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Post by Night Ranger on Jan 4, 2014 16:58:51 GMT -5
Pennsylvania skip to South Carolina - Saturday Jan 04, 2014
I modded my Midland 13-853 this week to include a crystal filter in the 1st IF chain in hopes of reducing the co-channeling from channel 11 when the skip is running. I can tell a pretty big difference so far. The stock adjacent channel rejection as listed in the owners manual is 55db @ 10kHz. The stock radio has tuned IF coils at the 1st IF and a ceramic filter at the 2nd IF.
Night Ranger
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Sandbagger
Administrator/The Boss 
Posts: 6,246 
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Post by Sandbagger on Jan 4, 2014 22:08:44 GMT -5
Pennsylvania skip to South Carolina - Saturday Jan 04, 2014 I modded my Midland 13-853 this week to include a crystal filter in the 1st IF chain in hopes of reducing the co-channeling from channel 11 when the skip is running. I can tell a pretty big difference so far. The stock adjacent channel rejection as listed in the owners manual is 55db @ 10kHz. The stock radio has tuned IF coils at the 1st IF and a ceramic filter at the 2nd IF. Night Ranger Tightening up your I.F. won't help much when the people you are hearing are splattering. Their signals are so wide that they are actually on channel, so you'll never get rid of them. You might knock it back a little and it'll help with truly adjacent channel signals that are strong but not wide. The other thing to keep in mind is that the ceramic filter has insertion loss, and it will reduce your receiver sensitivity a bit unless you up the gain of the I.F. to compensate for the loss. |
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Post by Night Ranger on Jan 4, 2014 23:48:45 GMT -5
The other thing to keep in mind is that the ceramic filter has insertion loss, and it will reduce your receiver sensitivity a bit unless you up the gain of the I.F. to compensate for the loss. Yep. The insertion loss appears to be between 3 and 6 db, but there is plenty of untapped IF gain. There is an IF gain adjustment after the 455 kHz ceramic filter, so I'll just turn that up from the service manuals suggested 1.7 volts to something that brings the S meter back in line with the pre-mod level. Still it knocked one of the high power locals down from several channels of bleedover to about 1 channel. Unfortunately there was also some guy on channel 6 putting my needle past the 30 mark. I could still hear him splattering up on channel 13 despite the improved selectivity. Night Ranger |
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Sandbagger
Administrator/The Boss
Posts: 6,246
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Post by Sandbagger on Jan 5, 2014 10:24:42 GMT -5
The other thing to keep in mind is that the ceramic filter has insertion loss, and it will reduce your receiver sensitivity a bit unless you up the gain of the I.F. to compensate for the loss. Yep. The insertion loss appears to be between 3 and 6 db, but there is plenty of untapped IF gain. There is an IF gain adjustment after the 455 kHz ceramic filter, so I'll just turn that up from the service manuals suggested 1.7 volts to something that brings the S meter back in line with the pre-mod level. Still it knocked one of the high power locals down from several channels of bleedover to about 1 channel. Unfortunately there was also some guy on channel 6 putting my needle past the 30 mark. I could still hear him splattering up on channel 13 despite the improved selectivity. Night Ranger And there isn't much anyone can do on the receive end to eliminate splatter, other than just turning down the RF gain. We have a guy up in our neck of the woods that runs high power stuff (and class "C" amps unfortunately). His signal is so wide (in-band spurs?) that even his carrier leaks over into the adjacent channels when he's not talking. You can see this on a radio with a spectrum receive scope. |
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