This section is part of DSP-10 User's Manual, Chapter 2 - Weak Signal Operation
This mode is a study tool for the use of Long-Term Integration to enhance the apparent signal to noise ratio of the received signal. It is capable of working with signals in the -180 to -190 dBm range, which is 30 to 40 dB weaker than can be copied by ear in CW. One may devise ways to use this mode to perform extremely slow communications, but the primary purpose is to explore the nature of difficult propagation paths and to allow determination of the potential for using other codes such as PUA43, LHL7 and CW.
In transmit, this mode is very similar to CW, except that provision is made for randomizing the frequency. In receive, the processing of the data and its presentation is very much like EME-2. The duty cycle in LTI can be raise to almost 100% which allows better sensitivity than can be achieved with EME-2, which is set at 40%.
The following material should be helpful in using the LTI mode. The emphasis is put on the controls and the functioning of the mode, rather than a specific "how to use it." This is primarily a measurement tool and the applications for the tool are mostly left to the creativity of the user!
Reception takes place with untriggered FFT spectral processing. Two displays are presented in the upper spectral display. The conventional "white" trace is at the bottom. Above this is a "yellow" trace with the LTI results.
A "Long-Term trace" is produced on the DSP-10 screen by simply averaging the total of the spectral signal powers that have been received. With the default DSP-10 colors, this produces a Yellow trace, and this is the common nickname for the upper display in this mode. The yellow trace can be set to not occupy the full width of the screen to reduce the amount of processing to update the display. The center operating frequency is marked by a tall red-line (when using the default colors). Detailed data is taken for 10 bins above the center frequency and 10 bins below. In addition, numerical data is available in a "data box" on the returns received for the 21 bins of special interest (activated by Alt-A).
Receive frequency correction for EME Doppler is available for LTI . Doppler correction for the EME path is applied to the receiver frequency so that the return will always line up with the center marker. The correction can be toggled on and off by ALT L, or ALT l. It is in effect only when an EME path is being displayed on the bottom line. An 'E' below the frequency readout means the feature is enabled.
It may take many thousands of FFT processes to produce an unambiguous signal trace. The yellow trace will start as noise and, in time, this will average towards a smooth line. At any frequency where a signal is present, there is a sum of signal and noise power and the trace will show a spike over the noise curve. This continues to be more obvious as time progresses. Because of variation in the receiver response across the frequency band, the noise trace will not be straight as one might wish. A flattener function fits the noise curve with a fourth order polynomial which can be subtracted from the yellow and white traces to make them nominally straight and horizontal.
Item by item in the Alt-B Box:
Display Fr Low, Hz - This is the lowest frequency that is displayed in both the white and yellow traces. It must be consistent with the SpecAnl width or a warning message will be generated and the value will be adjusted.
Center Freq, Hz - This is the frequency in the display that is over the tall red line. Detailed data is shown for this frequency along with 10 spectral bins above and below (see Alt-A box below). In transmit, this is the FSK offset above the Base Transmit Frequency before modification by a random frequency. It must be consistent with the SpecAnl width or a warning message will be generated and the value will be adjusted.
Display Fr High, Hz - This is the highest frequency that is displayed in both the white and yellow traces. It must be consistent with the SpecAnl width or a warning message will be generated and the value will be adjusted.
Random Frequency Spread, Hz, - The maximum total range that the transmitter and receiver frequencies are shifted to minimize problems due to "birdies" that can corrupt the data. The spreading changes each minute and is based on the same file of random numbers that is used in PUA43 mode. Too little spread allows birdies to cluster near the center and to be a source of interference. Too much spread makes it difficult to find an operating frequency that does not include birdies. The selection of this parameter depends on the operating environment. A value of 200 Hz is often adequate. To disable the random feature set this value to zero.
Record Data to Disk - Provides for a disk file that saves the important spectral data for later analysis. This feature is not normally used, but it gives a way to reanalyze the received data after Moon echoes have been received. The data format for the resulting files is in the appendix.
LTerm Trace max x dB/ - This series of radio buttons prevent the hiding of details in the yellow trace by a strong, off frequency signal. The yellow trace is self scaling and sometimes it is desirable to allow portions of the trace to run off the screen. Normally this is not a problem and the max can be left at the first button, 10 dB/.
Noise Blank, dB - Wide band noise can disrupt the LTI results by adding a large amount of power to all bins. This tends to cause the signal to sink into the noise. The average of the 18 noise measurement bins is compared with a running average of these same bins. If the current average exceeds the running average by more than the "Noise Blank, dB," all the data from that 5 second period is discarded. A value of 0 dB will exclude roughly half the data. A value of 99 dB will never exclude any data. Values around 1 dB are good starting points. When the data is discarded, the yellow trace is not shown and a note "*NB*" appear on the left side of the upper spectral box.
In LTI mode the audio portions of the receiver are fully operational, but have no interaction with the display. Controls such as Filter, LMS and AF Gain can be changed at will. The same is true of the settings that affect the upper and lower spectral displays, such as Contrast, Brightness, Trace Normalization, AutoDisplay, dB/div, dB Offset and SpecAve.
The transmit base frequency is displayed in the Transmit Freq Box and should be chosen to find a "birdie" free frequency.
Xmit Pwr - the duty cycle in the LTI mode can be adjusted by the message being sent. In turn, this adjusts the duty cycle duty cycle being transmitted. The transmit power should be kept consistent with the cooling capabilities of any following amplifiers and the selected duty cycle.
Clock Set - The software clock is needed if Randomization is applied to the transmit/receive frequencies. The accuracy requirements are relaxed from that required for some of the other modes and 0.1 seconds is quite sufficient, since the randomization only changes once per minute. See the Clock Setting section below for further information.
The windowing function (Alt-W, Alt-w or Scrl-W with kbd_alt2 = 1 in .CFG file) is programmable for None, Tukey-25 dB, Hamming or Blackman-harris 92 dB. These windowing functions allow trading off the selectivity of the FFT relative to the off frequency rejection. No windowing function gives the best sensitivity and can often be used. If there are problems with strong signals or birdies, one can use a windowing function such as Tukey-25 dB, or possibly Hamming.
The width of the spectral display is set with Alt-J. All FFT's are 1024 point, but the sample rate is 2400, 4800 or 9600 Hz. This produces bin resolution bandwidths, without windowing functions, of about 2.3, 4.6 and 9.2 Hz. The display in the left hand column is "SpecAnl Window Width" where Window is the windowing functions and Width is the 1200, 2400 or 9600 Hz. Experience to date shows that the 1200 Width can be used at frequencies up to 1296 MHz. It gives better sensitivity because of the reduced noise bandwidth. As an aside, when one cuts the bin bandwidth in half, the noise bandwidth is half and the S/N of a single FFT measurement improves by 3 dB. But, each FFT measurement takes twice as long and so in any given time there is only half as much non coherent integration. This decreases the advantage of the narrower bin bandwidth to 1.5 dB.
EME Doppler correction (Alt-L or At-l) is available in the LTI mode. To allow the Doppler correction, it is necessary to have a "Moon:" line at the bottom of the screen (Scrl-F3). This designates the latitude and longitude of the station or stations being used. The coordinates are set in the .CFG file as is explained there, and in the previous software notes. The receive frequency is shifted for Doppler correction and is shown just below the Transmit Frequency Box, reflecting the affects of both randomization and EME Doppler. To indicate that these corrections are being made, there is an 'R' on the left (if the randomization frequency spread is non zero) and an 'E' on the right (if EME Doppler is enabled).
The transmit frequency is the "base frequency" and the FSK modulation that modifies the transmit frequency is shown just above the Transmit Frequency Box. This system allows one to tune the frequency being used without ambiguity.
A non modal information box is available for the LTI display. This occupies the right edge of the spectral waterfall. As the display scrolls the data up, it gets covered by this box, but no data is actually lost. The 21 bin area is far enough to the left to never be covered by the Alt-A box. This box is both opened and closed by the Alt-A keyboard command.
The box is updated along with every screen update. The top line shows the number of data points that have been averaged together and a note "Rcd = N" or "Rcd = Y" to indicate the status of data recording. The equivalent amount of effective Signal to Noise improvement is shown on the right "xx.xdB." The third and fourth lines indicate the amount of signal found at the center frequency. The display shows "fffHz S+N/N=y.yyydB" and the next line is "Cntr Sig= -zzz dBm." The signal + noise/noise values are determined from the running average of the 18 noise bins. Calculations for each of the 21 center bins is made to determine the presence of a signal. The S+N/N value will be both + and - across the set of bins. Negative values are noise generated errors and correspond to signals that have negative power. Positive values allow the calculation of a signal level as shown in the "Cntr Sig=" line. An assumption needs to be made about the noise power density and this comes from the eme2_te entry in the .CFG file. The default value for this noise temperature is 290 degrees K and corresponds to -174 dBm/Hz. One can fill in a different value for the effective noise temperature, if it is known. This signal level takes into account the noise bandwidth of the FFT and windowing processes as well as the estimated noise power density.
Only the S+N/N values are shown for the 10 bins above and the 10 bins below the center frequency. These are shown to the left and right of N1, N2, ..., N10.
Finally at the bottom of the Alt-A box are the Randomization frequency shift in Hz and in the bottom line the Running Average and the Current Noise power values, in relative dB. This last line allows one to observe the operation of the Noise Blanker. The difference between the values is compared with the "Noise Blank, dB" setting from the Alt-B Box to see if noise blanking will occur.
This mode is capable of considerable sensitivity when operated for sufficient periods of time. It produces an estimate of the actual returned signal strength. It is very useful as a test device between two suitably equipped stations to insure that all elements are operating properly. This can be applied to terrestrial or EME paths.
The single biggest hardware requirement for the LTI mode is the frequency reference. The spectral bins are narrow and the frequency control for both transmitting and reception must be tight relative to the bin width. As is the case for the PUA43 mode the accuracy, at the final operating frequency should be 0.5 Hz or better. This can be relaxed by a factor of 2 or 4 if the SpecAnl width is 2400 or 4800 Hz, but the requirements are still tight.
To use the LTI mode, first select it (Alt-M) and open the Alt-B Box. Select parameters as described above. Close this box (Enter) and open the Alt-A box to show the result data. Reception begins immediately. When the transmitting station starts to send, you should clear the long-term data with a Ctrl-W. The yellow trace will first look like the white trace and probably start at 5 dB per division. After a few transmissions the yellow trace will begin to smooth into a better defined curve. As it does this, the scale of dB/division will reduce automatically and the noise will be reducing in magnitude. The signal will appear as a narrow spike at the tall red line. The S+N/N numbers and the center estimated signal levels will be displayed in the Alt-A box.
For the transmitting station, the Home key puts the transmitter into operation. This then behaves very much like CW. Hit a key for a CW character and it is shown on one of 3 lines at the top of the screen. This is sent in CW as soon as it is typed. To make a more useful LTI signal, the loop feature ('#' pound signs) should be used, along with the '{' character that will hold the key down for 2 seconds and '}' that will leave the key up for 2 seconds. So to continuously transmit 20 seconds of key down followed by a CW identification, one could enter "#{{{{{{{{{{ DE W7XYZ #". This can be entered while in receive, but the entered data will not show since it is being overwritten with spectral data. The ~ (tilde) is also available for longer key down time and is 10 seconds by default. This can be defined in the UHFA.CFG file to be any desired length. The previous message could be done as #~~ DE W7XYZ # , or if the interval has been set to 20 seconds, #~ DE W7XYZ #.
Let the process run until the signal becomes well defined from the noise. The amount of time required for this varies greatly depending on the station capabilities. Depending on the time available and the system noise temperature, it generally is possible to see good traces on signals in the -170 to -195 dBm range (audible copy for CW is about -145 to -155 dBm).