You NEVER choose a 3rd order product as the desired signal. You choose
either the upper sideband or the lower sideband output of the
mixer.........the undesired sideband is commonly called the image. 3rd order
products are a different story......it is not a matter of semantics in this
case.

Pete

"dave" wrote in message
...
Pete KE9OA wrote:
I have been an RF Engineer, specializing in low level RF design since
1992. Why are we having this conversation in the first place? The reason
for my post was to clarify which mechanism was responsible for the
manifestation that the original poster was experiencing.
To glibly state that this was an image problem is clearly misinformation.
Too much of that going around these days. Mini-Circuits has a very good
tutorial on this subject on their website.
Now, what does being a professional frequency coordinator have to do with
RF engineering principles, specifically, small signal RF design? I have
no doubt that you have experience in this area, and the 900kHz frequency
that the original poster mentioned can confuse the issue if is is assumed
that the receiver has a 450kHz I.F. which it may or may not
have..........many receivers over the years used a 455kHz I.F. which
would place the primary image 910kHz below the actual frequency. A simple
analysis of this problem clearly shows that it is a 3rd order IMD
problem.
For other folks out there that may be interested, 3rd order
intermodulation products don't exclusively occur only in
mixers.......they can occur in any nonlinear system, such as oxidized
electrical connectors, saturated ferrite cores in front ends of
receivers, bandpass filter switching diodes, crystal filters, and AGC
diodes. It can also occur in rusty rain gutters, or anywhere else that
galvanic corrosion can occur.
I am sorry if you took offense at my post, but next to doing everything
to preserving life, truth is the most important thing in this
world......... thus, the reason for my reply to the original poster.

Pete

"dave" wrote in message
...
Pete KE9OA wrote:
This can also occur in multiple conversion receivers, depending on the
ultimate rejection of the roofing filters.
In this case, image rejection has nothing to do with it. What is being
experienced are 3rd order IMD products. Do a Google search on 3rd order
IMD products in mixers, and it will become very clear.............

Pete

"dave" wrote in message
...
gccradioscience wrote:
If you live in a city area or some rural area with a real strong AM
station its likely it will be found on the longwave bands from 150 to
519 kHz (1050 kHz to 1419 kHz) the 900 kHz carrier image. Please
understand that this station is not a real longwave radio station.
Its a image from the broadcast station when the station is strong
enough to bleed on other longwave frequencies. The radios that I
have is the DX-380, DX-390, DX-392 and the yes the new Grundig G5.
It has do something with the dual conversion circuitry . I am
hoping that manufactuers, hobbyists, and technicians will use the 900
kHz formula to see other image stations to show that these AM
stations
should not be on the LW bands.

Desired AM Station (kHz) - 900 kHz = Image Station

The images I am getting here locally in Virginia Beach, VA on LW is


150 kHz = 1050 kHz WVXX -AM
210 kHz = 1110 kHz WYRM-AM
330 kHz = 1230 kHz WJOI -AM
410 kHz = 1310 kHz WGH-AM
450 kHz = 1350 kHz WGPL -AM
500 kHz = 1400 kHz WPCE-AM

AM Band Images that are annoying

550 kHz = 1450 kHz
650 kHz = 1550 kHz

Adam E.






It's a single-conversion phenomenom usually. Two times the IF, + or -
the FOI.

Sir, I am a professional frequency coordinator and need no schooling on
3rd order IMD.

My trusty Philips AE-3805 (Sangean innards) has a big gap in coverage
(7300 kHz-9.500 kHz) that can largely be overcome by tuning 900 kHz
either side of the frequency of interest. 900 kHz is twice the IF. This
is the exact mathematical relationship described in the original post.
I suspect we are arguing over word usage, not the science.

A mixer simply produces a desired intermod product.

One of the few positives of a dual-conversion receiver is their ability
to keep the first image out of the downstream bandpass.


Like I said, it's semantics. The mixer produces two 3rd order products.
One you want and one you call distortion.