Superheterodyne Receiver

[FreshmanNewbie]

In a Superheterodyne receiver, let us assume, I have a signal in the RF Range of 88MHz to 108MHz.

From what I have read, my understanding is that by tuning the local oscillator frequency, we can make the incoming RF signal to fall within the IF frequency range which we have designed.

My questions :

Suppose, I have a frequency of 95MHz. And My IF filter is designed at 98MHz (Am I correct in assuming this value for the IF Filter?). So, the signal after going through the filter, will come as a signal having 98MHz, right? But my actually signal is 95MHz. And all my modulation is done at 95MHz. Not after the filter, the signal is converted to 98MHz. Won't my actual signal present in the 95MHz, be lost or tampered with filter?
In other words, how is the modulation retained even after passing through the IF Filter?

Does high carrier frequency imply that more information can be carried and Low carrier frequency imply that less information can be carried per unit time? Can someone explain.

 

[Easy peasy]

The IF is tuned so that the difference freq is the one of interest, 455kHz for the AM band

for 88-108 FM the IF can be tuned over the same range to get the PLL to generate the audio directly ... ( direct conversion ).

 

[stenzer]

Hi,

basically you are mixing down the high input frequency (f_RF) towards a lower IF frequency (f_IF) by your mixer operated by a tuneable local oscillator frequency (f_LO) which results in

f_IF = f_RF − f_LO

According to wikipedia "a FM (Frequency Modulation) broadcast band receiver covers 88 MHz to 108 MHz band with a 10.7 MHz IF frequency", thus you can see your IF is significantly lower than your input RF.
So your signal after the IF filter has a frequency of 10.7 MHz. The information transmitted by the RF signal lies within its bandwith (200 kHz for FM). This information is modulated by e.g. Frequency Modulation, Amplitude Modulation, ..... Have a look on modulation types. Thus also the information within the bandwith of 200 kHz is mixed down ( 10.7 MHz ±200 kHz). Depending on your used modulation and the characteristic of your implemented filter your IF signal (and information) is altered slightly due to the imperfectness of your IF filter in some kind e.g. attenuated or a phase shift. Neverteless, your information does not get lost. Assume an AM where the information content depends on the received amplitude i.e. an audio signal will sound quieter if it is attenuated by the IF filter (but the information is still there).

greets

 

[betwixt]

For FM broadcast band 88-108MHz the choice of 10.7MHz is a good one. What matters is the bandwidth and image rejection and there will always be some compromise between them. Certainly using 98MHz as IF is not a good idea because to receive 95MHz the local oscillator will be at 3MHz and the image will only be 3MHz the other side of the IF at 101MHz which is still 'in-band'. In other words, unless you used so selective filtering at the signal frequency, you would pick up 95MHz and 101MHz equally well and at the same time. Using a higher IF moves the image further away from the signal frequency making it much easier to eliminate.

The idea behind superheterodyne 'superhet' is you track the input frequency with a local oscillator frequency so the difference between them stays constant. The mixer product from Fsignal - Foscillator stays constant regardless of what you tune to. The input signal, including its sidebands is 'mixed down' to the intermediate frequency where it is easier to filter with at a fixed frequency. The IF filters are wide enough to allow the sidebands (and hence the modulation) to pass through but not wide enough that adjacent stations get through. Basically, you shift the station you want down to a lower fixed frequency where it is easier to work with.

So in a typical receiver, picking up 95MHz would involve mixing it with a local oscillator at (95-10.7MHz) 84.3MHz. The two mixer output components would be the signal that was at 95MHz and the image at (84.3-10.7MHz) 73.6MHz. Hopefully they are far enough apart that the 73.6MHz would already have been eliminated by the front end filters.

Once at 10.7MHz, the filtering is easy because being fixed frequency, the tuned circuits or ceramic filters do not have to be adjusted and their bandwidth is easy to define. A typical FM receiver has a bandwidth of about 200KHz so using your original 95MHz signal and mixing it down with an 84.3MHz local oscillator would result in reception of 94.9MHz to 95.1MHz (100KHz each side of the carrier) which is exactly what you need to carry the width of FM broadcast signals.

Brian.

 

[FreshmanNewbie]

Thank you very much for the clear and detailed answer.

Could you please provide the definition of image frequency and could you also please provide an answer to my second question, "Does high carrier frequency imply that more information can be carried and Low carrier frequency imply that less information can be carried per unit time? Can someone explain."

 

[betwixt]

I will try:

Most mixer circuits have two inputs, usually the signal you want to monitor and the local oscillator. There are several types of mixer, some using cancelling techniques (balanced and double balanced mixers) to eliminate some of their output signals but for simplicity, the ones normally used in domestic receivers are not balanced. They produce typically four output frequencies:
1. a small amount of input signal leakage
2. a small amount of local oscillator leakage
3. a signal at the sum of the signal and local oscillator frequency
4. a signal at the difference of signal and local oscillator frequency.

You can use 3 or 4 as the signal you want to select, the choice is yours, the only difference it makes is whether the LO is on the high side or low side of the frequency you are receiving.

You can look at 3 and 4 the other way around - for a given local oscillator frequency, the output frequency (the IF) can be made from two different input frequencies, it is up to the selective circuits at the input of the receiver to block the unwanted one as much as possible. You will come across the term "image rejection" used to describe how well the unwanted input is reduced relative to the wanted one.

Regarding the frequencies that can carry information, yes there is a relationship but one isn't directly proportional to the other. Modulation is the name for the process of adding information to a carrier, in doing so it produces a shift in the phase (PSK, QPSK etc) the frequency (FM etc) or the amplitude (AM, DSB,VSB, SSB etc). All of these make the required bandwidth greater than a single unmodulated carrier. The amount the bandwidth increases by depends on the kind of modulation and the amount of information the modulation carries. It follows that you can't use a low frequency carrier to carry a wide bandwidth signal, it simply runs out of space in the radio wave spectrum. In general, as long as the carrier frequency is high enough to carry the modulation it will work but given there are so many users of the spectrum it makes sense to keep the low frequency bands for narrower signals and higher frequency bands for wider signals.

For example, a digital TV transmission channel is typically 27MHz wide, it wouldn't make sense to place it in the 0.5 - 1.6MHz AM radio band! There are still many narrow band signals on higher frequencies though.

Brian.

 

[danadakk]

https://www.rfcafe.com/references/electrical/mixer-spur-web.htm

https://www.rfcafe.com/references/electrical/image-frequency.htm


Some useful ref info.