causeitso
Newbie level 6
Hi , I have exactly three questions about superheterodyne receivers .
They may seem at first like simple ordinary questions , but I searched left and right and didn't find any satisfactory answer.
Question (1)- In a superheterodyne receiver I know that to convert the RF to IF we mix the RF with an LO of a predetermined frequency , that's ok.
But during my simple idealistic simulation of an RF modulated signal of say (110 MHz) multiplied by an LO of (100 MHz) the result (information) is at (10 MHz).
The simulation ran so smoothly and I could recover the information exactly as it was transmitted (whether it is ASK , FSK or PSK).
But to test the non-ideal behavior of the receiver I changed the LO frequency very slightly (100.001 MHz) , this very slight change caused a disastrous effect on the demodulated (information) signal , which manifests it self as if the information signal is now riding (or being multiplied by) a very low frequency , and this low frequency periodically Nulls the information signal.
The more accurate the LO frequency say(100.0001 MHz) the lower the frequency of this interfering signal .
The problem is that getting so incredibly unrealistically close to a LO frequency of (100 MHz) doesn't even begin to solve the problem , as this creates a so incredibly low frequency signal that will Null the information sooner or later.
This Nulling happens when the RF and LO are out of phase by 90 degrees.
Do you see what I am talking about?
So how could we avoid this problem (knowing that I didn't see any tutorial on superheterodyne receiver address this problem as if it doesn't exist).
Plz , Correct my understanding if I am wrong.
Question (2)- In mixer design they talk about LO input power and IF input power which affect the 1-dB compression and TOI.
How could the input LO power be calculated , I mean the LO has an output resistance and the mixer have a nonlinear input resistance , so is the LO input power is the power dissipated on the output resistance of the LO?
Or is it the power that get dissipated on the input resistance of the mixer or what?
Same goes for IF input power.
Last Question- If I am set to design a Double-balanced mixer (4 diodes) , then what BALUN to use (1:1 , 1:4 etc..).
I know that the choice depends upon the required impedance matching , but the problem is that the diodes have nonlinear impedance , so how could I know the input impedance of the diodes?
Is this DBM balun design has a rule of thumb or something , because I figured that different diodes are not really that different .
So is there a general Balun ratio used for DBM that use silicon diodes (0.7 volt forward bias voltage).
Sorry for these long questions , but I wouldn't ask if I had of any other possible option.
Thank you.
They may seem at first like simple ordinary questions , but I searched left and right and didn't find any satisfactory answer.
Question (1)- In a superheterodyne receiver I know that to convert the RF to IF we mix the RF with an LO of a predetermined frequency , that's ok.
But during my simple idealistic simulation of an RF modulated signal of say (110 MHz) multiplied by an LO of (100 MHz) the result (information) is at (10 MHz).
The simulation ran so smoothly and I could recover the information exactly as it was transmitted (whether it is ASK , FSK or PSK).
But to test the non-ideal behavior of the receiver I changed the LO frequency very slightly (100.001 MHz) , this very slight change caused a disastrous effect on the demodulated (information) signal , which manifests it self as if the information signal is now riding (or being multiplied by) a very low frequency , and this low frequency periodically Nulls the information signal.
The more accurate the LO frequency say(100.0001 MHz) the lower the frequency of this interfering signal .
The problem is that getting so incredibly unrealistically close to a LO frequency of (100 MHz) doesn't even begin to solve the problem , as this creates a so incredibly low frequency signal that will Null the information sooner or later.
This Nulling happens when the RF and LO are out of phase by 90 degrees.
Do you see what I am talking about?
So how could we avoid this problem (knowing that I didn't see any tutorial on superheterodyne receiver address this problem as if it doesn't exist).
Plz , Correct my understanding if I am wrong.
Question (2)- In mixer design they talk about LO input power and IF input power which affect the 1-dB compression and TOI.
How could the input LO power be calculated , I mean the LO has an output resistance and the mixer have a nonlinear input resistance , so is the LO input power is the power dissipated on the output resistance of the LO?
Or is it the power that get dissipated on the input resistance of the mixer or what?
Same goes for IF input power.
Last Question- If I am set to design a Double-balanced mixer (4 diodes) , then what BALUN to use (1:1 , 1:4 etc..).
I know that the choice depends upon the required impedance matching , but the problem is that the diodes have nonlinear impedance , so how could I know the input impedance of the diodes?
Is this DBM balun design has a rule of thumb or something , because I figured that different diodes are not really that different .
So is there a general Balun ratio used for DBM that use silicon diodes (0.7 volt forward bias voltage).
Sorry for these long questions , but I wouldn't ask if I had of any other possible option.
Thank you.