In article ,
"Brenda Ann" wrote:

"dead of night" wrote in message
oups.com...
Hi. I recently posted that I heard an interesting station at 2305.
Many replied and told me it was an image of WCCR at 3215. I checked
and indeed, the 2305 broadcast was much clearer at 3215. What does it
mean when you get an image? How do you know if it's an image or the
actual station broadcasting at its frequency? Does getting an image
indicate that you have a lesser quality radio? I have a 7600GR.


An image, in most cases, is a signal being received on the wrong side of the
local oscillator frequency, in reference to the IF frequency. In this case,
you were receiving the signal on the low side of the local oscillator, when
it is meant to be received on the high side.

Basic heterodyne explanation:

This is the proper math.

Local Oscillator= 3670 KHz
Incoming signal = 3215 KHz
IF Frequency= 455 KHz


Your radio was doing it like this:

Incoming signal= 3215 KHz
Local Oscillator= 2760 KHz
IF Frequency= 455 KHz

I know my explanation is far from clear.. perhaps someone can put my math
into plain English for you.


Lets say the IF is 455 KHz and the LO is 455 KHz higher than the station
to be received so then if the radio is tuned to 2305:
station 2305
LO 2760
Difference 455
sum 5065

When two frequencies mix the 2nd image products are the difference and
the sum. In radio circuits only one product is used and the other
ignored. Here we use the 455 KHz and ignore the 5065.

but this also happens at the same time
station 3215
LO 2760
Difference 455
Sum 5975

If the local oscillator is at 2760 when the radio is tuned to 2305 the
difference of 3215 and the LO of 2760 is also 455 KHz so that product
will pass through the IF stage.

Not that this will help a lot but it looks like 2305 is also a 3rd image
product of the local oscillator and station frequency assuming an IF of
455 KHz.

Station 3215
Local osc 2760

Math
(2 x F1) - F2
(2*2760)-3215 = 2305

I don't know the radios design but if the 2nd image product is
effectively blocked by say band pass filters then maybe the 3rd image
product is being received.

The way this third image product could happen is that the local
oscillator leaks out the antenna port or connection mixes, with the 3215
station and the 3rd image product can go right back through the radio
input.

--
Telamon
Ventura, California