FM Receiver that produces no interference to other FM receivers.

[Nigel Goodwin]

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.

Why would you place two radios right next to each other tuned to different stations? - I also suggest you check what the station spacing is for FM radio

 

[smanches]

Nigel, the only reason you think it's pointless is because you "know" the outcome. For someone that doesn't it's not pointless. It's learning.

Don't ever discourage learning or invention.

 

[JimB]

Lots of good banter here regarding to desirability of such performance in radio receivers.

JimBs view:

For the average broadcast receiver, simple circuit design and unscreened mechanical construction are perfectly OK for 99.99% of users.

But where communication receivers are concerned, where several receivers are located in close proximity and using a common antenna, or, separate but closely space antennae, such as on an aircraft, high performance, including very low oscillator radiation is very important.

JimB

 

[Sceadwian]

Hugo, just because it's a 'flaw' in your mind doesn't mean it is, sure you could in theory remove it. To what ends? It's the outcome of the common systems in use. Not a flaw. I would side with Nigel at this point. Not to dissuade Hugo but but you have to weight the cost/benefit of doing ANYTHING. What does 'fixing' this 'problem' 'solve' ? Nothing.

 

[tcmtech]

How many unesisarily over complicated and just pointless "construct a device to use a complex circuit to do a simple process" threads or "theoreticaly nearly impossible to actualy happen" threads are there on this site now? I dont think that this one would be much different.

Just pick a genralized catigory name from below and file it under that.

1: How can I tell if my lights are out without getting out and looking.
2: How can I use way too many components to charge a simple battery.
3: How can I use a microprocessor to blink a LED.
4: How can I get the last nanowatt of energy out of a AA battery for under a $50 investment.
5: How can I build a common device without actauly using common parts.
6: How can I build a store bought device using salvaged parts from several of the same store bought devices I already have that still do work.
7: How do I run some device off a power source not designed to run it.
8: I am cheap and have no common sense can some one help me out.
9: Do you have my home work done yet.
10: How do I make something work when I dont even know how to describe what it is.
11: How do I solve an unlikely problem that is realisticaly never going to happen.
12: How do I convince others that what I think is right without evidence or proof to support my opinions.
13: If I took my OCD meds would this senario I am so worried about still seem important to me.

 

[smanches]

And how many side inventions were discovered because people were trying to solve an equally pointless problem and discovered it could be applied to a completely different problem?

Can anyone say Microwave Ovens?

As he mentioned before, he's curious. He's not trying to revolutionize the FM receiver industry.

This thread has degraded into nothing now. Thanks for the discouragement.

EDIT: I guess Microwave ovens are a bad example as the original idea was successful. How about Post-it-Notes then?

 

[Sceadwian]

I chose number three.

I would like to add to number 6. "in 5 minutes"
Number 9 made me laugh.

 

[audioguru]

A half-decent FM radio tunes its input, tunes the output of the RF amplifier and tunes the local oscillator. The tuned input eliminates an image interference from another radio.

El cheapo clock radios do not tune the input so of course they pickup interference and are overloaded by all the stations in town.

 

[kchriste]

You can minimize the problem by using a mixer with better port isolation. The mixer in a regular broadcast FM radio is usually the simplest, crappiest and cheapest available: a single transistor stage. A double balanced mixer would be better but is seldom, if ever, used in a cheap FM radio.

 

[hugoender]

Thank you JimB and the rest of the guys who added constructive posts to this thread. Your replies have helped me figure out where I can look to improve the interference as well as where I should not look. I appreciate your input.

As for the rest of you.... yeah.

 

[Mikebits]

When I was on board a Navy ship we had some EW gear that could detect and localize LO emission from a receiver. I was never told how it works as I did not have the need to know as they would say. During some training drills we had to go into radio silence including powering down receivers.

Considering today's satellite technology I should suppose that such measures might be outdated, but in the advent that the satellites are brought down, then perhaps it may be useful.

Like was said earlier, most high end radio's like that used by the military have a strict spec on LO leakage through the antenna. Low noise amps with good isolation and attenuators in front of the mixer are usually needed. Oh and a preselector is often used as well. The preselector must be low loss, as this will greatly impact the radio's noise figure. A loss of < 3dB is typical.

 

[flat5]

To possibly add to Jim's list:
Use a mixer designed for the lowest LO injection requirement.
This may be a bad idea because I can't think of one that has good isolation. All the balanced mixers require a high LO injection level, right?

 

[Mikebits]

To possibly add to Jim's list:
Use a mixer designed for the lowest LO injection requirement.
This may be a bad idea because I can't think of one that has good isolation. All the balanced mixers require a high LO injection level, right?

Low LO injection has poor Intermodulation performance.

 

[flat5]

"Low LO injection has poor Intermodulation performance."

Even if the LO level is correct for the mixer design?
Dual gate fets can have a good crosstalk immunity.
can handle relatively strong signals.

 

[mneary]

Thank you JimB and the rest of the guys who added constructive posts to this thread. Your replies have helped me figure out where I can look to improve the interference as well as where I should not look. I appreciate your input.

As for the rest of you.... yeah.

I don't think you'll find any stations assigned 10.7 MHz apart, since channel spacings are 200 kHz. I'll bet this wasn't accidental.

 

[smanches]

In Europe they are spaced an odd frequency apart though, which could land on the 10.7Mhz boundry. Don't remember the exact amount so I'm relying on a Euro to fill in the blank.

 

[Leftyretro]

I think there have been some good topics raised here despite a somtimes low signal to noise ratio. The best information on receiver design tends to come from some of the ham radio sites. Improving the dynamic range of RF front ends has had great progress over the last several decades.

Today direct conversion using DSP has reduced the RF portion of a state of the art receiver to just a few well designed stages and all the rest done in software, no hardware IF or demodulation or AGC needed! Kind of takes the fun of studing receiver circuits but that seems to be the trend in high performance receiver.

Lefty
WA6TKD

 

[Mikebits]

I think there have been some good topics raised here despite a somtimes low signal to noise ratio. The best information on receiver design tends to come from some of the ham radio sites. Improving the dynamic range of RF front ends has had great progress over the last several decades.

Today direct conversion using DSP has reduced the RF portion of a state of the art receiver to just a few well designed stages and all the rest done in software, no hardware IF or demodulation or AGC needed! Kind of takes the fun of studing receiver circuits but that seems to be the trend in high performance receiver.

Lefty
WA6TKD

All those functions you mentioned above are still needed, only they are done in code...

Front end design is still all done in analog hardware though.

 

[Leftyretro]

All those functions you mentioned above are still needed, only they are done in code...

Front end design is still all done in analog hardware though.

Isn't that what I said? Must be getting late

Lefty

 

[Mikebits]

Isn't that what I said? Must be getting late

Lefty

Reading back, I see you did say that, my oops...