Hey,
I have a simple superhet SSB radio receiver, the IF is 10MHz. The Fvfo is 6.5MHz, and the input frequency is 3.5MHz.
In this case Fin < IF (3.5 < 10)
Fin = IF - Fvfo
3.5 = 10 - 6.5
I want to convert it to receive 14MHz or so: Fin = 14MHz. I want IF to stay 10MHz.
I heard that's possible to add frequency in mixer, instead of subtract it.
Fin = IF + Fvfo
14 = 10 + 4
In my receiver frequency mixing is done with transistors.
This is the schematic:
My question is, how to modify the receiver (specifally, the first mixer) to receiver 14MHz correctly?
PS:
I am aware that I also have to adjust input bandpass filter for 14MHz.
The reason of using high-side LO injection is to get the image frequency far away in frequency, to be easier to be rejected.
In your case fImage=fRF+2*fIF = 3.5+2*10 = 23.5MHz
Placing a simple low pass filter in front of the mixer (series with C26) it will improve the receive sensitivity at least 3dB.
In 14MHz band the image frequency will be even further, at 34MHz.
The reason of using high-side LO injection is to get the image frequency far away in frequency, to be easier to be rejected.
In your case fImage=fRF+2*fIF = 3.5+2*10 = 23.5MHz
Placing a simple low pass filter in front of the mixer (series with C26) it will improve the receive sensitivity at least 3dB.
In 14MHz band the image frequency will be even further, at 34MHz.
Personally, I would connect ALL the crystal filter and ALL the decoupling capacitors to ground rather than the +12V line. Apart from the risk of noise on the +12V line, it makes construction with a ground plane almost impossible. Note that a good ground plane and possibly shielding the ladder filter is essential to get good selectivity.
Personally, I would connect ALL the crystal filter and ALL the decoupling capacitors to ground rather than the +12V line. Apart from the risk of noise on the +12V line, it makes construction with a ground plane almost impossible. Note that a good ground plane and possibly shielding the ladder filter is essential to get good selectivity.