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Since I’m just a noob when it comes to spectrum analyzers, I have a question when grabbing real-time IQ data from the BB60C.
In the example MATLAB code in bbgetiq.m provided in the SDK:
iqlen = 1024; % Flush IQ data filter ramp up time [status, iqdata] = bbgetiq(handle, iqlen, true); % Get actual data [status, iqdata] = bbgetiq(handle, iqlen, true);If I’m passing true as the purge argument, then why does the example call the bbgetiq function twice? According to the BB API manual, specifying purge = TRUE discards “any samples acquired by the API since the last time and(sic) bbGetIQ function was called.
It seems that if it were set to false you’d have to call it twice, but if true wouldn’t that accomplish the intent here?
Hi Jgauthier,
This is a good question. The reason this is called twice has less to do with the purge parameter, and more to do with wanting to discard the first chunk of I/Q samples in the stream. Your assumptions about the purge parameter are correct. There are potentially several FIR filters (depending on settings) that are in the signal processing chain when I/Q streaming. These filters take hundreds/thousands of samples to ‘charge’ and until they do, the I/Q samples will not be amplitude accurate. By calling the function once with the purge parameter, this ensures that the first large segment of I/Q samples subject to this charge time are discarded. This is not necessary, but if you do this, doing this only once after each configure is adequate.
Another interesting component of this, the first call to GetIQ might not purge any samples, as there may not be any samples collected yet on the PC. So the worst case scenario is that the first call is just waiting until some samples arrive on the PC, and the second call is the one that actually purges that first chunk of I/Q to arrive.
Feel free to try just 1 call and see if that affects your application.
Also, waiting some small amount of time (100ms) before calling getIQ after configuring the receiver would ensure that the first call to GetIQ purges the first samples.
We chose to give these samples to the user instead of discard them in the API as they could still contain information just at a reduced amplitude.
If you are using any sort of triggering to acquire your waveform, calling the function twice is probably not necessary.
Regards,
AndrewJGauthier,
You will need to write a dedicated function for that. A wrapper function around the GetIQ functionality should be adequate for performing any triggering on the I/Q data. You could perform video or external triggering with this function. (you didn’t specify trigger type in your message) If external triggering, there is a C++ example showing you how to detect an external trigger event, you would need to then align your capture on the trigger position and collect any additional samples required (with purge disabled to ensure continuity)
The API does not perform any triggering for the user, only provides the facilities for triggering to be performed.
Let me know if you have specific questions as you work through this.
Regards,
AndrewThanks for the clarification.
Yes, you would need to call the vsgSubmitTrigger function from Matlab which you should find easy if you glance at the other wrapper code in VSG60 matlab class.
You could create a loop like this
while(forever) {
vsgSetFreq
vsgSubmitTrigger
vsgSubmitIQ(your_iq_cw_array)
}The trigger is submitted inline with any other function calls to the API.
Hope this helps.
Regards,
AndrewJon,
Here is some code to get this going.
First, you will want to ensure you configure the external port to accept a rising edge (or falling) external trigger.
calllib('bb_api', 'bbConfigureIO', handle, int32(0), int32(64));The best time to call this would be right after opening the device and only once. The 64 is taken from the header, it is the value for BB_PORT2_IN_TRIGGER_RISING_EDGE (0x40).
Then you will want to create a trigger array and count just like the iq data.
% Trigger parameters triggerptr = libpointer('int32Ptr', zeros(4, 1)); triggercount = int32(4);In this instance I am creating an array of 4 integers to store trigger positions. If I am only expecting 1 trigger, then an array of one will work fine.
Now I need to call bbGetIQ with these new parameters.
status = calllib('bb_api', 'bbGetIQUnpacked', handle, ... iqarrayptr, iqcountint, triggerptr, triggercount, purgearg, ... nullptr, nullptr, nullptr, nullptr);And pull out the array when it returns
triggers = triggerptr.Value;If a trigger occurred during this capture of I/Q, you will see it in the trigger array.
disp(triggers);Might show
230722 0 0 0Meaning an external trigger event occurred at sample index 230722 (zero-based).
Regards,
AndrewJon,
vsgSubmitTrigger and vsgSubmitIQ are part of the VSG60s streaming interface. If you call those functions it will push those actions into the queue to transmit ASAP. For example, if you wanted to transmit a single short waveform synchronized with a trigger, you would call in sequence
vsgSubmitTrigger
vsgSubmitIQ(yourWaveform)
vsgFlush/vsgFlushAndWaitThe vsgOutputWaveform/vsgRepeatWaveform functions are just convenience functions that cannot be combined with others. vsgOutputWaveform transmits your waveform exactly once. It simply calls vsgSubmitIQ and vsgFlushAndWait for you. No triggering is possible with this function. vsgRepeatWaveform is simply a convenience over calling vsgSubmitIQ in a loop. It spins off a thread to do this for you and returns control to your application. If you perform any other action, the waveform being transmitted by vsgRepeatWaveform is interrupted.
If you want to re-transmit a waveform with thg trigger, you will need to recall the submitTrigger/IQ/flush sequence again.
I hope this helps.
RegardsI should add that:
*I’m collecting at the highest resolution: decimation = 1 and bandwidth = 27MHz.
*The VSG60’s signal output is connected directly to the BB60 signal input (with a physical cable and 10dB attenuator).I’ve tried varying iqlen from 4096 [2^12] to 1048576 [2^20] with no real difference – I don’t see a trigger event. I’ve checked to make sure I have the VSG port2 (trigger out) connected to BB60 port 2, and the BB60 is set to trigger on the pulse leading edge…
I have the trigger length set to the default of 10 usec.
I’ll have to check and see if the PC’s SSD can sustain writes of 250MB/sec…
JonG
Are you setting purge to true in that example? Effectively flushing any data and triggers that have accumulated?
I see only one GetIQData function call. The BB60C is a streaming device, so how much data do you have accumulated before the actual trigger occurs? Are you sure the trigger isn’t buffered in the API and you just haven’t polled it yet through the getIQ function?
The BB60C API only stores about 1/2 second of I/Q data. If this buffer fills, it will be forced to wrap it’s internal circular buffer. If it takes you 1/2 second to call getIQ after configuring you might have discarded that I/Q data.
Ideally, you would be polling the BB60C in another thread and waiting for the trigger to occur or transmitting the VSG60 waveform and trigger in another thread while you poll the BB60C in your main thread. That way there is no worries about any I/Q accumulation buffers overflowing. If your VSG60 waveform is short, that is probably not the case here, but worth noting.
Yes – purge is set to true.
The VSG waveform for now is short – it’s the simple CW waveform (iq = complex(1, 0) ) from the MATLAB examples in the SDK. The elapsed time from the start of transmitting the waveform from the VSG to returning from the GetIQData function (calling bbGetIQUnpacked and converting the returned array to complex) is < 0.20 seconds, so I’d think I wouldn’t need a separate thread just yet… But I’ll investigate doing it if I have to.
I’ve made the attached script just about as simple as I can get it. I’ve allowed for up to 4 triggers when grabbing I/Q data.
So, you would want to set purge to false. Purge tells the API to throw away any accumulated data in the API, which would include your waveform and trigger location. Once you set purge to false, you will also need to poll the API until you see the trigger position.
Regards
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