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Presented here spectrum analyzer is based on TV head. The device has a wide measure range and developed software for PC and microcontroller. TV head operates in the range of 45-860MHz – the device was adapted to operate within 45-2000MHz. To make it possible, the spectrum analyzer also uses a head pf satellite tuner, which operates within 950-2050MHz. Thus, you get two non-overlapping measure ranges.
The device is a triple/double superheterodyne. Used SAT head from Samsung is not equipped with PLL loop inside. There is also a MB506 prescaler. In the original device it divided the signal by 128, but it needs to be changed to 64. TSA5512 acts as external PLL loop. Additionally, discriminator CXA1165 was disconnected. Intermediate frequency signal was moved out of the head via drilled hole. Strengthening both heads and thus the whole system is regulated in a promitive way through the AGC pins. SAT and TV head is followed by the intermediate frequency filters. There is a line filter added to the ouput of SAT head. It consists of three resonators, shorted by capacities. The coupling with the source and load is based on induction loops which results in unbalanced edges. Filter added to the TV head is a normal centered filter, 4-circuit. The coupling between the cells is capacitive. Both filters were implemented as Chebyshev filters. The second transition of the frequencies for both heads and switching between the bands was made on the basis of TDA5630. It is not a perfect solution because of the high gain of TDA5630, but certainly it is a simple one.
TDA5630 implements the following transitions of the frequencies:
- 38,9MHz/10,7MHz (for the TV head)
- 476,62MHz/10,7MHz (for the SAT head)
Presented above solution provides an intermediate frequency signal for both measure bands. There is a ceramic filter at the output of the TDA5630. Then there are three independent paths. The first of them is a synchronous detector on television TDA2541 (broadband path – 300KHz bandwidth). The second consists of third transition of the frequency 10,7MHz/455KHz (NE612) and identical synchronous detector (narrowband path – 15KHz bandwidth). The last one is a discriminator (TDA1047) and low frequency path – it is an ordinary FM listening.
The whole is controlled by Atmega16 via I2C. Read of the Y axis is performed linearly or logarithmically (more dynamic). Logarithm is performed in software, with a simple array. Adequately prepared signals from the sunchronous detectors are processed by the A/C converter of Atmega and presented in the form of amplitude spectrum on the graphic display LCD with T6963C controller. Program for Atmega was written in C (AVR-GCC). Communication with PC is done via RS232, which allows you to make display screenshots and save them in BMP file. The device also uses a simple protocol of „Q-R”.
Schematic diagram of the analog part:
There is a PLL loop for the transition after the SAT head. It is a loop with a premixer. The signal from the output of the heterodyne of TDA5630 undergoes a transition of the frequency to 32MHz. The heterodyne is made on a SAW resonator – 433,92MHz. Switching the measure ranges is implemented by shorting the voltage divider via BS pin of TSA6057.
Parameters and features of the device:
- input frequency rages: 45-860MHz; 950-2000MHz
- bandwidth of the measure path: 15KHz; 3000KHz
- tuning step: 1,25KHz; 12,5KHz; 62,5KHz; 250KHz; 4MHz; 8MHz
- the maximum width of the spectrum displayed on the screen: 1048MHz
- sensitivity: about 15uV
- power voltage: 230V 50Hz
- power consumption: less than 30W
- input impedance: 75ohm
- control: pulser and keypad
- work with PC: RS232C interface
- speed: about 50 measurements per second
- measurement results presentation: graphical LCD display
- resolution of the read box of the LCD display: 240x128 pixels
Pictures of the device:
And measurements made using the device.
Spectrum of signal with FM modulation. From the left: each parameter settings, without modulation, spectrum for Δf=3KHz, spectrum for Δf=10KHz.
The above pictures are screenshots sent from the LCD display to the PC and saved in BMP. You can create such printscreen any moment.
Spectrum of the signal with AM modulation. Frequency of the modulating signal: 24KHz. Carrier: 204,99375MHz, the upper band: 205,015MHz, the bottom band: 204,96625MHz. m=70%.
Spectrum of the signal with AM modulation. Frequency of the modulating signal: 1MHz. Carrier: 266,55MHz, the upper band: 267,4855MHz, the bottom band: 265,4875MHz. m=50%.
Spectrum of the signal of TV channel. From the left: analyse parameters, spectrum in logarythmic scale, spectrum in linear scale.
Both spectra were made with the same gain of the device. You can see the main advantage of the logarythmic scale. It enables better identification of low amplitude components and you can see chroma subcarrier more clearly.
Another spectrum of the signal of different TV channel.
And other pictures:
You can see here double-band modulation, lack of audio and chrominance.
Spectrum of the signal with CPFSK modulation.
It is a FSK with continuous phase. It was made by adding rectangular signal 1MHz to the input of FM modulation in generator G-4-116. In that way you can gain a symbol, bit rate modulation of 2Mb/s. The spectrum consists of bands, because the modulating signal is deterministic. Almost the whole analyzed signal is periodic, but it does not matter.
Spectrum of UKF band in linear scale.
Measurement in range of 900-2000MHz.
Link to original thread (useful attachment) - Analizator widma - wersja II