Rule of thumb for receiver filter design
I'm designing a superheterodyne RF receiver with a unique downconversion stage.
I'm trying to understand how to choose the filters, such as image-reject filter before the mixer, harmonic-reject filter for local oscillator harmonics, and IF filter after mixer.
Do you have any advices for me? Any rule of thumb for these filters?
How do I do to know what is the IRR of my receiver? How much IF-spurs can degradate the performances?
Help me.
Thank you
There are no rules of thumb, it is all a compromise.
The filter requirements are based on the use of the receiver. What signals will be present at the image frequency and what level will they be in reation to the signal you are trying to receive. How much interference from the image is alowable, it may be you can get away with 20dB or so rejectio or maybe you will need 80 or 90.
The LO rejection filter requiremtns will depend on the isolation provided by your mixer and RF amplifier, if any. The image filter may provide adequate rejection dependign on the frequencies involved.
The IF filter depends on the bandwidth of the signal you are receiving. It need to be just wide enough to pass the signal with minimal distortion and adeqately reject the adjacent signals.
Most of the high-end HF receivers use the first IF much higher than the RF signal (45MHz, 69MHz, 73MHz, 110MHz, etc), placing the image frequency well above the RF frequency.
But in the last years old approaches with relative low first IF (9MHz or 10.7MHz) are returning. In this way you get the advantage of using high-quality multi-pole roofing filters. In approaches like this is mandatory to use high-side LO injection, to be able attenuating well the image signals.
The problem with image frequencies is that, even if you don't have any interferer placed on the image frequency, the image noise it will affect the noise figure of the receiver by at least 3dB. So, an image filter placed before the first mixer is mandatory in a superheterodine receiver.