I suspect that the stop band rejection of the front end filter is at best not very good anyway, so even when the local oscillator is at 118Mhz, it will still pass to the antenna with minimal attenuation.The reason that one FM receiver can interfere with another is because the VCO is generating a frequency that can get past the band pass filter (between 88-108MHz).
Low side injection can have its problems too.What if I make the IF frequency higher than 10.7MHz so that the VCO never has to produce a frequency higher than 88MHz and thus the leakage from the VCO never gets past the Band-Select Filter?
I dont know why 10.7Mhz either. I can only summise that in the early days 10.7Mhz was picked because off-air signals would be weak enough not to cause IF breakthrough.Is this possible or do I need an IF of 10.7MHz (I am not sure why 10.7MHz is the standard for FM receivers)?
Also, the average FM broadcast receiver has no screening whatsoever, (open circuit boards and plastic case) so there will be a lot of direct radiation from the components around the local oscillator.
Low side injection can have its problems too.
Consider an IF of 50Mhz, to receive 100MHz to local oscillator would have to run at 50Mhz.
Here there are two problems. The LO will pass straight through the mixer and swamp the IF amplifier, and the second harmonic of the LO will fall on the signal frequency.
It is generally better to go for high side injection. I just checked a portable radio which I had to hand and the LO is above signal frequency.
I dont know why 10.7Mhz either. I can only summise that in the early days 10.7Mhz was picked because off-air signals would be weak enough not to cause IF breakthrough.
10.7Mhz is common but other IF frequencies are used, 21.4Mhz is often used to improve 2nd channel rejection in UHF (and some VHF) communication radios.
I dont know if it is used in any broadcast receivers.
This however leads me to the question: why?
Have you come across a real-world problem with LO radiation?
Enquiring minds wish to know.
Have you ever asked why FM receivers are prohibited on Commercial Aircraft? Have you ever wondered how (in those countries that license operating a receiver) the radio police can track you down?
Question, what would be the reason for such a stealth receiver? What possible niche market could it fill? The world seems to have gotten along with them fine so far.
I am not trying to fill anything. I have no reason to do this besides the fact that I want to see if I can. What is up with the paranoia!?
It's very simple:
1. Someone showed me a flaw in regular radio receiver.
2. I want to see if this flaw can be fixed/circumvented.
That's it. I don't even plan on actually making this receiver, just simulating it in ADS. Is there something I am not seeing here?
I can't say I've ever seen, or heard of, such a flaw - obviously LO pickup could be a concern, but not in any sensible use of a radio.
Only that you don't appear to have little basic knowledge of radio?, you are perfectly free to choose any IF frequency you wish, or even use double (or triple) conversion - 10.7MHz was chosen for wideband FM because it has sufficient bandwidth for the signals, and sufficient image rejection. A decently designed receiver should have no problems.
If you really want to remove the possibility of LO interference, then use a VERY much higher LO, the maths is simple enough.
Try it for yourself. Grab two hand held radios and tune one to 102.3MHz and the second to 91.6MHz. You will see that the first radio (102.3MHz) goes silent when you orient the antennas the same way and bring them near each other.
I would like to believe that I do have basic knowledge of radios. What I do not have is knowledge of the more complex aspects of a radio such as why 10.7MHz and 21.4 MHz seem to be the norm.
A decently designed receiver should have no problems but the ones I have tested this with do have it. And I consider the two radios I used of medium quality (not super fancy and not super cheap).
The math is indeed simple. What I do not know is what I can get away with when trying to use commercially available components. I want this to be realistic so I am not going to choose an IF frequency that cannot be filtered by commercially available filters.
And why would you consider that a sensible way to use a radio?
But why do it?, it's a silly thing to do.
deliberately doing something pointless isn't a concern. If you smash a radio with a large hammer it will break, so don't do it - the same applies to your deliberate problem making.
Why do it:
1. To learn more about radios (as you clearly pointed out I am not an expert).
2. To see if I can since it doesn't seem like anyone else has.
Is that good enough? I am pretty sure that what I am inquiring about is very much related to electronics and our understanding of them which, if I am not mistaken, is the purpose of this forum. If this forum prohibits or frowns upon individuals seeking knowledge for the sole reason of enlightening themselves and seeing what they can or cannot accomplish with electronics then I apologize for posting this thread. Feel free to delete it -.-
You are free to do anything you wish, but to try and correct a 'problem' that doesn't exist seems pretty pointless?.
Like I said, it's like trying to design a radio that can't be smashed with a hammer - what would be the point?.
The problem does exist (under the right conditions).
But under artificial conditions, deliberately engineered to cause the problem - have you ever had this effect in any normal use of a radio?.
I am not trying to fill anything. I have no reason to do this besides the fact that I want to see if I can. What is up with the paranoia!?
It's very simple:
1. Someone showed me a flaw in regular radio receiver.
2. I want to see if this flaw can be fixed/circumvented.
That's it. I don't even plan on actually making this receiver, just simulating it in ADS. Is there something I am not seeing here?
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