This lab is different from previous labs in that it is not a step-by-step experiment, but rather a design project.
GOAL: You have to design a receiver that has an output power level of 0 dBm and SNR > 9 dB.
Challenge: Choose amplifiers and filters that have the following restrictions:
- Amplifiers – 5 total.
- Amplifier NF/Gain (max gain is 30 dB from single amplifier)
- 0-10 dB Gain, NF=3dB
- 11-20 dB Gain, NF=6dB
- 21-30 dB Gain, NF=9dB
- Amplifiers – P1dB =10 dBm
- The signal will not reach this level, but spurious tones might. Make sure to filter any large, unwanted tones so as not to saturate the amplifiers.
- Filters – 3 total
- Filter specs
- > 500 MHz, BW=20%, insertion loss=2dB
- < 500 MHz, BW=20% insertion loss=1 dB
- Mixer – use the provided mixer with NF=10 and do not adjust any other parameters, e.g. conversion gain, LO Feedthrough. You may change the LO power, however the output signal will also be affected.
As in any engineering task, there will be trade-offs and you should clearly motivate your choice of frequencies. NOTE that there is no unique “Right” or ‘Wrong” solution to this assignment. If your answer fulfills the requirements, it is obviously a correct design. Bonus credit for the highest SNR achieved (492 and 592).
Please download the NI/AWR Design Environment file called “MY_NF_Start.”
The node “Ant” is the signal received from the antenna. You need to downconvert that to below 100 MHz and amplify to meet the specs listed.
The system diagram looks like this:
To calculate the SNR you will need to calculate the total noise, which is the integral of the noise over the bandwidth. In NI/AWR Design Environment, you can select SYSTEM-POWER->PWR-MTR. It integrates all of the signals in the BW specified. In your start file this auto selects the center frequency and sets the BW=fs, which is the sampling frequency and is set in System Options. In your file it is set to 10 GHz (1 Ghz Symbol rate and 10x oversampling). An example of the measurement is in your startup file.
The power of your signal can be calculated from the spectrum plot with a marker.
AWGN Noise. You will notice that your AWGN is set to: PWR=-75 dBm and PWRTYPE=NORMALIZED N0/2, where N0 would be the PSD of your noise spectrum. The -75 dBm represents that total power of the noise, i.e. N0/2*BW. In this case the BW=fs. You should see that the total power is equal to approx. 75 in your PWR_MTR graph.
If you look at your spectrum plot, it does not show -75, n fact it only shows ~-80 dBm and is moving around. This is because the Resolution Bandwidth is set to 1 GHz. A spectrum analyzer (SA) is just a simpler heterodyne receiver.
|Figure 1: Block Diagram for a spectrum analyzer. RBW and VBW are KNOBS on the SA. Screen of SA plots the integrated power over the RBW @ single LO, and then increases LO to next frequency.|
The spectrum analyzer’s antenna is its input port. It amplifies the signal and then sweeps the LO across the desired frequency span. At each LO value, it calculates the TOTAL POWER out of the filter. In the case of a SA, it is a BPF. The SA then plots that TOTAL POWER on the screed for the LO frequency. It then goes to the next LO. Every signal in the BPF BW gets integrated, signals, interferers, noise, etc. Thus, the point you see on a spectrum analyzer is not necessarily your actual signal. By making the filter BW narrower, you exclude other signals and reduce the total noise power. This filter is called a Resolution Bandwidth Filter (RBW) as it sets the resolution of your measurement.
Change the RBW of your Spectrum Antenna graph from 1 GHz to 1 MHz. This reduces the integrated noise by how much? (BW reduces by 1000: 1Mhz/1Ghz=-30 dB) What is the drop you see in the spectrum pot? (30 dB). You can see that there are now finer measurements on your spectrum plot. You could reduce the RBW further. What might the tradeoff be? (time – try setting it to 1 kHz and see how long it takes to respond).
You may still seem some fluctuations on the screen, which can be removed by the Video Bandwidth Filter, which is a filter that comes after the power meter. It is just a LPF of what is on the screen. Averaging of the plot would do the same function. You can adjust the VBW also in NI/AWR Design Environment, but it slows down your measurements too.
Assignment (40 pts):
- Insert a system diagram of your final design such that all paraemters are visable (5 pts).
- Write 1-2 paragraphs about how you optimized your design. What methodology did you use, what were the tradeoffs, etc. (10 pts)
- Plot the signal power and total noise power at each node of your system diagram. You may do this by calculating them in NI/AWR Design Environment and then using Excel to plot. You may also investigate options in NI/AWR Design Environment that may do this for you (check by hand to make sure it is calculating what you think) (8 pts).
- Calculate the SNR at each point. Is it increasing or decreasing? (2 pts)
- Calculate by hand the NF of your final receiver. (5 pts)
- Plot the frequency spectrum of your signal at each point where you change circuit element. For example after an amp and before a filter. But not, between two amps (same element). Plot signals of like frequency on the same graph, but separate each axis to make signals clear. (6 pts)
- Plot the time domain waveform at the antenna. Can you see your signal? (2 pts)
- Plot the time domain waveform at the output. Can you see your signal? (2 pts)