Loopback Mode
There are seven loopback modes available:
No Loopback
Under this mode, the Signal Generator and the DAQ-related instruments (e.g. Oscilloscope, Spectrum Analyzer, Multimeter, Spectrum 3D Plot, Data Logger…) work independent within the software.
However, you can still establish a loopback via external means:
- Hardwired loopback via an external loopback cable
or
- Mixer-level loopback established via selecting "Wave Out Mix" or the like as the input source in the Recording Control under Windows Control Panel.
The "Wave Out Mix" may not be listed as an input source in the Recording Control by default, but you should be able to find it in the Options of the Recording Control. This method feeds the sound card DAC output directly to its ADC input through the sound card mixer, so there is some hardware involved. Be sure to adjust the "Wave Out Mix" volume properly to avoid possible input saturation.
iA=oA, iB=oB
Under this mode, a full loopback digital route is established at software level between the output channel and input channel, such that the signal from the Output Channel A is fed into the Input Channel A and the signal from the Output Channel B is fed into the Input Channel B.
During this mode, the Trigger Parameters and Sampling Parameters will be automatically set by the software, and will be disabled and remain not adjustable until the end of the signal output. The frame size of the Oscilloscope has to be 1 second. It should be noted that the signal fed into the input channels is "ideal" in the sense that it does not pass through any hardware circuit, neither the sound card output channels, nor the sound card input channels. This is different from the hardwired loopback and mixer-level loopback modes described previously.
iA=oA, iB=oA
This loopback mode is similar to the previous one except that both input channels is fed with the signal from the Output Channel A only. This mode is especially useful when you want to check the characteristics of a digital filter in the Oscilloscope and Spectrum Analyzer. In such cases, you should generate a proper stimulus from the Signal Generator and apply the digital filter to the Input Channel A only.
iB=oA
Under this mode, a partial loopback digital route is established at software level between the Output Channel A and the Input Channel B, such that the signal from the Output Channel A is fed into the Input Channel B and the Input Channel A is still available for field input. Only the first second of data from the Output Channel A will be fed into the Input Channel B. Note that the Oscilloscope should be set in dual channel mode in order to see the signal from the Signal Generator.
Sync. No Loopback
This mode is the same as the No Loopback mode, except that the start of the Signal Generator and the start of the Oscilloscope are synchronized. The synchronization works as follows:
- Stop the Oscilloscope manually if it is running
- Start the Signal Generator
The Oscilloscope will be started automatically after the Latency Time + the Delay Time specified by “Start OSC after (s)”. The latency time should be calibrated according to the procedure described previously in the section “Latency for Synchronized Output/Input”.
If you tick "Echo Only", the delay time will be automatically filled with the duration of the output signal, so that the Oscilloscope will start sampling just after the direct output signal finishes, i.e. only the echo will be captured.
Sync. iB=oA
This mode is the same as the iB=oA mode, except the synchronization between the Signal Generator and the Oscilloscope as described previously.
Sync. iB<--oA
This loopback mode requires an external cable to feed the signal from the Output Channel A into the Input Channel B. Note that as far as the loopback modes are concern, “=” means a software-level loopback. And “ß” means a hardwired loopback.
You can specify when to start the Oscilloscope after the signal has been output from the output channel by the signal generator. The Latency Time has no effect on this mode, because the software uses the actual signal acquired from the Output Channel B to determine when the front of the output signal actually arrives. So this mode is more accurate than “Sync. iB=oA” mode, but the drawback is that it requires two physical input channels with two input jacks and a hardwired loopback. Note that the Trigger Edge, Trigger Level and Trigger Delay must be set properly to allow accurate detection of the front of the output signal. The Trigger Mode will be forced to “Single” and the Trigger Source will be forced to “B” automatically by the software.
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