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FM Receiver that produces no interference to other FM receivers.

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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. :)

Neither do I, but they don't fall on a 10.7Mz boundary, which is why I've said all along it's trying to resolve a 'problem' which has never existed.
 
I suspect most cheap radios spam the spectrum...
 
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?

Absolutely right!
Low LO power would indeed help the spurious radiation problem, but the most "rugged" balanced mixers do require several mW of LO power.

There you go, a bit of praise!

Tot siens!

JimB
 
Why you don't think to a different approach?
Nobody forces you to have an IF stadium, as the technology is currently much better than in the past, you can use a pre-amplifier/filer and then you can digitally sample the signal and make the whole work by means of Digital Signal Processing.
You can find a good example at HPSDR site:
HPSDR - High Performance Software Defined Radio
 
And how many clock oscillators are there in all that computing?
All battering away at various frequencies plus harmonics of those frequencies.

Have you tried listening to a radio (with built-in antenna) which is sitting near a computer?

JimB
 
Those software defined receivers still frequently need IF stages.
 
My clock radio sits on top of my computer. The computer has a plastic case so it must be radiating into the clock radio. FM is perfect but of course AM has interference.

The FM circuit does not tune the input and the RF amp does not have AGC. So to avoid overload by strong local stations its sensitivity is reduced. It picks up only strong local stations.
 
Not so much the praise I was after (look at me! look at me!)
but I asked a somewhat relevant question.
Since the LO injection required in a dual gate fet mixer is much lower than a Mini-Circuits diode mixer (with it's good port isolation), would the LO feed back to the RF amp be less using the fet? Let's assume both mixers use the same number of signal input & output tuned circuits.
Perhaps this is hard to figure without testing.
 
Anyone know if there is a website with a database of components (LNA's, Mixers, BPF, etc.) that are available from companies? I know that in digi-key you can specify parameters and it supplies you a list of components that meet those parameters but they don't have everything I would need.

I need to find the components that give me the correct isolation, gain, etc. so that I can look at their datasheets and input the values into my simulator.

Thanks again for the help.
 
actually the flaw in the radio that lets LO signal bleed out is that the mixer has poor isolation between the LO input port and the IF input port. to keep the LO out of the antenna, the mixer stage needs to be isolated better. this costs a few pennies more than manufacturers are willing to part with, so they go with the cheaper mixer stage that provides just enough isolation that the radio still works. the best isolation would be from the use of a Doubly Balanced Mixer. the LO isolation in a DBM can be as good as -70 or -80 db. the cheap mixers using emitter and base of a transistor as the input ports only get -20 to -30db isolation. the simplest DBM is 3 balanced rf transformers, and a diode ring. depending on how well you match the diodes (Vf and Cj) the isolation between input ports can be as good as -60 db. a DBM using transistors can get even better results, especially with matched transistors. ironically, it was spurious RF emission from radio receivers becoming a big problem that was one of the reasons that superhet radios became the standard, regenerative receivers radiated detector signals with almost no isolation at all, and this was a BIG rfi problem. some countries even passed laws requiring all radio receivers sold to the public be of a superhet design. this didn't do away with the problem entirely, but did reduce it a lot. so now there are cheapie superhet devices that have -30db isolation (which is better than no isolation) but there are ways of improving on this, but the only people that will improve on it are engineers that see a need for it. if i were to build my own receiver, you betcha i would use a mixer with high isolation. not only does it reduce the LO emissions, but it improves the performance of the receiver drastically.
 
actually the flaw in the radio that lets LO signal bleed out is that the mixer has poor isolation between the LO input port and the IF input port. to keep the LO out of the antenna, the mixer stage needs to be isolated better. this costs a few pennies more than manufacturers are willing to part with, so they go with the cheaper mixer stage that provides just enough isolation that the radio still works. the best isolation would be from the use of a Doubly Balanced Mixer. the LO isolation in a DBM can be as good as -70 or -80 db. the cheap mixers using emitter and base of a transistor as the input ports only get -20 to -30db isolation. the simplest DBM is 3 balanced rf transformers, and a diode ring. depending on how well you match the diodes (Vf and Cj) the isolation between input ports can be as good as -60 db. a DBM using transistors can get even better results, especially with matched transistors. ironically, it was spurious RF emission from radio receivers becoming a big problem that was one of the reasons that superhet radios became the standard, regenerative receivers radiated detector signals with almost no isolation at all, and this was a BIG rfi problem. some countries even passed laws requiring all radio receivers sold to the public be of a superhet design. this didn't do away with the problem entirely, but did reduce it a lot. so now there are cheapie superhet devices that have -30db isolation (which is better than no isolation) but there are ways of improving on this, but the only people that will improve on it are engineers that see a need for it. if i were to build my own receiver, you betcha i would use a mixer with high isolation. not only does it reduce the LO emissions, but it improves the performance of the receiver drastically.

Thank you very much for this detailed response. I greatly appreciate it.
 
Two brute force methods are:

1. Put an RF circulator (sometimes called an isolator) after the RF preamp. That is a unidirectional RF device. Can give as much as 30 dB isolation per device and .5 dB loss in the thru direction.

2. Make a tracking preselector filter. That is, a filter at the RF input that is very narrow (150 KHz or so) and tunes in tandem with the LO freq. Elaborate, but you asked for ideas.
 
I would think a circulator at 100 MHz would be hard to come by. I have only seen them used in microwave frequencies.
 
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I think as frequency gets lower the size increases proportionally thus making lower frequency devices very large. I once used a circulator for a PCS (1.8GHz) design and it was about 1 inch^2.
 
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I think as frequency gets lower the size increases proportionally thus making lower frequency devices very large. I once used a circulator for a PCS (1.8GHz) design and it was about 1 inch^2.

Yes, one I just found is 3in x 2.5in x 1.4in. (without the load attached, I think). I wonder how much of that bulk is due to the huge connectors.

**broken link removed**

I found this VHF circulator that uses op amps. Could be made pretty small. Causes me to wonder why not just use an op amp for the RF preamp.

https://www.wenzel.com/pdffiles1/pdfs/RFDesign3.pdf
 
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there is an interesting bit of history about the reason most military radio equipment began using balanced mixers rather than single-ended mixers. it seem that during WWII the germans discovered that they could DF local oscillator emissions from american radios, and by doing so locate american units. if they knew the IF frequency being used, they could also guess the frequency the radio was tuned to and listen in. so american military radios began to be built with balanced mixers and double conversion.
 
Use could also use a high Q LC trap at the antenna tuned to the LO frequency and gang tuned with the LO.
 
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