Superheterodyne receiver - The MF
I'm currently studying some Computer- and Telecommunications. I found this forum to be a great source of help and information. Although I cannot provide much to this forum because of my lack of expertise in this very field, I'm very helpful if there is someone who could help me achieve me to become one as this is a very interesting field.
I'm reading a bit about superheterodyne receiver and the double superheterodyne receiever. My questions here is that depending on the MF frequency, you adjust local oscillator to it. What brings to my mind is that 10,7 Mhz is a prefered frequency. Why is that? Is there some kind of "logical" proof or mathemathical proof for that for that certain frequency? Or both?
I'm really grateful for an answer! Thanks!
Do you mean IF and not MF? I assume so, since 10.7 MHz is a common IF.
There's no theoretical reason a specific frequency should be an IF, so 10.7 MHz is not been settled on because of some theory that says 10.7 MHz is optimal. The attraction today of using 10.7 MHz is that there are lots of components at that frequency. Getting filters made for special frequencies is expensive, so there's an obvious attraction to use something that is available off the shelf.
The 10.7 MHz IF frequency was originally chosen for FM broadcast receivers. It was a good compromise between bandwidth and selectivity. 10.6 or 10.8 would have worked as well, were it not for this: FM station frequencies are 200kHz (.2MHz) apart. If you had two strong FM signals nearby, one at 99.9 MHz, the other at 89.3 MHz and your IF was 10.6, you'd hear one station or the other no matter where you set the dial.
If I understand you correctly.
1) Since the FM stations are 200 kHz apart, the mixer product (difference) will be an integer multiple of 200 kHz.
2) By using an IF half-way between them, you would always be able to resolve one without interference to the other.
3) In that case, 10.3, 10.5, 10.7, 10.9 MHz would all be equallly good, but 10.4, 10.6, 10.8 or 11.0 MHz would be a bad idea?
Deborah
If I understand you correctly.
1) Since the FM stations are 200 kHz apart, the mixer product (difference) will be an integer multiple of 200 kHz.
2) By using an IF half-way between them, you would always be able to resolve one without interference to the other.
3) In that case, 10.3, 10.5, 10.7, 10.9 MHz would all be equallly good, but 10.4, 10.6, 10.8 or 11.0 MHz would be a bad idea?
Deborah
You got it.
I have an old radio magazine from 1927 that has an article about the "Improved Remler 45-kc. superheterodyne receiver". It explains that the usual IF frequency at the time was 50 KHz, using a single-stage regenerative detector. It says that such receivers were useless in a place like Chicago, where there were stations 50 KHz apart, as it would receive stations even if you removed the local-oscillator tube entirely. Using 45KHz as an IF was a way around this. Of course, the ultimate solution was to use an IF of 455KHz, with multiple tuned stages at that frequency to attain the desired selectivity.
I am not sure these arguments are entirely correct. The image is 2xIF from the desired frequency for one thing. For the FM band of 88-108MHz the image is not even in the band (which may be why an IF >10MHz was chosen). Also, there is some front end selectivity to provide image rejection.
Keith
Keith-
You're talking about images generated by the local oscillator. Assuming a 10.7MHz IF and high injection, to receive 99.9, the LO will oscillate at 110.8 and any signal at 121.5 MHz will also appear at the IF frequency of 10.7. As you say, it would have to be a very strong signal to cause problems, because the front end tends to reject such out-of-band freqs.That's not what I'm talking about, though. I'm talking about intermodulation products from signals in the same band separated by the IF frequency. 10.7 was probably chosen back when the FM band was 42-50 MHz, a spread of only 8MHz.
---------- Post added at 21:31 ---------- Previous post was at 21:30 ----------
Keith-
You're talking about images generated by the local oscillator. Assuming a 10.7MHz IF and high injection, to receive 99.9, the LO will oscillate at 110.8 and any signal at 121.5 MHz will also appear at the IF frequency of 10.7. As you say, it would have to be a very strong signal to cause problems, because the front end tends to reject such out-of-band freqs.That's not what I'm talking about, though. I'm talking about intermodulation products from signals in the same band separated by the IF frequency. 10.7 was probably chosen back when the FM band was 42-50 MHz, a spread of only 8MHz.
Please excuse my double post. Also please excuse the error in my example. The LO and image frequencies correlate to 100.1 MHz, not 99.9