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Crystal filter question

I just finished the alignment. I couldn't get USB to 7.8025 (only 7.80241), so I went with 7.8023. LSB was able to go to 7.7975. I changed the VCO to 34.9873 for USB and 34.9825 for LSB (CH 19). The peaks in the adjacent sideband are now gone.

loosecannon said:
Brandon i think the filter you chose is too wide for the 148 chassis.
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Yes it was, but it worked out nicely after changing both carriers to move the IF to the edge. Although it stays in its own sideband now, the audio bandwidth that gets through is about 200Hz to 4.4kHz. Thats a little wide. Maybe I can compensate for that with audio filtering...
 
brandon7861 said:
Markers 1 and 2 are where 200Hz into LSB would show up on USB and where 200Hz into USB would appear in LSB, respectively. For this filter to not allow audio into the opposite sideband, this is where the rolloff should be rolling off, but its not.
View attachment 66274
View attachment 66276
EDIT: turned peak table off to remove all those numbers in the trace and reuploaded pic
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You should reduce the span and look at the individual peaks and dips. Then look at the 30 dB points. I can see that the filter is bad, but the span reduced to 5 kHz will reveal how bad it is. You may look at the original filter the same way. Perhaps you can determine what is bad in that filter and perhaps repair it.
 
SuperLid said:
You should reduce the span and look at the individual peaks and dips. Then look at the 30 dB points. I can see that the filter is bad, but the span reduced to 5 kHz will reveal how bad it is. You may look at the original filter the same way. Perhaps you can determine what is bad in that filter and perhaps repair it.
Click to expand...
I think a little ripple is normal in these filters. It sure doesn't seem bad. I cannot narrow the span as it is already in the radio and I don't want to remove it again.

The filter that came out of the radio originally was definitely bad. I did a sweep on that one yesterday too.
IMG_20231111_212021866.jpg
IMG_20231111_212010279.jpg
 
tecnicoloco said:
the filter belongs to a radio like Cobra 140,Uniden Washintong,Etc

is a 5 Khz filter,

148 is narrow filter 1..5 on each sideband=3Khz
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Am I disgracing the 148 by making it a wideband radio? The airwaves are full of radios with the same bandwidth, what the face plate says doesn't really matter. The guy getting it knows it has the wider filter and doesn't mind (that's what he bought to begin with, the junk stalker), The filter may not belong to that cobra, but I don't see that it is hurting anything running it that way. Its not like I am trying to pass it off as a narrowbander.
 
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Unless you set the carrier 2.5 kHz above/below center frequency you will have excess carrier on sideband transmit. The edges of the filter's "window" serve to suppress the carrier. The balanced modulator removes most of the carrier, but the filter does the rest of the job. But only if the carrier is set far enough above/below the filter's center frequency. The wider filter won't knock down the carrier if it's only 1.5 kHz away from center.

The 148 won't care how you set it up. And you'll have learned something by the time you're done with the project.

Makes the effort worth it, most likely.

73
 
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nomadradio said:
Unless you set the carrier 2.5 kHz above/below center frequency you will have excess carrier on sideband transmit. The edges of the filter's "window" serve to suppress the carrier. The balanced modulator removes most of the carrier, but the filter does the rest of the job. But only if the carrier is set far enough above/below the filter's center frequency. The wider filter won't knock down the carrier if it's only 1.5 kHz away from center.

The 148 won't care how you set it up. And you'll have learned something by the time you're done with the project.

Makes the effort worth it, most likely.

73
Click to expand...
I did my best there without changing parts in the oscillator circuit. I was able to set the LSB carrier to the 7.7975 the filter was designed for (instead of the 7.7985 called for in the cobra manual), but I could only get to 7.8023 on USB, not 7.8025. From the last sweep I did, it looks like the carrier would be around the -20dB part of the filter response on USB. -20dB on top of the work the balanced modulator is doing should get it down there quite low.

The radio board has been removed from the mobile chassis so I cannot test this until I get it installed into the base chassis (pots hanging free, no coax connector etc). I did recall seeing a carrier when looking for spikes in the other sideband yesterday, but with the mic gain all the way down in SSB, it was down at the same height as the noise that made it through the filter.

I will check this again once I get it together, but it will take some doing because the boards were not the same size. It might take a couple days to figure out the best way of making it fit. If I have to change trim caps to get that extra 200Hz higher on the USB carrier, I can cross that bridge when I get there.

Thank you for pointing that out! I will get some screenshots of the output posted here when I get that far. Learning is the only reason I help my friend fix these broken radios he keeps buying. That's literally all I get out of it, and I'm more than happy with that until I earn my golden screwdriver certificate and have the confidence to charge something.
 
brandon7861 said:
Am I disgracing the 148 by making it a wideband radio? The airwaves are full of radios with the same bandwidth, what the face plate says doesn't really matter. The guy getting it knows it has the wider filter and doesn't mind (that's what he bought to begin with, the junk stalker), The filter may not belong to that cobra, but I don't see that it is hurting anything running it that way. Its not like I am trying to pass it off as a narrowbander.
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I don't think so. I think you saved it and if it is used for SSB most of the time, it might be an improvement. There is also a chance, that the filter you used, has a better shape factor than the stock filter. You need to focus on the 30 dB points. Some people will put in a filter like you had on hand for higher bandwidth.

Good work my man!
 
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The software-defined radio crowd are doing just this but increasing the signal's bandwidth with software. Calling it "ESSB" for extended single sideband. I guess they forgot that making the bandwidth narrow was to increase the system's signal-to-noise ratio for any given transmit power level.

But hey, it's a hack. Might sound better, even.

Might

73
 
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Just an update. I haven't forgot to come back with the results of the carrier suppression, I just haven't got that far yet. I ended up getting sick, and then not feeling well, I was staring at several issues I wasn't expecting. The board didn't fit, so I had to install the entire cobra chassis. then I was faced with wire spaghetti knowing it would look like crap if I just spliced them longer, so I have been slowly replacing wires with braided sets for each control. After that's done, I need to add a separate heat sync for the PS transistor as I don't want it by the final where it was before as the radio ran room temp and rather it stays that way. The meters will be another project because I cannot find anyone that prints business cards in fluorescent UV ink :) (I have UV LEDs waiting in the post office).

Still looks like crap, but it's getting there.
IMG_20231120_014844172.jpg
 
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nomadradio said:
Best way I know to set carrier balance is with the S-meter of another receiver on the same channel. AM works fine. The carrier null will usually produce the loudest white noise from the receiver speaker, even if the null is a bit broad on the receiver's S-meter.

73
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That sounds like a neat trick! I usually use the FFT on my scope and it does produce a reasonably sharp null, but I love learning tricks that make this stuff possible without having access to complicated equipment. I will definitely give that a try! Thanks!
 
That carrier oscillator issue has been resolved. I swapped the USB carrier crystal cap (C125 installed parallel to trimmer) from 10pF to 7pF and that allowed the oscillator to easily be adjusted to 7.8025 (no more of that 7.8023 nonsense). Now the crystal filter is getting exactly what it was made for.

But now I have a new problem (as is always the case), the PLL ref crystal is over 1kHz high and there is no trimmer, so thats the next project. Still waiting on a new pot as one is a little scratchy and won't clean up.

I need to glue the crack in the face and give it a better coat of silver paint too, but here is how the meters turned out. Purple in the back, UV on the sides to hit the needles. Camera don't pick it up well. Nothing is too bright, the new tx/rx lights are matched in brightness, gonna be a nice radio in the end if I ever find a 40 ch knob. Looking like I will need to make one.

IMG_20231209_011721069.jpg
B
 
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