Radio Mixer IF spectrum

Talking about superheterodyne discrete transistor mixers. IF is supposed to be at 10.7MHz.

*) Suppose my RF amp feeds the signal to the mixer transistor.
*) LO feeds the same transistor from 98-118MHz. This is my assumption.

My question is if I want to get 10.7MHz IF, I need to oscillate LO at 98.7-118.7MHz?
*) If so, what are all the image frequencies generated? Will they be continues spectrum? What's the range of IF's generated by this setup?
*) If I oscillate LO at 88-108MHz(same broadcast band), will I get 455KHz IF as a image frequency?

I don't have frequency counters. So Please enlighten me with a detailed explanation of IF range spectrum generated in both cases.

FM receivers mostly use low side injection, so the LO operates 10.7MHz below whatever frequency the receiver is tuned to. e.g., if the receiver is set to 100.3MHz, the LO is at 89.6MHz (100.3-10.7). If they used high-side injection, then the LO would have to be at 100.3+10.7=111.0Mhz. The reason that FM tuners rarely use high-side injection is because 111.0MHz is smack in the middle of the Aviation Navigation band of 108-117.9MHz, so radiation from the LO can be a serious source of interference to aircraft navigation.

In the example above, if you have an FM receiver tuned to 100.3MHz with low-side injection, the image would be at 100.3-2*IF = 100.3- 2*10.7 = 100.3-21.4 = 78.9MHz. It is up to the selectivity of the tuned stages between the antenna and the mixer to suppress the image frequency so that it does not get to the mixer...

Thanks Mike ML, really enlightened me ...one more question, what's special with that 10.7 digit or why not just 10MHz?

transistor495 said:

Thanks Mike ML, really enlightened me ...one more question, what's special with that 10.7 digit or why not just 10MHz?

Click to expand...

One main reason and may not be the historical reason, 10.7 Mhz filters are now common and an off the shelf item. Can be easily ordered without long lead times and by multiple vendors. Try ordering a 10.125 MHz filter...

There are technical reasons such as even harmonics and stuff, but I am sure the other Mike can explain better

Mikebits said:

One main reason and may not be the historical reason, 10.7 Mhz filters are now common and an off the shelf item. Can be easily ordered without long lead times and by multiple vendors.

Click to expand...

OK, that was my thought too...

transistor495 said:

OK, that was my thought too...

Click to expand...

MikeMI can give you the techy mumbo jumbo, but from the business end, 10.7 MHz is very common and cheapest to get. I have worked on special government programs where special IF's were needed and filters cost thousands of dollars. For commercial use this is not the way to go...

My PLL dual conversion FM receiver is capable of receiving 76-108MHz range. So as I don't have a frequency counting device, I can easily catch the LO low-side injection range in between 77.3-97.3..that is currently my plan ..hope this won't make any problem for sure.

Any opinion about that..or am I following a stupid procedure?

While testing, I could receive the LO signal of a cheap FM receiver exactly 10.7 MHz behind on the dial of PLL digital RX placing close. So that one would be my frequency counter for alignment