IF Freqs: Why particular freqs are used

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N9JIG

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In re-reading a bunch of old threads about various scanners I got to thinking (dangerous I know).

The question is why are specific frequencies chosen for IF's? Why 455 KHz. vs. 450? 10.7 vs. 10.8 or 10.85?
 

Hatchett

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~455Kc and ~10.7Mc are commonly used as IF frequencies because at those frequencies, a resonant circuit of a modest Q value will produce a commonly used selectivity.

Those frequencies pretty much became standardized with the advent of AM and FM broadcast radios.

455Kc produces a selectivity of about +/-10Kc and is used in AM and narrow band FM receivers.

10.7Mc produces a selectivity of about +/-200Kc is used in wide band FM broadcast radios.

Scanners usually use a dual conversion system with both 10.7Mc and 455Kc. 10.7Mc is used for the first IF, so that the input frequency is far enough away from the LO frequency to reduce image frequency problems. Then that 10.7 Mc signal is converted to the 455Kc second IF frequency for final detection to produced the desired selectivity.
 

Ubbe

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You missunderstood Rich question. Why did some scanners use 10.8 or 10.85MHz and 450KHz instead of the more common avavailable 10.7MHz and 455KHz filters?

/Ubbe
 

jonwienke

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Changing the IF frequency changes the location of birdie frequencies. Some scanners have a way of changing the IF frequency on specific frequencies so you can avoid birdies if they happen to be on a frequency you need to monitor.
 

Ubbe

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What you are saying are that when a scanner manufacture in the old days run their prototype they discovered a birdie and to solve the problem they slapped in an odd ball 10.8MHz IF filter in their final production line?

/Ubbe
 

jonwienke

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No.

Changing IF frequency changes the location of birdies. If you have a selection of IF frequencies to choose from, you can move a birdie away from a frequency you want to hear. For example, if you have a birdie at 460.000 MHz and want to monitor that frequency, changing the IF frequency from 10.80MHz to 10.85MHz would move the birdie to 459.950MHz. That allows 460.000 to be monitored without interference.

The x36 scanners have this feature, it is under the Miscellaneous tab of the Profile Editor, the button labeled Intermediate Frequency Exchange (IFX). You can list frequencies that have birdies using the default IF frequency, and the alternate IF frequency will be used for those frequencies.
 

wa1nic

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Am AM it puts the LO on a frequency that is not a real AM channel... it won't interfere with a legitimate channel on your neighbors radio.

It also puts the IF lower than the bottom of the broadcast band by enough to make a reasonably linear local oscillator tuning chart. Going too low makes it hard to get an adequate passband response.
 
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N9JIG

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No.

Changing IF frequency changes the location of birdies. If you have a selection of IF frequencies to choose from, you can move a birdie away from a frequency you want to hear. For example, if you have a birdie at 460.000 MHz and want to monitor that frequency, changing the IF frequency from 10.80MHz to 10.85MHz would move the birdie to 459.950MHz. That allows 460.000 to be monitored without interference.

The x36 scanners have this feature, it is under the Miscellaneous tab of the Profile Editor, the button labeled Intermediate Frequency Exchange (IFX). You can list frequencies that have birdies using the default IF frequency, and the alternate IF frequency will be used for those frequencies.
The question however is why they selected 10.7 etc. in the first place. Would 10.5 have worked, or 10.9 etc. The IF switch feature is a great tool but I don't know why specific freqs were chosen in the first place.

WA1NIC explains well the reason for 455 KHz. so I assume there was some similar thought behind the 10.7 (Regency) and 10.8 (Electra) choices made for scanners back in the day.
 

nd5y

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The question however is why they selected 10.7 etc. in the first place. Would 10.5 have worked, or 10.9 etc. The IF switch feature is a great tool but I don't know why specific freqs were chosen in the first place.

WA1NIC explains well the reason for 455 KHz. so I assume there was some similar thought behind the 10.7 (Regency) and 10.8 (Electra) choices made for scanners back in the day.
Crystals for one manufacturer's scanners will not be on the same frequency in another manufacturer's scanners that used a different IF. I don't know if that was intentional.
 

jonwienke

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Probably. Intentional incompatibility has always been a side effect of competition.
 

wtp

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my version

i heard in high school that the frequencies were left unoccupied so that they could be used for just this reason.
 

Ubbe

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i heard in high school that the frequencies were left unoccupied so that they could be used for just this reason.
I've heard/learnt the same thing but for 10.7MHz and I guess that's a reserved frequency for europe and 10.8MHz are for US? So when Uniden makes a european model they change the IF from 10.8MHz to 10.7MHz and that's the major change between EU and US models? Nowadays the scanners are so good RF screened that it's seems unecessary to do that.

/Ubbe
 

N9JIG

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Crystals for one manufacturer's scanners will not be on the same frequency in another manufacturer's scanners that used a different IF. I don't know if that was intentional.
Back in the crystal scanner days we used to chart what freqs Regency (10.7) crystals would result in a Bearcat scanner and vice-versa. Usually it resulted in something 5 KHz off but that was close enough in those days. When you have a lot of crystals but they were expensive ($5 then, like $30 or so in todays dollars) that meant you could hear a different channel if the freqs lined up.

I used to keep a list of all my crystals and the freqs they would work on in the various scanners. Sometimes they worked on local channels or for a channel used where I was traveling to. I put a dab of red nail polish on Regency crystals and blue for Bearcat ones.
 

N9JIG

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It was a group of engineers who decided that 455 Kc and 10.8 Mc should be the freqs used for the IF circuit. More or less a "gentleman's agreement". This was the reason given to me many years ago. True or not, I'm not sure.
Thats as good an explanation as I ever heard, makes more sense than anything else.

I suppose it developed into an unwritten standard as manufactures could order crystals off the shelf instead of having them made special.
 
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