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FM Receiver that produces no interference to other FM receivers.

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hugoender

New Member
As most of you probably know, when you tune an FM receiver to a certain frequency and then place another FM receiver next to it tuned to another frequency (10.7MHz difference), you can interfere with the signal being received by the first FM receiver (the FM receiver that was receiving a radio station signal goes silent). This is caused by the LO leakage of the second FM receiver that gets transmitted through the antenna.

That being said, I am trying to design the architecture of an FM receiver that does not produce interference. I will then try and find some manufacturers that make the components and use their data to simulate the receiver in ADS.

I just want some general ideas on methods in which I can accomplish this.

Thanks in advance for any suggestions.
 

hugoender

New Member
I was just brainstorming a bit and I came up with this (let me know if this would not work):

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). 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?

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)?
 
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JimB

Super Moderator
Most Helpful Member
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).
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.

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.

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?
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.

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)?
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.

JimB
 

hugoender

New Member
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.
I am not worried about radiation from the circuit itself. I am only concerned about radiation from the antenna.

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.
Yeah I was also thinking about going with an LO above the RF frequency. However, this would still not allow me to increase my LO frequency to stay away from the passband because of the reason you have stated above.

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.
If I cannot increase my IF frequency then what other method(s) would you suggest for accomplishing my end goal? The interference I care about is the interference being radiated by the antenna (not the circuit itself since the two receivers would have to be VERY close together for the circuit radiation to cause interference).

Thanks for your help and very clear explanation.
 

JimB

Super Moderator
Most Helpful Member
It is creditable that you should be seeking to reduce spurious emissions from a receiver, when half of the posters on this board are seeking to vomit all over the VHF broadcast band with "FM transmitters" of various dubious designs.

This however leads me to the question: why?

Have you come across a real-world problem with LO radiation?

Enquiring minds wish to know.

JimB
 

hugoender

New Member
This however leads me to the question: why?

Have you come across a real-world problem with LO radiation?

Enquiring minds wish to know.
Well someone showed me this "phenomena" by putting two radios together and tuning each one and asked me (being that I am an electrical engineer) the question of why it happens. I was able to answer them but then that lead to me wondering how this could be prevented. So now I am on the hunt for answers and want to simulate my design to see if it truly does get rid of this interference (being that I do not have the time, equipment, nor patience to physically modify an FM receiver).

A search for this topic results in ZERO results that actually talk about a concrete way of preventing this interference from occurring. Most just spit out random equations and theory. I want to know how I would go about doing it!

EDIT: What if I have two mixers instead of one? That way I do not require such a high Q for the channel select filter and I can have good image rejection as well as good channel selection. I am not quite sure how making my architecture incorporate two mixers would help with the spurious emission but just throwing it out there in case it sparks any ideas in someone.
 
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MikeMl

Well-Known Member
Most Helpful Member
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?
 

hugoender

New Member
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?
... No. Not really. I know FM receivers have spurious emissions. What I don't know is how to go about preventing these emissions in a regular radio receiver.
 
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hugoender

New Member
Alright well looking at different manufacturers of filters, I can see that 10.7MHz and 21.4MHz are the standard and so I need to stick with those frequencies as my possible IF frequency.

That being said, with an IF frequency of 21.4MHz, I get LO frequencies of:
66.6-86.6MHz and 109.4-129.4MHz. These frequencies are out of the passband filter range but not by much so now I need to see if I can find a bandpass filter with a really sharp cutoff. If I cannot, then I am going to have to look for another way.
 
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Sceadwian

Banned
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.
 

hugoender

New Member
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?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
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.
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.

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?
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.
 
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hugoender

New Member
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.
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.


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.
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).

If you really want to remove the possibility of LO interference, then use a VERY much higher LO, the maths is simple enough.
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.

Thank you for your help though.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
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.
And why would you consider that a sensible way to use a radio?.

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.
What's complex about that? - never seen or heard of 21.4MHz though.

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).
But why do it?, it's a silly thing to do.

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.
Commercially available filters do the job, so nothing else is needed - 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.
 

hugoender

New Member
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 -.-
 

Nigel Goodwin

Super Moderator
Most Helpful Member
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?.

If you want to learn more about radio, I suggest you try looking in to taking your RAE (Radio Amateurs Exam) which (hopefully) will still inform you of the reasons for such design choices.
 

hugoender

New Member
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).

And the point of a radio that cannot be smashed with a hammer would be just that... so that the radio is rugged enough to withstand a hammer hit. I bet the person who's radio breaks because a hammer or similar object fell on it would appreciate having had the durability of such a radio.
 

hugoender

New Member
But under artificial conditions, deliberately engineered to cause the problem - have you ever had this effect in any normal use of a radio?.
What if you wanted to listen to two radio stations at the same time and they happen to be 10.7MHz apart? THEN YOU ARE SCREWED! No artificial conditions there. Now you are limited in what you can do because you cannot grab two radios and set them in front of you to listen to two different radio stations 10.7MHz apart. :D
 

JimB

Super Moderator
Most Helpful Member
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?
OK, lets try a few ideas.
Mixer, use a balanced mixer with good isolation from the LO port to the RF port.
RF amplifier, use one. Ensure that it has a low reverse gain to minimise backfeed from mixer to antenna.
Filtering. Use high Q tuned circuits between antenna and RF amp and between RF amp and mixer.
The passband of the RF amplifier and its input/output circuits will be much narrower than 88 to 108MHz, and it will be necessary to tune these circuits as the local oscillator is tuned.
Local oscillator. Run it above the signal frequency.
Intermediate frequency. Higher is better, puts the LO frequency further down into the stop band of the RF amplifier circuits.
Mechanical. Mechanical design is just as important circuit design in high performance RF applications.
Ensure that the circuit layout does not allow LO energy to leak past the filters back to the antenna.
Ensure everything is screened.
Supply wiring well bypassed, ideally with feedthrough capacitors where wires pass through the screening enclosure.

JimB
 
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