TJZ's D-Star DV adapter and would be handy to have test equipment to see how the system performs when complete. Having worked on SDH digital microwave the important check with that was when the BER dropped to 1% threshold and guess the same would apply to the GMSK (FM) modulation used in a D-Star radio. Have noted there has been a suggestion that there may be a pin on some of the D-Star modules where the BER may be checked, though may have been only for the later UT-121/123. Does the earlier Icom UT-118 which I will be using have this facility? To a degree the normal analog radio SINAD test should correlate to show the overall sensitivity of the radio using the normal FM mode. As a side issue we are presently shifting over to terrestrial digital TV in the UK and many other places in the world. For this purpose relatively cheap antenna test sets are starting to appear on the market which measure BER and signal quality for the COFDM digital modulation system used. Would be interesting if these could possibly be adapted as an external measuring unit similar to the Sinadder unit was for analog radio. Not seen any information as yet on these in how they work to be any the wiser. Perhaps some of the technicians working on digital Cable TV in the USA may have some answers on the subject, though understand that uses yet another system of digital modulation. In earlier days the only test we had in the field for digital signal quality was to measure the "eye" height of the VITS (Vision Inserted Test Signal) modulation provided by the studio/broadcaster with an o'scope to check for problems at the receive end. Had to do this on cable systems where drop out of the digital teletext signal was a problem due to group delay and similar effects. Regards, Bob, G3VVT (MSCTE) [Non-text portions of this message have been removed]
