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455Khz. Ceramic filters & bleedover in Exports...

The CFW455HT is a 6Khz. wide filter and ideal for AM so the problem may not be as widespread as I first thought. The 2950 must use some simple LC filtering in the FM IF strip because I don't think there is a wider FM filter in the radio and that one won't pass FM.

Would a 3 or 3.5 be ok for am? To eliminate noise from out side sources.
 
Would a 3 or 3.5 be ok for am? To eliminate noise from out side sources.
Once you start going below 5 or 6 Khz. the filter cuts off too much treble in the receiver audio and everyone starts sounding muffled on AM. Better results are obtained with higher quality filters, having more poles and sharper bandwidth skirts.

I'm more familiar with the old school Murata filters in a metal case for 455 Khz. and seen good results using them in many receivers. These had 9 poles and many different bandwidths covering everything from SSB to FM. The one Kop has posted is interesting because it looks like it has a pair of tuned matching transformers.

That gives some flexibility in bandwidth adjustment if you can sacrifice a bit of gain. The input could be tuned a little higher in frequency while the output could be tuned a little lower, adjusting the overall bandwidth. I suspect it may work well when just tuned normally. The addition of tuned circuits alone, sharpens the bandwidth.
 
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Once you start going below 5 or 6 Khz. the filter cuts off too much treble in the receiver audio and everyone starts sounding muffled on AM. Better results are obtained with higher quality filters, having more poles and sharper bandwidth skirts.

I'm more familiar with the old school Murata filters in a metal case for 455 Khz. and seen good results using them in many receivers. These had 9 poles and many different bandwidths covering everything from SSB to FM. The one Kop has posted is interesting because it looks like it has a pair of tuned matching transformers.

That gives some flexibility in bandwidth adjustment if you can sacrifice a bit of gain. The input could be tuned a little higher in frequency while the output could be tuned a little lower, adjusting the overall bandwidth. I suspect it may work well when just tuned normally. The addition of tuned circuits alone, sharpens the bandwidth.

I was under the impression that the H are advertised as 3khz ?
 
The same letter in different types or series of filters does not always indicate the same bandwidth. To be sure, we need to check the data sheet for the part in question. We also have to consider that while the CFW455HT is a 6Khz. filter for AM while it's only + or - 3Khz. from the center frequency and that's all the usable frequency response it can provide on AM.
 
From what folks were saying they were showing more like 8khz when swept. I just see the lescomm is doing narrower 10.7 and I think 455mhz as well to cut back on the hash and trash noise that's my goal to improve the rx on the ranger export I have.
 
Before you change AM filters to try and reduce noise, is the noise acceptable on SSB? If the signal to noise ratio has not noticeably improved on SSB with half of the receiver bandwidth, than we should not expect narrowing the AM filter to improve the results there either.

On the other hand, if most of the noise is high frequency content, narrowing the receiver filter removes some of that hash and you may find a better trade off between fidelity and signal to noise ratio, using a narrower AM filter.
 
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Before you change AM filters to try and reduce noise, is the noise acceptable on SSB? If the signal to noise ratio has not noticeably improved on SSB with half of the receiver bandwidth, than we should not expect narrowing the AM filter to improve the results there either.

On the other hand, if most of the noise is high frequency content, narrowing the receiver filter removes some of that hash and you may find a better trade off between fidelity and signal to noise ratio, using a narrower AM filter.

It's that Rci63ffc2 no ssb just am/fm cool loud radio for am which is pretty much where most if the locals stay here all 12 or so of them. Like I say was think the the 455i would be the way to go. What about narrowing the 7.8 how narrow can it go before degradation ?
 
It's that Rci63ffc2 no ssb just am/fm cool loud radio for am which is pretty much where most if the locals stay here all 12 or so of them. Like I say was think the the 455i would be the way to go. What about narrowing the 7.8 how narrow can it go before degradation ?
I can't find a schematic for that radio or even an internal picture and I'm a bit surprised that it would be using a 7.8 Mhz. filter without SSB. These filters are the crystal type rather than the 455 Khz. ceramic and have much sharper bandwidth skirts. It would be unusual for a CB or 10 meter rig to be using both IF filters simultaneously on AM. Although, many HF rigs do exactly that to provide the best performance. If this radio did, it should also have very good selectivity as a result.
 
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I just wanted to show there were options. I do not however think that the 10.7~455 is all of the problem.

http://ps-2.kev009.com/jshorney/2510.htm Thanks to Jim Shorney we have this observation of the HR2510 filters compared to the HTX-100.
I reasoned that the 2510, being an 'all-mode' radio with AM included, may use the common CB radio design practice of having a wider 'compromise' IF crystal filter to improve the RX audio response on AM. The center frequency of both filters is 10.695 MHz, so assuming that the carrier oscillator signal is placed on the slope of the filter, a check of the carrier oscillator alignment frequency for USB should tell us the approximate bandwidth of the filter. Sure enough, the USB carrier oscillator frequency is different in the two radios! Shown below is a chart of the carrier oscillator frequencies for both rigs, with a LSB value extrapolated for the HTX100.
uniden.jpg
So far it seems we're all on the same page.
If we assume that the USB and LSB carrier frequencies represent the edges of filter, we can see that the 2510 has a 5 KHz wide filter, while the HTX has a 3 KHz filter! This is enough to make a noticeable difference in the performance of the receiver.
Okay, still with us but not the whole story.

The difference between my communications receivers and a "CB-Export Radio" is what is allowed into the radio in the first place. We'll exclude the high IF modern radios and look at only 34-38MHz 10.7MHz mixes with a nod to the 7.8MHz mixes.

Let's get the brain strain out of the way first.
http://www.rfcafe.com/references/electrical/image-frequency.htm
image-frequency_small.gif
as well as
http://www.arcticpeak.com/radiopages/imagefrequencies.htm
It does the math for you (y)
and on to mixer spurs for the next part of the problem.
https://leleivre.com/rf_mixerspur.html

Now that we have an idea what can and does leak into our receiver via image and spur we need to examine just how much more of the radio spectrum we are listening to that is absolutely unnecessary.

Generally a CB radio listens to just .45MHz plus the skirts for a little over .6MHz at the 6db down points. Add the six or more "bands" added to an "Export" radio and we begin to have problems.

So it becomes a question of ratios and percentages, Listen through 600KHz of HF noise to get to your desired 450KHz bandwidth cut down to 6KHz at the 10.7 MHz (or 7.8MHz) filter and further at the 455KHz filter...

or

Expect a 6 "band" export radio to sample near 3MHZ of HF noise to get to your desired 450KHz bandwidth cut down to 6KHz at the 10.7 MHz (or 7.8MHz) filter and further at the 455KHz filter.

Do you think modern radios could benefit from a little band pass filtering on the front end ? Simply the bigger sample you take to get what you want the more crap you're going to get!

This isn't a how to or even a complete definition of the problem I have more questions than answers and only a few common sense practices to create a livable signal to noise ratio.

Comments ?
 
Since a bandpass filters only rejects out of band signals, there would have to be an out of band signal strong enough to be causing problems in the receiver like desensing, before adding one would help noticeably. CB radios and exports already have a fixed tuned circuit in the front end, tuned for the center of the band. HF rigs have one for each band.

The best receivers adjust the resonance of the tuned filter circuits before the front end transistor, as the frequency of the receiver is changed. This allows the filter to track with the receiver frequency and the bandwidth of the filter can be much tighter than one passing the entire band. Then the front end can also reject strong, adjacent in band signals.
 
"If we assume that the USB and LSB carrier frequencies represent the edges of filter, we can see that the 2510 has a 5 KHz wide filter, while the HTX has a 3 KHz filter! This is enough to make a noticeable difference in the performance of the receiver."

That's exactly what they did with the Cobra 142 GTL in order to get away with one 5 KHz filter in the single conversion design. It provides poor sideband rejection and lousy high frequency response on AM. When the sideband offsets are more than 1.5 Khz away from center, you know you have a compromised crystal filter in use.
 
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I can't find a schematic for that radio or even an internal picture and I'm a bit surprised that it would be using a 7.8 Mhz. filter without SSB. These filters are the crystal type rather than the 455 Khz. ceramic and have much sharper bandwidth skirts. It would be unusual for a CB or 10 meter rig to be using both IF filters simultaneously on AM. Although, many HF rigs do exactly that to provide the best performance. If this radio did, it should also have very good selectivity as a result.[/QUOTE

Yep I definitely misspoke I did in fact mean 10.7 crystal filter I did pull one from an older cobra or something and swapped it in think better part not made in china. Didn't really notice much change. So yes I would like to narrow both the 10.7 and 455 if it will help. Also yes I can't get a schematic for it either I've called and emailed Rci with no luck its shares similarities with both the rci6300f 150 and the rci2950dx.
 

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