Time-domain scope

Still under construction !
Last modified in May 2004

Contents

• Introduction
• Oscilloscope mode
• Trigger modes
• Phase meters
• Presets
• Loran table  (for precise-sync trigger)

See also:    SpecLab's main index , Component window , sample rate calibration .

Introduction

Besides the analysis of signals in the frequency domain, you can use Spectrum Lab for an analysis of waveforms in the time domain (through I highly recommend to use a real oscilloscope for this purpose ;-).

To open the time-domain scope in SpecLab, select "View/Windows"..."Time Domain Scope" in SL's main menu, or click on the Time Domain Scope box in Spectrum Lab's circuit window. Please note that even if the time domain scope's window is visible, the scope function itself may be turned off to save CPU power (especially on slower machines you will see that the scope is a big eater of calculation power). You can turn the scope function on and off through the scope's "Mode" menu.

One or two input channels may (or must, depending on the mode) be connected to the scope. You can select the sources in the combo boxes on the left side of the scope screen, but it's easier to select the inputs in the circuit window, where you can also see all available sources.

Note: If any of the inputs is connected to a "currently unavailable" source, the scope display may pause (for example, if one of the scope's inputs is connected to the right input channel of the soundcard, with the card not running in stereo mode). Usually, only available sources are listed in the combo box, but if you change the audio settings afterwards these sources may become unavailable.

The scope can be triggered either with its built-in trigger (which is quite simple) or with the external "Universal Trigger Block" (part of the test circuit, offers more flexibility). The universal trigger can be used to trigger the scope and the spectrum analyzer simultaneously, which is impossible with the scope's built-in trigger.

All scope settings can be saved as part of a Spectrum Lab configuration file (*.ini or *.usr), so you can quickly switch from one configuration to another through SL's Quick Settings menu. For many common applications, there is a set of preconfigured settings (scope settings only) in the scope's menu ("Presets"). There is ...

• a two-channel oscilloscope (which you can turn into a single-channel scope easily)
• several frequency-selective phase meters
• a very special application called "Loran monitor"
• and another preconfigured setup called "Lissajous figure", which is in fact the X/Y scope mode without trigger, used for phase/frequency comparations in the old days of analog oscilloscopes. Use it for nostalgic reasons or educational purposes - remember the lissajous figure of two signals of same amplitude, same frequency but 90° phase difference ? Some radio amateurs use similar displays to tune the mark/space-filters in their RTTY decoder.

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Oscilloscope mode

In "oscilloscope" mode, a signal can be plotted as a function of time. It's possible to

• display one or two channels with common or individual vertical scaling

• use one of different trigger modes

• reduce the bandwidth of the input (by decimation)

• display peaks and/or average value, depending on the timebase settings

Like most oscilloscope, this scope can also be switched into X/Y mode. In this mode, both channels must be active (turned on in the "Mode" menu). The first channel is used for X-deflection, the second for Y-deflection. The trigger is pretty useless in X/Y mode, though it works. An important parameter for X/Y mode is the persistance, which can be set on the "Display" tab under "Miscellaneous". A persistance of zero means eternal persistance, which means pixels will not disappear once they are plotted !

Trigger modes

There is a simple trigger function, mainly intended to be used in Y(t) mode ("oscilloscope") but you can use the trigger also in other modes. The trigger settings can be defined on the scope's "Acquisition + Trigger" tab, where you will find these options - or maybe more:

Trigger Mode : Select one of the following:

• OFF (untriggered roll)

• AUTO: If a trigger signal is detected with the current 'level' setting, the display is triggered, otherwise the display will be updated without a trigger. This is also the default mode for many analog oscilloscopes, because "you always see something even with the wrong trigger settings".

• NORMAL: The display will only be updated if there is a trigger event. Without a trigger signal, the display will freeze (usually due to a wrong setting of the trigger level). As with analog oscilloscopes, the AUTO mode is more frequently used than the NORMAL mode.

• SINGLE SHOT: The trigger only fires once, then the display will freeze until you start it again (manually).

Slope: Allows you to select

• Rising or

• Falling edge of the trigger signal.

Source: Defines the source of the trigger signal..

• Sync Interval Generator (internal)

• Channel 1 (internal)

• Channel 2 (internal)

• External Trigger: If this option is selected, the "Universal Trigger Block" (part of the test circuit) replaces the scope's internal trigger. In this mode, all other controls in the "Trigger" control panel have no function - including the trigger mode, slope, coupling, etc because the Universal Trigger has its own control panel (a popup menu in the circuit window).

Trigger coupling: Select

• "DC" for direct current, or better

• "AC" for alternating current here, because some soundcards have a quite large offset error, which means the permanent level is not zero if there is no input signal. It may be as high as "300" on an ADC value range of +/- 32767 !

Pretrigger: Most analog oscilloscopes start the scan in the moment when the trigger fires, so the trigger event can often not be displayed completely. This is the case when the pretrigger is set to 0 % (percent). With 50 percent, the trigger event will be visible in the middle of the scope area. With 100 percent pretrigger, you will only see the PRE-TRIGGER history (pre = "before"), and the trigger event would be at the right edge of the screen.

Level: The trigger will fire when the input signal crosses this level (~voltage). The possible range is +/- 32767, because mostly 16-bit A/D converters are used (and, if not, the value range will be internally scaled to +/-32k as floating point numbers so there will be no difference when using a soundcard with 24 bits of analog resolution.

tSync: Interval time of the sync generator in seconds. Only has an effect if the trigger source is set to "Sync Interval Generator". This can be used if to observe periodic signals with well-known frequency, for example the 50 (60) Hz mains, etc. Below the edit field, the reciprocal of the interval time (= the frequency) is displayed. Remeber that -like in many other input fields of SL- you can not only enter a fixed numerical value but also an expression ("formula"). So, for example, if you want to set the sync interval generator to 60 Hertz, enter "1/60" in the field and hit the Enter/Return key to apply the edit. After evaluation, the calculated result will be visible in the edit field.
The sync generator was a nice tool to play with when hunting for worldwide Loran stations as explained here.

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Phase meters

In the scope's "phase meter" mode, amplitude and phase are plotted as a function of time. It's possible to

• display one or two channels

• use individual or common 'oscillators' for both channels

• set the bandwidth for both channels (realized by low-pass filtering decimation)

The basic function of a phase meter (as implemented here) is this:

• Multiply the incoming signal with a local oscillator, here: a numerical controlled oscillator with two outputs (90°)

• Low-pass filter and decimate the mixed signal until the required (low) bandwidth is reached

• Calculate the amplitude and the phase angle of the decimated signal

Usually, the 'reference' clock is the soundcard's internal sample rate. It must be accurately set. Spectrum Lab offers two different ways to "calibrate" the sample rate. For long-term phase observations, you should use the "permanent sample rate calibration". It is possible to use any reference frequency, not necessarily the frequency you want to observe with the phase meter ! Example: Use a weak 15625 Hz-signal from your TV to permanently calibrate the sample rate, while plotting phase graphs for a 600 Hz- and a 850 Hz- signal on the screen. (Note: in older versions of Spectrum Lab one of the two phase meters had to be used as 'reference' to calibrate the sample rate. This limitation does no longer exist, now both phase meter channels can be used to watch 'unknown' signals. However, you can still use one of the phase meters to monitor your 'reference signal' to detect phase jumps etc).

See also: Preset phase meters

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Presets

For a quick start, there are a couple of 'preset' configurations for the time domain scope.

Here just some of the presets, there may already be more:

Two Channel Oscilloscope

a dual-channel oscilloscope. Channel 1 coupled to the left audio input (ADC), Channel 2 to the right channel. Both vertical scales are set for 'maximum range' (+-32767) so a common vertical axis appears. Timebase settings are set for 'no decimation', trigger is enabled, trigger source is channel 1. If no trigger signal is detected for some time, the scan starts automatically ('auto-trigger').

Phase meters for 15625 Hz, 800 Hz and 650 Hz

Turns the time-domain scope into a phase meter with one or two channels. The 'local oscillator' for the first channel is set to 15625 Hz, which is the TV 'line' sync frequency. If there is a TV running close to your PC, a piece of wire connected to the left audio input should be enough to see the phase and amplitude of the sync signal. You can use this to verify the calibration of the soundcard, if your local TV broadcaster has the sync signal locked to an atomic clock (which is the case for the German ZDF program).
The '800 Hz' and '650 Hz' phase meters run at slower sampling rates (better for slow PCs, 11025 Hz audio sample rate is enough for most shortwave and longwave receivers).

Loran Monitor

A special application for longwave listeners. Tune your AM receiver to 100kHz and connect it to the soundcard. The preset 'Loran Monitor' is a special Y(t) diagram with the trigger driven by a sync-interval generator. The interval is (by default) set to 0.07499 seconds, which is the Group Repetition Interval (GRI) of the Sylt and Lessay transmitter. The display runs in '64 value average' mode to reject noise. After a couple of seconds, the Loran pulse train should appear. With an other GRI value (can be found in the appendix), you may be able to dig other Loran stations out of the noise. If required, set the 'average' value higher. If the pulse train slowly moves left or right, the sampling rate of your soundcard is not properly calibrated. The Loran transmitters (if they still exist) are used as navigation aid, their stability is excellent, similar to the more recent GPS system (but Loran is much simpler to receive).
Note: The average value is drawn as a line, while the min+max values are only drawn here as single dots. This avoids a crowded screen, but you can still see the pulses from other Loran transmitters (with different GRI's).

Lissajous figure

Channel 1 is used for 'X' deflection, channel 2 for the 'Y' deflection. This can be used for crude phase comparation (circle = 90° phase difference etc), or as a RTTY tuning scope (connect Ch1 to a MARK frequency filter and Ch2 to a SPACE frequency filter, realized either by software or hardware).
Note: The 'persistance' value of the display can be adjusted, the optimum value for a good and not too crowded display depends on the signal frequency and the amount of noise.

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Appendix

Loran table

If you have a longwave receiver for 100kHz (AM or SSB), you can use the preset "Loran Monitor" to watch the pulses on the screen. The following table has the "Group Repetion Intervals" for most Loran stations worldwide:

Loran Table

Note: A GRI of 7499 is a group repetition interval of 0.07499 seconds.
This value must be entered in the 'tSync' field on the trigger sheet if you want to dig such a signal out of the noise with high "average" value..