Memoria 2 (2015)Pràctica Inglés
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CHARACTERISTIC PARAMETERS OF A
1 Leyre Mendiolagaray Oscar Liñan ACTIVITY 2.1: Measurement of the Receiver Sensitivity. All the receivers, including spectrum analyzers, generate some internal noise that limits its capability to deal with small signals. The Sensitivity is the parameter that characterizes this feature, and is given by the noise floor in dBm of the instrument for a particular IF filter bandwidth, usually the smaller Resolution Bandwidth setting. In this activity we ask you to measure the sensitivity of your SA acting as a receiver of 10 kHz bandwidth signals, with a carrier frequency between 500 and 1500 kHz and with a minimum Signal-to-Noise Ratio (SNR) of 30dB at the demodulator output. Follow the next procedure to measure the Sensitivity.
Configure the SA to operate with the signals with a carrier frequency between 500 and 1500 kHz and 10 kHz of bandwidth. Set the attenuation to 0dB. Select your own carrier frequency.
Without connecting any signal to the receiver input, decrease the Reference Level until you observe the noise floor.
Measure the noise level setting Span to Zero and decrease the Video Filter bandwidth until the trace is almost flat. You may use Display Line.
The Receiver Sensitivity for a 30dB of SNR would then be the noise floor level plus 30dB. Write down the Receiver Sensitivity of your SA and justify the procedure followed.
First we set the span to 2 MHz, the central frequency to 1 MHz, and the bandwidth to 10 KHz. Then we put down the reference level until -30dB, and set the zero span mode on, to see the envelope of the received signal.
After that, we decrease the video filter bandwidth, in order to measure the noise level we use the option “Display line” so we get a measure of -108.1 dBm. The receiver sensibility is of -78.1 dBm.
ACTIVITY 2.2: Measurement of Third Order Intercept point. The objective of this activity is to compute the TOI point of the SA using the equation TOI=Pi+Δ/2. For this purpose, generate two tons of 900 kHz and 950 kHz frequencies using the signals generators of 600Ω and 50Ω internal impedance, respectively. Combine these signals using a power splitter for this exercise, which consists in a resistive combining network matched to each generator’s internal impedance. Apply the sum of these tones to the receiver RF input. Note that the TOI value is highly sensitive to the frequency range; therefore, the TOI measured in this exercise is valid for the AM range. Then, configure the receiver in swept mode with an Attenuation of 0dB, and select the Center Frequency and Span to visualize both the two tones and the third order intermodulation products on the screen. Finally, adjust the level of the two input tones to -20dBm.
At this stage, you might have to reduce the noise level that masks the third order intermodulation products to properly measure the power level. If so, reduce the bandwidth of the Video filter and the Resolution filter to set the noise level at least 60dB below the input tone until the two intermodulation products are clearly visible.
2 Leyre Mendiolagaray Oscar Liñan Another aspect to consider before computing the TOI with the equation is that the SA is working well below saturation. Note that equation is valid only for medium input power values where the SA works below saturation since a compression gain effect is given otherwise. To verify this, switch the RF Attenuator from 0 to 10dB. You should observe a change in the power level of the input tones, it means that the SA is in saturation and you have to decrease the input tones power well below -20dB until this effect is not observed. Explain in your report why this procedure is useful to verify whether the SA is working in saturation or not.
Compute the TOI point value using the equation and explain this activity in your report.
This is the image that we observe in the SA: To measure the TOI, we generate two tons of 900 KHz and 950 KHz using the signals generators of 600Ω and 50Ω internal impedance respectively then we adjust the amplitudes to have power output of -20 dBm.
To visualize this input we set the span to 250 KHz and the central frequency to 925MHz and reduce the VBW until 30 Hz to see the received signal properly. To obtain the TOI we measure the amplitude of the 3r order harmonics which gives us the result of -79,4 dBm at 1,091 MHz and -77,8 dBm at 846 KHz, then we found the ratio between Pi and the power of the third order intermodulation products (Δ).
The TOI that we can measure here is: TOI = Pi + 𝛥⁄2 = 8.6 ACTIVITY 2.3: Measurement of the Receiver Selectivity. Following the procedure described in Q2.2, measure the Selectivity of your SA with the Resolution Bandwidth set to 10kHz as the ratio between the -50dB bandwidth and the -3dB bandwidth. Additionally, measure the bandwidth of the Resolution filter at [-5, -10, -20, -30] dB. You may use the SA function “N dB Down”.
The resolution filter of the SA has a Gaussian transfer function as the following one: 3 Leyre Mendiolagaray Oscar Liñan Using the bandwidth measurements of the Resolution filter, adjust these measurements to a Gaussian shape using the polyfit.m function of Matlab and determine the variance σ2. This activity should be done at home and included in the report.
For the measurement of the Receiver Selectivity of the SA, first we set the span to 50kHz, the central frequency to 900kHz, and the RBW to 10kHz. Then we measure the RBW at -3 dB, -5 dB, -10 dB, -20 dB, -30 dB, -50 dB.
Bw=13kHz Bw=18.5kHz Bw=25.9kHz Bw=31.9kHz Bw=10.2kHz Bw=41.9kHz 4 Leyre Mendiolagaray Oscar Liñan N dB down Frequency -3 10.2kHz -5 13kHz -10 18.5kHz -20 25.9kHz -30 31.9kHz -50 41.9kHz This was the Matlab program we used with the polyfit function to find the value of the variance 𝜎 2 : The second coefficient calculated by the function is equals to − 10·log(𝑒) , therefore the variance is 8,68 · 10−5 𝜎2 5 Leyre Mendiolagaray Oscar Liñan QUESTION 2.3: Interference caused by image frequencies. Solve the following exercise on interference caused by image frequencies at home and include it in the report.
Exercise. The figure shows a two-stage spectrum analyzer: (Solution in the next page) 6 Leyre Mendiolagaray Oscar Liñan 7 Leyre Mendiolagaray Oscar Liñan 8 ...