Intermediate frequency and input frequency
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.
A simple mixer always generates frequency sum and difference, no changes required except adjusting the VFO and input filter frequency.
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.
Thanks. I must have misunderstand the theory, because I tought that there are TWO TYPES of mixers. I will need to reread that stuff again!
There is already a bandpass filter in front of the first mixer... can you explain how the second filter can improve the reception even more?
Do you have any other ideas how to improve the schematic from the image?
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.
Brian.
Sadly I have already ordered the PCBs according to the schematic from the article, where the +12V line is used.
I already have 80m version, but I am going to make 20m and 40m version as well, by using the frequency difference from mixer (14MHz-4MHz=10MHz).
Also I am thinking about adding an AGC circuit at the audio frequency stage. Would it bring any advantages?