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NI PXI-5422 Setting Carrier Frequency Amplitude

Hello all,

 

I hope I have the right forum. I hav ea NI PXI-5441 and am trying to run the example "Fgen OSP AM.vi". Using this example VI, how do you change the RF Carrier Amplitude and the Modulation index for this HW? Does this HW support this option?

 

Thanks

 

Minesh

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Hello Minesh,

 

The title of this thread indicates a 5422, but the body of your post mentions a 5441.  Which one are you working with?  The 5422 is not nearly as fully - featured as the 5441.

 

Since you're working with AM modulation, setting the modulation index should be relatively straightforward, but I need to dig around in the API a little more.  In a worst-case scenario, we could define a waveform in an hws file before starting the transmisison, and streaming that from disk.

 

Hopefully that won't be necessary.  I'll post again after I've had some more time to poke around NI-FGEN.

 

Best regards,

Matthew H.
Applications Engineer
National Instruments
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Hi Mathew,

 

I have a PXI-5441 and want to do Amplitude Modulation. I can vary the modulated signals Amplitude and Frequency as well as the Carrier's Frequency.  But do not see anywhere in the example and digging through the help files on where to set the Modulation Index or set the Carrier's Amplitude.

 

Thanks for any help!

 

Minesh

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Hello Minesh,

 

Digging through the NI Signal Generators Help, I found that the following Carrier properties are exposed:
NIFGEN_ATTR_OSP_CARRIER_ENABLED

NIFGEN_ATTR_OSP_CARRIER_FREQUENCY

 

So if we're only working with the FGEN API, we're only able to set the carrier frequency and enable/disable it.  If, you're looking to increase broadcast power, we're not going to be able to meet that with just a 5441.  If you're looking to see how different carrier powers effect signal readability/reliability, we could step backwards into that by modifying our  I and Q gain (apply the same less-than-unity gain to each).

NIFGEN_ATTR_OSP_PRE_FILTER_GAIN_I

NIFGEN_ATTR_OSP_PRE_FILTER_GAIN_Q

 

Both of those properties can be accessed through an NI-FGen property node.

 

Where setting the modulation index is concerned, the only way I've found that to be directly modifiable is by using our Modulation Toolkit.  If you already have the modulation toolkit installed, there is a shipping example that allows the modulation index to be modified.  In the Example Finder, navigate:

 

Hardware Input and Output » Modular Instruments » NI-FGEN (Signal Generators) » Modulation Tookit » MT niFgen AM Signal Generation.vi

Best regards,

Matthew H.
Applications Engineer
National Instruments
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Hi Mathew,

 

Using the 5441 VI, what is the Amplitude of the carrier frequency set to intially?

 

And not sure if I competely follow. So there is no property for setting the carrier frequency amplitude. But we can affect the carrier power (which is the amplitude) by ajusting the pre filter I/Q properties. Is that correct?

 

Thanks

 

Minesh

 

 

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Hello Minesh,

 

For all of our FGENs, the waveform is formed at 1Vpp, and then scaled up or down in the analog output path.  The amplitude of the carrier isn't documented in any of the materials I can find publically, but given the OSP's position in the signal path, I assume it is generated at similar levels.

 

If you are interested, you may examine the block diagram, OSP diagram, and Analog Output Path.

 

After re-examining the block diagrams, I think my initial suggestion may have been in error.  I wasn't thinking that we'd be able to increase the carrier ampitude, but rather decrease the amplitude of the signal being upconverted to it.  Increasing total output power while decreasing the base-band signal's amplitude would have a similar effect on the signal's appearance as just increasing the carrier frequency amplitude.  However, from the OSP diagram, it looks like the upconversion happens prior to the application of I and Q gain values.

 

Our 5441 is being used by another group right now, so I am unable to test out that idea.  If you have one on hand, you could test out adjusting the I and Q gains to less-than-unity to see how that alters the outgoing signal, but you will still be limited to the card's output limitation of 12Vpp.

 

Best regards

Matthew H.
Applications Engineer
National Instruments
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