Michael Black wrote:
ken scharf ) writes:
Uncle Peter wrote:
"ken scharf" wrote in message
. ..
Well a friend of mine once wanted to build a mobile transmitter for
455khz AM and 10.7mhz FM so he could tell the bozo in the car ahead of him
where to go after he cut him off.....
I hope you clued him in.. (re. 262 kHz...)

Pete


I'm not too sure, but I think that today's car radios DO use 455khz.
I think they stopped using 262khz some years back when Detroit started
having the radios made in China and got rid of the RF stages.

Actually, most of them use 450KHz nowadays.

The synthesizer ICs are designed for 450KHz, presumably because of
some fact that makes it easier to design for that IF, such as it
allows an existing common crystal or something.

I can't think of any car radio I've opened up that had digital tuning
and not a 450KHz IF.

It's not so absolute in other types of radios, but it is still a common
IF (which can be a real bother if you've got a shortwave receiver
and thus can't improve selectivity with a good and relatively cheap
used 455KHz mechanical filter).

262KHz IFs in car radios did pretty much disappear with the coming of ceramic
filters. And no car radio is being made without ceramic filters. Though,
I think I did see one car radio, it must have had analog tuning, that did
have a 262KHz ceramic filter.

Michael

The choice of an IF frequency makes an interesting bit of radio history.
The first BC superhets used IF's from 90khz to several hundred khz.
Eventually 135khz, 150khz, and 175khz became "standard" (I remember
seeing replacement IF cans with these frequency numbers in J W Miller's
catalog.) These sets had an RF stage so were not bothered much by
images with such a low IF. Later during the depression when radio
manufacturers had to cut costs the RF stage went away (along with the
power transformers) and the IF frequency was raised to 455 (+/- a few).

Car radios needed more sensitivity and selectivity to handle fringe area
reception so they retained RF stages and used a 262khz IF.