Dopplergrams on the EVM

This program operates in conjunction with a Motorola DSP56002EVM card to display very narrow frequency-domain spectra of audio signals. This is intended to be used for examining doppler-shifted radio signals reflected from moving objects or ionospheric layers.

Click one of the following topics or keep selecting >> to view all topics in turn.

Dopplergrams overview

Setting up the EVM board

Connecting to the receiver

Running the Dopplergram program

Viewing dopplergrams

Saving dopplergrams

Interpreting dopplergrams

The PLX boot ROM


Dopplergrams overview

In the study of propagation of high frequency (short-wave) radio signals, it is possible to gain some insight into the properties of the reflecting layers by using the fact that a moving layer will impart a small but measurable frequency-change to the radio signal reflected from it, known as a Doppler shift.

A dopplergram is a record of the doppler shift, or change in frequency, of a signal over a period of time. In this program, radio signals received on an SSB receiver are fed into a DSP card (the Motorola DSP56002EVM), which connects to the PC serial port, and the audio signal from the SSB receiver is analysed and displayed on screen as a chart with time represented along the X-axis. frequency along the Y-axis, the blackness of the display at the point (X,Y) showing the signal strength of the signal at that time and frequency.

By tuning in to the carrier frequency of a beacon or AM broadcast station or any number of other carriers and pilot tones of other signals in suitable locations and monitoring them over periods of time, many aspects of HF propagation can be studied with could never have been done before by amateurs.


Setting up the EVM
Setting up the DSP56002EVM

This program works only with the Motorola DSP56002EVM card. Consult the documentation with the EVM card for full instructions on the use of this card in general. Become familiar with the debugger program, especially the operations of loading and running DSP programs. To run this program, the following points should be noted:

>	A DB9 socket, identical to the one used for the OnCE port, should be fitted into the HOST P5 position.

>	A total of four jumpers should be fitted in the J8 position, all parallel with the short edge of the card.

>	The J12 jumper should be in the 16k position.

>	Jumpers should be fitted in all positions of J10 except PC3.

>	To speed up the start-up of the Dopplergram program, you may wish to fit an EPROM or FLASH ROM in the U10 position, so a 28-pin DIL socket can be fitted. See the PLX boot ROM topic for further details.


Connecting the receiver

Connect a screened audio cable from the auxiliary audio output of your SSB receiver to the IN jack (J9). The braid connects to the sleeve of the jack plug and the centre wire to the tip. This connects the receiver audio to the left-hand stereo input of the EVM. The EVM will overload if the input level exceeds 2v rms (5.6v pk-pk). The Dopplergram program can increase the input gain of the EVM by up to 22.5dB, thus increasing the sensitivity but at the expense of lower overload level. The Dopplergram software has a further dynamic range of over 60dB, so signals at least as low as 0.2mV rms will be clearly visible.

It will also be necessary to operate the receiver with AGC off to receive dopplergrams. At the very narrow bandwidths used, the AGC action in the receiver is effectively modulating the signal. This can give rise to spurious signals on the dopplergram.

The receiver should be set in USB mode, so that an increase in the received radio frequency will be reproduced as an increase in the audio frequency, and hence show on the dopplergram as an upward shift of the trace. Note that to tune in a radio signal on a specific frequency, the radio dial should be set lower than the desired frequency by an offset, to put the signal in the centre of the receiver passband, and the Dopplergram centre frequency should then be set to this offset value, for example, 1000Hz.


Running the Dopplergram program

To produce a trace on the dopplergram and spectrum displays, the EVM must be running the EVMDOP.CLD program and the host port on the EVM must be connected to a serial port on the PC, and the Dopplergram program must be set to recognise this serial port.

To set the serial port, select the SETUP menu, and click on the COM port number to which you have connected the EVM.

There are two ways to start the EVMDOP.CLD program running in the EVM:

>	Start the EVMDOP.CLD programming running on the EVM before you start to run this program, using the EVM56K or EVM56KW debugger program that came with the EVM. This will involve connecting the EVM OnCE port connector to the PC serial port, starting the debugger, and following the procedure to load and run the EVMDOP.CLD file. When you have done this, swap the serial port cable at the EVM end from the OnCE port to the HOST port, then start this program from inside WINDOWS.

>	If you fit an EPROM or FLASH ROM with the PLXBOOT utility programmed into it, then the EVMDOP.CLD program can be loaded and started from within EVMDOP.EXE without the time-consuming process of running the debugger and swapping the serial port cable, which can be left permanently in the HOST port socket. The startup of EVMDOP.CLD through the HOST port only takes about 1 sec compared with about 30 secs via the debugger. With the PLXBOOT ROM fitted, you can also restart EVMDOP.CLD without quitting this program, using the FILE+RELOAD menu.

When you have chosen how you wish to start the EVM, select the SETUP menu and if you have chosen to fit the ROM, enable the PLXBOOT box in the SETUP box. Don't enable PLXBOOT if you will be starting the EVM from the debugger.

The EVM will accept audio signals up to 2v rms without overloading the ADC. It is possible to increase the gain of the EVM by up to 22.5 dB, but with a consequent reduction in the overload level to 150mV rms. To do this, use the up/down buttons next to the INPUT GAIN box in the SETUP box to set the required input gain level.

When you exit the SETUP box, the chosen COM port, input gain setting and startup method will be recorded in the EVMDOP.INI file, or you can edit this file by hand.


Viewing dopplergrams

As you start the program, the dopplergram trace will start to build up on the screen. The horizontal centre-line of the dopplergram represents an audio frequency of 1000Hz, and the top-to-bottom extent of the dopplergram screen represents a span of 80Hz, that is a range of audio frequencies from 960Hz to 1040Hz. The dopplergram trace is building up at the rate of 1 sample every 1.28 secs. The upper spectrum display shows the signal levels over this frequency span. A signal which appears at the top of the spectrum display will give a black trace on the dopplergram display and a signal 60dB below it will appear on the bottom edge of the spectrum display and as white on the dopplergram. The frequency and signal level can be read off from any point of the spectrum analyser or dopplergram screen by pointing to it with the mouse cursor. The centre frequency, scan width, display gain, and sample interval can all be adjusted with the on-screen controls. To see more about each of these controls, click on a topic below

Setting centre frequency

Setting scan width

Setting display gain

Setting sample interval


Setting centre frequency
Setting the centre frequency

The centre frequency of the dopplergram can be set in several ways:

>	Get the flashing cursor into the white frequency display box next to the "Centre Frequency" label, by using the TAB key or by clicking the mouse in this box, then edit the displayed frequency. Hit the ENTER key to actually make the frequency change. If you type it wrong the computer will beep and the display will go back to the original

>	Use the up and down buttons next to the frequency display box to increase or decrease the frequency. These buttons work in steps of one tenth of the width of the spectrum display.

>	If you wish to centre the display to the largest signal visible on the spectrum, click on the CENTRE button in the Peak box.

>	If you wish to centre the display on any other point in the spectrum, click on that point in the spectrum display itself. You can also set the centre-frequency by clicking in the dopplergram display, but if you have previously changed the centre frequency or scan width while the dopplergram was running, you will not get the right result if you click to the left of the point where you made the last change.

Note that when you make a frequency change, it will take some time for this change to take effect. The narrower the scan width the longer it will take. See the scan width topic for more about this effect.


Setting scan Width

The scan width can be set to 80Hz, 40Hz, 20Hz, 10Hz, 5Hz, or 2.5Hz by means of the up-down buttons next to the Scan Width label.

The narrower you make the scan width, the slower will be the response-time. This is inherent in the physics and is not a program limitation. As you change the scan width, the program will automatically change the Sample Interval to the setting that is optimum for the selected scan width. You can override this if you wish, See the sample-interval topic for more about this.

When you change the scan width, the increment of the frequency up/down buttons changes so that these buttons always move the centre-frequency by one-tenth of the scan width.


Setting display gain

The display gain can be adjusted in 10dB steps by means of the up/down buttons next to the Display Gain label.

Note that when the display gain is set to 0dB, the spectrum display will show the signals in such a way that the top of the spectrum display represents the maximum signal level that can be handled by the EVM. If the spectrum display is off the top when the display gain is set to 0db, there is a risk that the signal will be distorted. This will show as spurious signals on the spectrum display and the dopplergram which are not actually there. If this happens, reduce the audio input level or reduce the EVM input gain setting in the SETUP menu.

Provided that no part of the spectrum display goes off the top with the display gain set to 0dB, there is no problem in turning up the display gain so that the largest signals go off the top, if this is necessary in order to get a good display of the weaker signals.


Setting sample interval
Settting the sample interval

The sample interval sets the speed at which the dopplergram trace builds up across the screen. It is normally automatically selected when the scan width is set, to the value which is optimum for that scan width.

However, it can be overridden manually by using the Sample Interval up/down buttons. These change the sample interval in the sequence 1.28, 2.56, 5.12, and so on in multiples of two. You may want to slow down the dopplergram trace speed by increasing the sample interval above the optimum setting if you want to monitor a wide scan width over a long period. However, this will effectively be taking separate "snapshots" of the signal rather than a continuous trace, so the penalty for doing this slower trace is that you might miss short events which occur between samples.

You can also select a shorter sample interval than the optimum, but there is probably no practical reason for doing so: the dopplergram trace just looks blurred.

The setting of the sample interval that is selected automatically by the scan width buttons, is half of the length of time that the system will take to respond to a frequency or scan width change. This means that any received event will be fully recorded. For example, a very short event such as a pulse, occurring exactly half-way between sample instants, will be recorded at half amplitude in both the preceding and following samples.


Saving Dopplergrams

You can keep the dopplergrams generated by this program for future reference either by saving the dopplergram to a file or by copying it to the clipboard.

>	To save to file, first get the flashing cursor into the Comments window by using the TAB key or by clicking in this window. Enter a title or comments about the dopplergram you wish to save, for example the radio frequency or transmitter name, then select the FILE menu then the SAVE sub-menu, and the familiar WINDOWS save dialogue box will appear. The dopplergram will be saved with the date, time, centre frequency, scan width and your comments added along the bottom. Although the dopplergram itself has 256 grey-levels, the saved file is in .BMP format with 16M colours.

>	The dopplergram can be copied to the clipboard using the EDIT then COPY menus. The dopplergram image can then be passed to another application in which it can be edited, saved, or converted into another format.

Previously-saved dopplergram images can also be viewed in this program by using the FILE OPEN menu, and dopplergram images on the clipboard can be pasted into the program with the EDIT PASTE menu. Be careful that opening or pasting images that are not dopplergrams may give unpredictable results, and that using the mouse to read frequencies and levels from a dopplergram imported in either of these ways will only give the right figures if the centre-frequency, scan width, and display gain settings are the same as they were when the original dopplergram was saved.


Interpreting dopplergrams

This is a subject that cannot be covered in a single help topic, and maybe not at all, since the whole area of Doppler study of HF propagation has not yet been fully explored. The interested user is referred to the article "Using Doppler DSP to study HF propagation" in the journal Radio Communication May 1998.

Here are a few short extracts from that article.

>	Dopplergrams of aircraft reflections can be used to give some idea of the distance of an aircraft. The area swept out between the centre-line and the aircraft trace over a certain time interval, measured in units of Hz*sec, is numerically equal to the number of wavelengths by which the path from the distant transmitter, via the aircraft, to your receiver, has changed over that time interval. Other techniques can be used to calculate the path distance via the aircraft at it's point of nearest approach.

>	Meteor scatter pings show doppler-shift which give a measure of the wind-speed in the E-layer. The spread of wind-speeds within an individual ping can give a measure of the wind-sheer, which may have significance for sporadic-E study.

>	Dopplergrams of reflections via the F-layer just above the critical frequency show vertical motion of the F-layer and the magnitude of the height-change can be calculated by the area-swept-out method. Dopplergrams of the F-layer as it fades in or out can similarly give information about the thickness structure of the layer.

>	There may be many as-yet undiscovered propagation effects which the dopplergram technique may help to identify.


The PLXBOOT ROM

Loading and starting the EVMDOP.CLD program in the EVM card via the EVM56 debugger is slow and cumbersome, requiring that the serial port cable be swapped between sockets. The PLXBOOT utility distributed with this program (as PLXBOOT.ZIP), contains a PLXBOOT.CLD program, which when loaded and run in the EVM under the debugger, can program a FLASH ROM which will enable the EVM to load and run the EVMDOP.CLD program via the host port rather than the OnCE port, and indeed much faster. The same ROM can be used to load and run other EVM programs using the LOADEVM.EXE utility distributed with this program. See the PLXBOOT.TXT file in the PLXBOOT.ZIP utility for further information.