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dual conversion vs. single conversion ????

B

BOOTY MONSTER

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looking at radios using the 8719 i see there are dual conversion and single conversion models . it seem dual conversion is better , but how much better ? more importantly , to me , whats the difference ? i know it has to do with the rx end and i think it relates to the ability to reject noise without sacrifacing being able to hear very weak/far away stations . when looking for an older radio is it worth the extra effort/cost to hold out for a dual over single radio ? also , are the lincolns/2510 radios dual conversion ?

cold someone give me the correct scoop on dual conversion ?

thanks .

http://books.google.com/books?id=Uv...=X&oi=book_result&ct=result&resnum=4#PPA63,M1
http://www.cbdomain.com/pll/mb87xx.htm
 
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Her's what you are looking for - I think...
http://www.bergent.net/SC-DC.pdf
Even though this is dealing with RC equipment, the principle is the same. Adjacent channel rejection and a cleaner recieve is the point. What I cannot understand, is why the Cobra 2000 has it and the Cobra 148GTL doesn't. It is supposed to be the same radio chassis as the 148 that is in the 2000.
Go figure...
 
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weasle .. LOL ;)

rob i saw the rc stuff when i googled for info . weather the RC thread applied to a modulated signal on am/ssb @ 11 meters i had no idea . but i wanted to ask to be sure . i think RC equipment is in the 11 meter area though as i think recall a post about 10k switches being on their channel and (lol) txing near the RC vehicle will "affect" it .
 
Her's what you are looking for - I think...
http://www.bergent.net/SC-DC.pdf
Even though this is dealing with RC equipment, the principle is the same. Adjacent channel rejection and a cleaner recieve is the point. What I cannot understand, is why the Cobra 2000 has it and the Cobra 148GTL doesn't. It is supposed to be the same radio chassis as the 148 that is in the 2000.
Go figure...

Don't know about the current 148s, but the Uniden manufactured 148 and the Grant XL/LT were definitely dual conversion.
 
also , are the lincolns/2510 radios dual conversion ?


thanks .

The CB Pll Data Book - Google Book Search
http://www.cbdomain.com/pll/mb87xx.htm

The Lincoln / 2510 chassis is single conversion, much the same as the Cobra 146. Also there is a addition for the single conversion chassis that allows the receiver to become more selective and give it a bit more gain, it is called the
Bleed over kit, here is a link to the page.....

Channel Guard Bleedover Filter for CB Radios

This works very well even in the 2510! (y)
mechanic
 
From "Understanding and Repairing CB Radios For the Professional Technician" by Lou Franklin:

(begin quote)

Single-Conversion VS. Dual-Conversion

Figure 4-2 shows block diagrams of the two basic CB receiver types: the single-conversion and double- or dual-conversion superheterodyne. ("Heterodyne" is another term for mixing.) The "superhet" has been around virtually unchanged since the early 1930s, which tells you something about its usefulness. Its most important advantage is that the bulk of the gain and selectivity takes placein a single narrow-bandwidth IF (or two, in the case of the dual-conversion) regardless of the actual input frequency band. This allows simpler and cheaper circuits, since gain blocks are most effective when designed for a single frequency.

Techs have argued for years about the relative merits of each, but in my opinion there's only one real determining factor: cost. The dual-conversion circuit is more complicated, requires more parts, and is therefore more expensive. The main advantage of single-conversion is that fewer stages are needed to produce a given signal-to-noise ratio, since each stage adds more noise along the way. The reason AM/SSB single-conversion types are found at all in today's crowded bands is because improved manufacturing techniques allow more selective crystal IF filters to be used. But you get what you pay for and to provie it, compare the receiver performance of chassis like the Cobra 140/142GTL or 146GTL/PC244 (single-conversion) to the Cobra 148/2000GTL (dual-conversion) chassis on AM.

As shown, the dual-conversion circuit requres a second Local Oscillator (L.O.) and a second mixer stage. The IF frequencies are usually 10.695 MHz and 455 KHz, univeral standards for which parts are readily available. Some popular chassis use a 7.8 MHz or 11.275 MHz IF in a single-conversion circuit, or as the high IF of a dual-conversion circuit with 455 KHz as the 2nd AM IF. There are a few unusual CB IFs, like the 4.3 MHz used in some Johnsons. The dual-conversion type has superior image rejection.

Incidentally, the reason for using the same IF path on AM and SSB in most single-conversion receivers is because only one of the two AM sidebands will be detected. Each sideband has exactly the same voice intelligence. Since SSB uses a very narrow IF filter, one AM sideband of up to 2.5 KHz width can pass through it, but not both. Another "economy" move... (end quote)

360-1239814355-47ab9bfd865245d7775d181994b54d8d.jpg
 
821/Mechinic your both right , Freecell and Mole basically did the math , So to answer the question which one is better , still presents itself. Although I personally do not always understand the math ? but hear tell single conversion will work just as well as dual.

quote .........Techs have argued for years about the relative merits of each. sounds like the single could and would just be fine and dandy. I suppose some just like to keep it simple. :)
 
In the context of “CB” radios (read cheap radio gear),

I seem to like the dual conversion receivers better.

But the RF seems to be a bit more exposed in the single conversion receiver, and

I may imagine that I can hear things that get drowned out with dual conversion receivers.

It is easy to quantify things like Image Rejection, Selectivity, Spurious Suppression,

But can you qualify what that really means to the listener?

Also, does anyone have a comprehensive list of “CB” radios listing their receiver type?
 
Only one reason to go with dual (or more) conversion: image rejection.

Is image reception really a problem with 11 meter operation?
Its only a problem if the image lands on a frequency range which is in use with strong signals, and its in the bandwidth of the RF front end.

Adding another mixer + L.O. adds degradation. It reduces the dynamic range of the receiver because every mixer has a 3rd order intercept point, and if its crappy then you've really made the receiver a piece of shit. The other thing is the L.O.: if its generated by a PLL then you have phase noise injected. Any noise on a local oscillator will end up on the output of the mixer. With PLLs there is a compromise between lock time (changing frequency so fast) and the noise it generates, its a loop-filter thing.

With multiple conversion receivers, they probably need to add another I.F. amplifier stage, which adds is own noise and its own shitty IP3 which limits receiver dynamic range.

With multiple conversion receivers, the phase noise of each L.O. adds noise onto the received signal. Ever listened to a radio with a full-quieting unmodulated signal, and you still hear noise? thats the L.O. noise you're hearing.

Its certainly possible to have a quality multiple conversion receiver, but it takes quality components and adds cost.
 
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