Icom IC-7600 use of 200 Hz filter on RTTY
Updated 06/01/2010 by Don AA5AU

On June 1, 2010, around 0200-0330Z, E4X was on the 30 meter RTTY.  His frequency was showing exactly 10140.00 kHz on the '7600 readout.  For an antenna, I was using my Cushcraft D3W WARC band rotatable dipole at about 50'.  This dipole is directional and it was "pointing" toward Palestine, which is 45 degrees northeast from my QTH.  I was using WriteLog for Windows with the MMTTY plug-in as the decoder.  There was a very strong steady interfering signal directly on top of his mark frequency making it impossible to copy.  I did make a contact early but only because I recognized my call "by ear" and saw on "AA" on the screen in his confirmation message.  There were several stations in the New Orleans area that wanted to work him, so I started turning knobs and changing filters in the '7600 to see what could be done to copy him better.  With the interfering signal directly on his mark frequency, there was nothing I could do to improve copy.  It was at this time that I discovered the '7600 has the ability to use the notch filter in the RTTY mode with or without the Twin Peak Filter (TPF) turned on.  The PRO III does not allow this.  Another great feature for RTTY on the '7600 even though it couldn't help with this particular situation.

Communicating with the members of the Delta DX Association via our 2-meter repeater, it was apparent everyone in New Orleans was hearing the interfering signal and no one was able to copy E4X on 30 meter RTTY. Eventually the interfering signal shifted down below the mark frequency by about 40 Hz, but it was still too close for copy with a 250 Hz filter because the interfering signal was still disguising as the mark tone.  Figure A and B show the MMTTY FFT and XY Scope displays using the 250 Hz filter in the IC-7600 with no RTTY signal present.   Figure A is with TPF turned off and Figure B is with TPF turned on.  You can clearly see the interfering signal as a "spike" to the left of the pass band.  The XY scope shows the interfering signal as a mark tone to the decoder with no signal present.  Figure C and Figure D shows the displays with E4X transmitting.  Even though there is a clear pattern in the XY scope, the interfering signal masked the actually mark tone and copy was only about 10% at best.

Fig. A - 250 Hz IF filter with TPF off and no RTTY signal present

Fig. B - 250 Hz filter with TPF on and no RTTY signal present

Fig. C - 250 Hz filter with TPF off and E4X RTTY signal present

Fig. D - 250 Hz filter with TPF on and E4X RTTY signal present

I then changed the IF filter in the IC-7600 from 250 Hz to 200 Hz.  When I turned on the 200 Hz filter, I could no longer hear the interfering signal in my headphones even though it still showed on MMTTY's FFT display when E4X was not transmitting.  Figures E and F show the MMTTY FFT and XY Scope displays when E4X was not transmitting.  Figure E is without the TPF activated.  Figure F is with the TPF activated.  You can see the interfering signal still shows on the FFT display in figures E, F & G even though I couldn't hear it anymore.  Figures G and H show the displays with E4X transmitting.  Figure G is with the TPF off and Figure H is with the TPF turned on.  With the TPF turned on, the interfering signal no longer shows in the display. What you don't see is the print so you'll have to take my word for it.  With the 200 Hz filter activated and TPF turned off, print was about 80%.  With the 200 Hz filter activated and TPF turned on, print was 90-100%.  It was a remarkable thing to see.

Fig. E - 200 Hz IF filter with TPF off and no RTTY signal present

Fig. F - 200 Hz IF filter with TFP on and no RTTY signal present

Fig. G - 200 Hz filter with TPF off and E4X RTTY signal present

Fig. H - 200 Hz filter with TPF on and E4X RTTY signal present

This experience shows the tremendous power of the IC-7600 on RTTY.  E4X did have a decent signal of S2-S3 without the preamp and with low band noise, so this demonstration does not show results had the signal been very low or had the interfering signal been closer than 40 Hz.  I also had the 3 kHz roofing filter in line for these screenshots but it didn't make a difference.  I changed the roofing filter from 3 to 6 and 15 kHz but the results were the same.  I had expected this.  I tried narrowing the IF filter to 150 Hz, but MMTTY could not copy with or without TPF.  The 150 Hz filter is too narrow for the 170 Hz shift on standard amateur HF RTTY.  Although the 200 Hz filter would probably not be my preferred filter for RTTY most of the time, just having the option of using it on RTTY is a valuable tool for fighting adjacent QRM.  I will gain more experience with the IC-7600 on RTTY during the upcoming DLDC RTTY contest and plan to report these results.