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Channel Guard Filters

J

JustinDePolis2

Guest
They're ok, I mean, they work. The idea behind using a crystal for a filter rather than alot of the low quality filters that are installed in the radio is an excellent idea. However, they don't work for their intended purposes, (bleedover) because the front end still has way too much gain.


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Hello Justin:



I modified a old Cobra 148 GTL, with the Lou Franklin Channel Gaurd IF Crystal Filter about 10 years ago and did notice some improvement in the receiver being narrower. And I think I saw less bleed over.



But I must admit I didn't sweep the reciver or make before and after formal measurements. Maybe it was me wanting a better receiver, after all that close in soldering.



Anyway so what is your advice on all this? From reading these posts here I take it you should have the receiver modified to a better Front end circuit, and have the channel gaurd installed?!?!?!?!?



I remember Bill Good saying something about using a Kenwood Crystal IF Filter for a radio modification, I think?!?!?!



What about a better first Mixer, is it possible to mod the receiver with a better Mixer like a Double Balanaced Mixer.or a upgrade first mixer, to reduce unwanted mixer output signals?!?!?!?!?!/



I am curious about all this. I am sure the mass quantities of radios manufactured are not up to par with todays technology, and neat-o-keen-o new stuff that is avialble.



Is it possible that a mod can cure all this?



Jay in the Mojave



www.a1antenans.com <img src=http://users.joplin.com/dutch64804/posticon44.gif ALT=":44">


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Highlander, I really think it's because alot of people don't know any better. The first problem is poor band-pass filtering. It all starts there. Next is the bipolar transistor in the front end which has a noise floor in itself . The gain is also so high here that the "front - end" gets saturated so easily, and it's just a mess from there, alot of unwanted products get fed right down the stages. Noisy situation. In my opinion, you want amplification AFTER the filtering, not before. I like a gasfet for the front end, or a mosfet. Also mosfets are great for mixers, and I.F. stage amps. They have a lower noise floor, and generate less internal "swiching" noise. Keep in mind, that even when a bipolar transistor is forward biased, it's a switch to a point.


Click Here To Go To FG Best Communications



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Man, I don't think you'll want a channel guard if Justin puts a GaAsFET front end on that LT!!


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Hello Moleculo CDX363:



Why would you NOT want a better filter in the IF section of the receiver. Em I missing something here?



If you had a better front end with a lower noise figure, a more selecitive receiver would be a help, especially on a CB radio.



I am now thinking that a better 27 MHz bandpass Filter, and lower noise RF Amplifier, maybe a better mixer??!?!? and a crystal IF filter would be a significant improvement.



One of the neat things to do was use a 10 or 20 dB attenuator in the receiver to allow for a quieter receiver in noisy condations. Especially when your trying to hear a station on a noisy and crowded frequncy. The Attenuator would decrease all incoming signals by -20dB, this placed the unwanted adjecent received signals to be place where the IF Crystal Filter could be more effective, and reducing all the incoming signals also all the noises and unwanted signals, which helped you hear the wanted on frequency signal, if it had enough signal strength.



The -20dB Attenuator on my old FT902DM was a life saver at times, as this allowed interfering stations signals to be attenuated even more when there where at a adjacent frequencys, or on the other sidband. I have seen radios modified that had a 0 to 30 dB step attenuator added to them.



The -20dB Attenuator worked really great with the beam antenna, but thats another story.



Jay in the Mojave



www.a1antennas.com


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=jayinthemojave>Jay in the Mojave</A> at: 11/25/04 1:33 pm
 
Jay, that sounds like a pretty good trick you made up there. I guess on my previous post, I should have said it different. On my Grant LT, the IF gain isn't cranked up so much like on the exports. That plus the GaAsFET front end, and the thing is a great little receiver. But if you were able to modify it for even better bandpass filtering at the IF stage, that would be even better! I guess that's why comanies like Icom make such better receivers in their radios. They take the time to engineer this stuff so it just works right out of the box...


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Jay, valid point, although in my opnion, the AMIN problem with the receivers isn't the width of the filtering, although while it is a problem, a lot of the problems that I've found come from the issues that I have previously stated. I also don't like the channel guard filters because once again, they have an "amplifier stage" right on the board, using 2SC1674's I believe.


Click Here To Go To FG Best Communications



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Hello Justin:



Ok good deal, glad to hear your thoughts on all this.



Speaking of receiver noise and such. I have seen Molculo's (highly modified) Jeep modified both mechinically and radio wise. I was impressed with his noise reduction modifications that he had installed trying to reduce both Alternator and Ignition noise being generated under the hood.



Interference or RFI (Radio Frequency Interference) need 3 things to be heard. 1 A generator. 2 A Path. 3 A receiver.



Trying to attenuate the noise source is the right place to start in my opinion. Mole has bypass capacitors and inline snap on ferrit cores installed that has reduced the nois levels somewhat.



Is there a modification that can be added into the receiver that would reject the audio frequency spectrum of generated noises?

To help reduce the noise in the reciver?



Jay in the Mojave <img src=http://users.joplin.com/dutch64804/posticon44.gif ALT=":44">


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Once again, good ideas, it would help with the issues like ignition noise and such, although the actual desensitization of the receiver from strong adjacent signals, of course would remain. Also, I have never had the pleasure of meeting you, whenever you get a chance, please by all mean come by the shop. I would like to meet the person behind all of the I-10K antennas, that I've heard so much about.



Take care,

Justin


Click Here To Go To FG Best Communications



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Giant is making the Cobra's now. Still plenty of new Maxon built radios still around though.




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