06-08-2012 05:02 AM
I am using a PXI-5650 RF Sig Gen to generate FM receiver test signals.
The receiver has a squelch facility using CTCSS (continuous tone squelch). This uses analog 127.3Hz on some channels and DCS 134.4 bps (i.e. a repeating data word pattern) on other channels.
The 5650 generator can generate single modulator FM but I don't believe it is capable of multi-frequency or arbitrary waveform FM modulation.
This means I am limited to generating a squelch tone with no audio test tone superimposed on it, or FM with an audio signal but no squelch tone. Also more fundamentally I can't apply DCS modulation.
One idea I had to get past this limitation (which may well not be viable depending on the ability of the generator clock PLL) is to clock the 5650 externally using an ARB? This would generate a 10MHz clock with a FM modulation squelch pattern i.e. a few hundred Hz of frequency deviation, analog or digital as required. Any chance this would work?
Would the generator track the clock modulation and represent that as a frequency modulated output, or would I maybe get a lock error?
If it should track, should I expect the FM output to be correct and as intended ie FM squelch + FM audio tone?
06-12-2012 07:39 AM - edited 06-12-2012 07:41 AM
You can try modulating the signal in software, and therefore the effects of squelching the signal, and have these delegated to the PXI-5650's output channels instead. This would work around having to use an external triggering system to drive the generation process since the single signal that we're driving out of the generator would already be have the effects of squelching superimposed onto it.
The module datasheet states that direct modulation of external signals is not supported by the PXI-5650, so this eliminates directly performing the modulation task.
In terms of synchronising the module externally in order to intermittently clock samples out of the device, the external clock source must be periodic in order for the generation task to function correctly. If the signal is aperiodic, issues will arise as the PLL is designed to work around a fixed frequency signal source since it always tries to match to the input signal frequency. With this in mind, varying the input frequenies, or the duty cycle, of the 10MHz clock can potentially cause some issues.
Our best bet would be to perform this modulation task in software and then generate the resulting waveform as normal. Does this sound like a suitable option?
06-12-2012 08:49 AM
Thanks, the external modulation idea was probably a long shot anyway - analog squelch would be periodic of sorts but DCS squelch would apply step changes to the clock frequency so I did suspect may well not track properly..
Regarding performing in software - when I tried RFSG Arbitrary Wave Generation or Script Generation I get an mode error back from the Generator saying only CW mode is available.
Is this kind of modulated output possible with this instrument? If more than one modulation sources can be achieved in software can you direct me to any LabView examples I should be looking at of how to do this?
Thanks again for assistance.