Dave Mueller:
Fovakis:
I don’t understand the combination of two stages and how a attenuator can minimize VSWR… i understand all these about reflections we said but not this one
The higher the attenuation value, the closer the attenuator becomes to a 50 Ohm load. Find an online calculator and check out the resistance values for a 40 dB attenuator. Also, the attenuator works in both directions, so the reflected power is reduced as well.
Here’s a worst case example: A 50 Ohm transmitter feeding a 3 dB attenuator, with nothing connected to the attenuator output.
If you put 1 W into the attenuator, you would think you would have 100% reflected and therefore 1 W reflected at the transmitter. However, the signal is reduced in each direction.
1 W into the attenuator, 1/2 watt out of the attenuator gets reflected back.
1/2 W coming backwards through the attenuator, 1/4 W reaches the transmitter. So a 3 dB attenuator made a 100% reflection seem like only 25%.
Using an attenuator between stages reduces your power, but improves the impedance for each device.
Many thanks about your reply !!! 
languer:
Look, it is somewhat evident from the questions that you’re not too confortable with RF in general. And that is just fine. If you want to really understand it, then you really have to read it and start doing the math. You can see it empirically (ie. hook up an SA with a directional coupler,measure transmitted and reflected power, and calculate VSWR), but until you do the math it really won’t click.
However, the big picture is this. If you don’t care much about reflections then you don’t really care much about reflections. You want to have as good a match as you can afford, but not much better really. If you are using pads to improve matching you are throwing power away. So you want to make sure you’re doing this only when you need to. Many times when we are measuring devices we use pads to isolate and improve matching between generators, devices, etc. But there are sometimes where we cannot tolerate the loss. In these instances we need other means (wheter an isolator, circuilator, or nothing). At the circuit level you try to use lossless matching when you can, you use pads when you can, and you use nothing when you can.
Back to your test case:
Ouput mixer:
LO => -60dBm
Desired => -18dBm
Undesired => -18dBm
Output of filter:
LO => -55dBm (~ 37dB below desired)
Desired => -18.50dBm
Undesired => -52dBm (~ 34dB below desired)
So the question you have to answer is: “are undesired signals 35dB below my desired signal matter to my receiver?”. We don’t know the answer to this because we do not know what your requirements are. The other question you may need to answer is: “will undesired signals 35dB below my desired signal meet my transmitter requirements?”. Before answering those questions you may want to take a look at these same signals out of your 2nd conversion to see how they fare? You then may want to analyze your frequency plan, figure out which frequencies may give you problems, and concentrate on those. You can use something as simple as Excel to analyze your frequency plan and which frequencies you need to concentrate on.
Thanks languer i appreciate your advices. I really love maths, i have done a little microwave theory during my master studies but i need to refresh them. I will come back with more specific question and some images from the lab during the next week.thanks for all