ELEC 432

In the Lab

Our plan for this week is to combine the analog RF processing of the Ten-Tec RX320 with digital IF processing in Labview. Although the RX320 is a complete receiver on its own, it only supports AM, SSB, and Morse Code (CW) signals.

Part 1: The RX320



Setting up.

 The RX320 must be connected to the PC in order to function. If not already set up, connect the PC serial port to the SERIAL DATA connector. Connect the external antenna to the ANTENNA connector and plug one of the speakers into the EXTERNAL SPEAKER connector. Start the Ten-Tec Radio Contol Panel program (Programs->Ten-Tec PC RADIO->Ttrcx).

Listening.

There's no instruction manual for the RX320, but the online help gives a quick tour of the functions. The quickest way to set a new frequency is to left click on the frequency display (the large yellow numbers) and type the desired frequency (in MHz), followed by return. If you are tired of listening to AM broadcast stations, a reliable station to try is WWV, the NIST time service at 5, 10, and 15 MHz. 15 MHz will be the strongest frequency during daylight, and 5 MHz can usually be heard only late at night. Strong AM signals can also be found on the international broadcast bands. There's a list of international broadcast and amateur frequencies under Introduction->Listening to the World in the RX320D help file. Again, frequencies above 10 MHz will be best during the day, those below best at night.

Characterize RX320 response.

 Using the appropriate adapter, connect the RX320 I.F. Out to the scope (with a BNC-T) and the spectrum analyzer. Configure the spectrum analyzer as follows:
Center Frequency
12 kHz
Span
24 kHz
Level
-20 dBm
Set the RX320 mode to AM, and tune to 9 MHz. With the RF off, set the signal generator to:
Frequency
9.0 MHz
Amplitude
-100 dBm
Modulation
AM, 80%, INT 400 Hz, Modulation ON.
Connect the signal generator output to the RX320 ANTENNA input.

Turn on the signal generator RF output. You should be able to hear the 400 Hz tone and see a peak at 12 kHz on the spectrum analyzer.

Turn the modulation off. Reduce the signal generator output until you can barely see the 12 kHz peak. This is the minimum detectable signal (MDS) level. Increase the signal generator output in steps of 5 dBm until you reach an output of 0 dBm. At each step, note the output level shown on the spectrum analyzer. Make a plot of the output vs. input level. Note the point at which the output stops increasing with increasing input. This is the AGC (automatic gain control) threshold.

Reset the spectrum analyzer to 24 kHz center frequency, 48 kHz span, -20 dBm reference level. Set the signal generator output to -80 dBm. Increase the signal generator frequency in steps of 1 kHz, noting the amplitude of the spectrum analyzer peak at each step, until the peak is no longer visible. Make a plot of the frequency response. What is the IF bandwidth?

Part 2: The RX-320 as a Labview Radio Front End



Connecting the RX320.

 Reset the signal generator to 9.0 MHz, -80 dBm, and turn the modulation on. Disconnect the RX320 I.F. OUT from the spectrum analyzer and connect it to the NI 6251 A/D input. Connect the 6251 D/A output to one of the speakers.

The Labview Radio VI.

 Load the lv_radio1.vi file from the directory C:/432/lv_radio.
The waterfall and spectrum displays are similar to the ni_proc VI used in the previous two labs. The Multi-Purpose Display shows the waveform or spectrum at selectable points in the system. Except for the tuning controls, all other controls are in the set of tabbed panels marked "Parameters" in the screen image above. Each panel contains the controls for the corresponding block in the block diagram. Examine the block diagram and note where the controls are connected.

Tuning

 Receiving a signal requires setting two tuning frequencies: the first local oscillator in the RX320 front end, and the last local oscillator in the Labview VI. To tune the RX320, enter the desired frequency in the Tune To box (in the Ten-Tec Tune block) and press the Tune button. If the frequency of the desired signal is a multiple of 2.5 kHz, then it will appear at 12 kHz in the lv_radio IF, otherwise it will be necessary to offset the VI's tuning frequency appropriately.

The main tuning control has been overlaid on the waterfall display. This makes it easy to tune to a signal which has been identified on the waterfall. This is especially useful for tuning single sideband (SSB) signals. The main tuning control tunes in steps of 50 Hz and the fine tuning control tunes continuously 100 Hz to either size of this value.

Listening to the signal generator.

 Start the VI, tune to 9.0 MHz, and turn on the RF output of the signal generator. You should hear the tone from the speaker and see the carrier and sidebands on the spectrum and waterfall displays. Use the multi-function display to trace the signal through the processing chain. Try all of the AM demodulators and experiment with the IF and AF filters.

FM.

 Set the signal generator to FM, with 400 Hz modulation and 4 kHz deviation. Set the demodulator to PLL FM. Vary the modulating frequency and the deviation and observe how the spectrum changes. How accurate is our rule of thumb regarding the bandwidth of an FM signal?

Listening to real radio signals.

 Disconnect the signal generator, connect the antenna, and go in search of signals to demodulate. As mentioned above, AM signals should be plentiful, but finding some of the others may take a bit of work. When listening to AM signals, compare the performance of the three different demodulators (envelope, product, and synchronous AM).

One fairly reliable signal is the New Orleans Coast Guard weather station at 8.502 MHz. This station alternates between voice announcements in SSB and FAX images in FSK. It transmits regularly, but not continuously, during the daytime. Voice weather anouncements in SSB can sometimes also be heard at 15.034 MHz.

There aren't very many FM signals below 30 MHz, but there are plenty of FSK signals, which are just binary FM. Use the FM demodulator with the AF filter bandwidth set fairly low. The 12 MHz band has a number of FSK signals, as well as other digital signals. Some frequencies to try are 12.015 and 12.120.

Listening to recorded signals.

 If you can't find any interesting signals on the air, here are a few pre-recorded ones to use instead, located in the C:/432/signals directory.
DATA5.SCL
FSK fax
DATA10.SCL
SSB voice
DATA13.SCL
FSK data

Part 3: A Design Problem

The signals from WWV are packed with a wealth of useful information: highly accurate carrier frequencies and audio tones, voice announcements of time, weather, and radio propagation conditions, a digital time code containing time, date, and a daylight saving time flag, and 1-second ticks. More information than you would ever need to know about the WWV signals is available in the NIST Time and Frequency Users Manual. A less scholarly, more colorful treatment is available in NIST Time and Frequency Services.

Your task is to design and build a sub-VI which separates and counts the 1-second ticks from the WWV signal. Your design should work reliably as the received signal strength varies due to changes in propagation conditions.