FMCW 2.4GHz Radar Gain Stage Help
I am currently working on the Gain Stage (see schematic in link above) and it is not working. I am using the MAX414 amplifier and a Tektronix Function Generation and Oscilloscope. I had been working on the Low Pass Filter the last couple weeks but I was unable to get that working, I was not getting the -3dB roll-off at 15kHz and after many days of troubleshooting my circuit and testing configuration and decided to move on to the Gain Stage and remove the LPF. I was told that I could take out the filter and I could still get ranging and Doppler data from my radar, but it would just be noisy and unable to filter out the higher frequencies.
So now to my questions. The picture below is a screenshot of the scope when I was testing the Gain Stage. The yellow line is the input and blue is the output. As you can see in the picture, the input is larger than the output which is not supposed to happen. I'm not sure if I have my probes in the right place, I have the input coming in at the 1uF capacitor which goes to pin 10 and the output at pin 8. Is that right? Also, can I still get data from the gain stage if I removed the LPF? I have verify that my power lines are both approximately 5V and 12V. If someone could help me soon, that would be great. I'm stuck and really need some help from people that know what they are doing. Thanks.
(the white jumper is the input and the yellow one is the output)
Here is a picture of the schematic if that helps:
Also, I had a question about the function generator. Would changing the amplitude of the signal affect the output results? Currently, I have it set on 200mV. Pk-Pk results on scope: Input = 4.4V and Output = 3.36V.
Are you using high impedance scope probe ? Some scopes have 50 Ohm input impedance and if it's so, the input impedance of the scope may load the output and output level can decrease.
From the attached screenshot I think you are using a TBS series Tektronix oscilloscope, which doesn't have possibility for 50 ohms input (only 1Mohm input).
So, your circuit might not function properly. Check it again, or replace the MAX414.
You should get at least 30dB gain with this circuit.
As a hint after you make the circuit working, replace the 1uF input capacitor, with 100nF. In this way you get a High-Pass characteristic which helps attenuating the first ground reflection.
I have already tried replacing the MAX414 and I did not change the output I was recording. I will check my circuit again and replace the input capacitor.
This is the scope probe I am using: https://www.digikey.com/products/en?...ds=501-1029-nd with https://www.digikey.com/products/en?...ds=461-1194-nd
Not sure if this is a high impedance scope probe or not...
The first digikey item is not a scope probe and is not high impedance. The second item is a coax cable, which is also not a scope probe.
That's what I am plugging into my scope and attaching to the circuit with the clips. What should I be using instead of this?
Using standard High Impedance scope probe..How did you connect these cables to scope ? The scopes have BNC input connectors and you used a proper adapted or something , didn't you?
Coaxial or not, it's not a big issue for 15-20 kHz but loading the output with low impedance/resistance will change entirely the signal amplitude.
It is a TDS 2024B
An actual scope probe...
https://www.digikey.com/products/en/...=scope%20probe
What you are doing is measuring a circuit with a ~50ohm load hanging off it.
If you want something to hold the scope probe then you can look at these results: https://www.google.com/search?q=hand...utf-8&oe=utf-8
An easy way is to solder a short wire to the signal destination pin and use one of the scope probe clips that attach to the end of the scope probe.
Okay, I will get that scope probe. Hopefully that was the problem. I've been testing the circuit the last couple weeks and the circuit was probably right the entire time and the probe was the problem, this is frustrating.
I got a new probe, it is a Tektronix P2220 voltage probe. Is this what I need?
Considering that you are looking at signals in the 10's of KHz range that should work just fine.
http://www.tek.com/datasheet/passive...e-probe-1x-10x
Just make sure you use the ground lead, which is shown as the left most item in the above link.
Given the issue you've had using the scope to measure things, perhaps you should read a tutorial on using scopes prior to continuing your measurements.
http://www.tek.com/learning/oscilloscope-tutorial
...for rather more simplistic basic tutorials...
https://learn.sparkfun.com/tutorials...n-oscilloscope
http://www.instructables.com/id/Oscilloscope-How-To/
I do not have the ground lead in that picture, can I just use a piece of hook-up wire?
Hey guys, so I tested the gain stage with the new high impedance probes and I got a plot that looks like this:
The output is much larger than the input which is good but is it supposed to be that noisy?
I installed the hardware of the radar (antenna, RF components, etc.) and soldered them together. I downloaded Audacity to record a .wav file from the radar and I am using the MATLAB code to process it on the MIT open courseware page for the radar class https://ocw.mit.edu/resources/res-ll...2011/projects/. Hoping to get some data from my radar this afternoon.
Another hint, when you record the audio file with Audacity, do this with maximum sampling rate that the software allow you (I don't remember now how much it is).
I have made some progress the last couple days. I set up my radar and was able to get simple ranging data.
That is the plot that MATLAB processed from the Audacity data. The interesting thing was that I would move and the audacity readings would spike and then when I stopped moving, it would level out. This can be seen in the MATLAB plot where it is cut off between different range readings. This was done in my office so you aren't able to see the range of the objects, I would have to also do a test in a much larger area, but at least the radar was able to see me.
Why is the radar not taking measurements when I'm not moving? That would make sense when I'm taking Doppler measurements due to a moving objected needed to calculate velocity, but why is it happening for the ranging test?
I will test again today with the higher sampling rate and see if that helps.
Also, I fixed the gain stage and it now looks like this:
The problem was that I used a 220kOhm resistor instead of a 220Ohm resistor, dumb mistake...