We need to demodulate a Continuous Phase Fast FSK (CPFSK) using the NI Lavbiew and RFSA PXI 5660.

[Sunilrajanna]

Following are the details:

Carrier Freq: 452.9375Mhz

Symbol Rate: 1200 Symbols per second

Deviations:
Space = 1800Hz
Mark = 1200Hz

Modulation Type: CP FFSK

We are unable to do this using standard vi. Any suggestions well be appreciated.

[Andy_Hinde]

The National Instruments Modulation Toolkit can perform demodulation of a continuous phase FSK signal. Here is a link to the product page online for the Modulation Toolkit:

http://sine.ni.com/nips/cds/view/p/lang/en/nid/12855

Regards,
Andy Hinde
National Instruments

[Sunil Rajanna]

Hello Andy Hinde,

We are already using the modulation tool kit, but the issue at hand is how do we demodulate unsymmetrical deviation which is fc - 1800hz and fc + 1200hz. There are no standard vis that i can use to do this they all have symmetrical deviations for eg fc - 1500hz and fc + 1500hz.
Further, we also need to transmit an similar (cpfsk) modulated carrier of 452.9375Mhz. Presently we can focus on the demodulation alone.

Pls check if these are possible.

thanks and regards,

sunil

[Andy_Hinde]

Hello Sunil,
The FSK software in Modulation Toolkit works with symmetrical frequencyt deviations, so inorder to analyze your signal you will need to compensate with your center frequency. Your total peak to peak devitaion is 3000 Hz, so your FSK deviation is 1500 Hz. You will need to set the center frequency of your 5660 to 300 Hz lower than what you are currently using. This will place the center frequency in the middle of your deviation.

As for transmission, you will need to do the same thing and lower your Tx carrier from 452.9375 MHz to 452.9372 MHz and use a FSK deviation of 1500 Hz

Regards,
Andy Hinde
National Instruments

[Sunil Rajanna]

Hi Andy,

Sorry that I did not reply earlier, did not visit the forum for long.

We were studying out DUT and found out that the method of modulation we need to actually work on is FM. The message bits are converted to continuous phase baseband FM signal using a chip, this signal is then upconverted to the 457.9375Mhz. Now our work has shifted to FM based vis.
The problem that we are now facing is that of converting our message bits to basedband frequencies in the following format:

Mark (1) = 1200Hz
Space(0) = 1800Hz

For the sake of testing, we have generated a waveform manually to resemble an FM Baseband with deviation of 300hz, that is 1200Hz and 1800Hz (the carrier maybe assumend to be 1500Hz). All that we now need to do is upconvert this baseband to our Tx frequency. we are trying to do this. Presently we are manually converting the waveform data to a suitable format needed by the niRFSG write Arb waveform.vi (which upconverts and writes into the Fgen for transmission at our carrier frequency).
We have not been successful as yet, we feel our method of conversion or something is missing in our conversion process.
We request a possible solution to this. I once again shall mention needs for clarity.

We have mapped message bits to form a continuous phase waveform consisting of 1200hz and 1800hz, this is now our FM baseband waveform, so no further FM modulation of this signal is required. This baseband waveform is now just to be upconverted to our carrier freq of 457.9375Mhz (just a multiplication operation of the two freqs).
We are looking for methods to do this.

Thanks and regards,
sunil

[Andy_Hinde]

Hello Sunil,
The NI-RFSG driver requries that you write baseband, complex IQ data for generating vector modulated signals with the PXI-567x signal generators. There are two ways to achieve your goals. First, you can create a baseband FM signal (modulating square wave for example) with 300 Hz FM deviation so that the high and low points of the square wave correspond to -300 and +300 HZ with a center frequency of 0 Hz. IQ signals typically have center frequencies of 0 Hz. The PXI-567x will then take this signal and whatever center frequency you request (say 100 MHz) will be the new center frequency after upconversion, creating an output of 100 MHz +/- 300 Hz. If you needed to offset this by 1500 Hz, you could change your requested center frequency by 1500 Hz, to 100.0015 MHz.

The other option you have is to upconvert the IQ signal created centered at 0 Hz to a new baseband center frequency of 1500 Hz, so that at baseband, you have a 1500 +/- 300 Hz signal. In order to do this, you must multiply the original IQ signal created by Modulation Toolkit by a complex sine wave of 1500 Hz. This will create a new IQ waveform centered at 1500 Hz that you can write to NI-RFSG. Then, when you request a center frequency of 100 MHz, you will get a signal centered at 100.0015 MHz +/- 300 Hz.

I have written a small exqample demonstrating option 2 with some LV screenshots. Please understand that the complex sine wave multiply I use is one of many possible methods and while it works for this specific case, it may not work for other cases. This is not becauise of the overall approach which is accurate, but the implementation I did was quick so you may have to spend some time making it robust.

Hope this helps!

Regards,
Andy Hinde
National Instruments

[Sunil Rajanna]

Hello Andy,

Thanks for the reply.

I have some issues to be addressed with the vi you have sent.

Your attachment for the second method seem to be the one which can suit our application since we cannot shift our tx carrier.

I have worked on your vi to try and suite to my application but faced an immediate problem which i have mentioned in the vi's block diagram.

Our problem is that of generating 1 cycle for a bit info of 1 ( ie, 1200hz) and a 1.2 cycles for a bit into of 0 (ie., 1800hz) within the same time span of 0.8mSec.

Presently the message signal is where we need to insert our original message signal of 288 bits length at 1.2k bits per second, which means, that our FM modulated signal after offsetting, should look like the screen shot (..I have attached) ie.

1. For a duration of 0.8mSec a bit 1 should come out as a single cycle.

2. For the same duration (0.8mSec) a bit 0 should come out as a cycle and a half.

    Please see screen shot to get a clear idea of what we are to actually send as baseband.

3. I have attached actual signal from our device, this data we are presently using to develop our demodulator and we are partially    

    successfully, i feel, it is the tx which is really a challenge.

I sincerely appreciate your efforts till date, hope you will be able to support further, especially now I see we are very close to a solution.

thks and regards,

sunil

ATS

[Sunil Rajanna]

Hi Andy,

please discard the earlier vi with this name. one control was not initialized.

thks

sunil

Message Edited by Sunil Rajanna on 07-04-2005 08:16 AM

[Andy_Hinde]

Hello Sunil,
Can you tell me more about the data in the Excel file? Most importantly, I need to know the sample rate at which this data was acquired, basically the dT in between each sample. Was this data acquired with the 5660 from the DUT?

Thanks,
Andy

[Sunil Rajanna]

Hello Andy,

The dT is (6.40e-05 Sec) and was acquired using an FM Demod example vi from the examples folder all other settings are in the bit map that was in the zip file and the hardware was PXI-5660.  

Yesterday there was a work we did to achieve that which i have mentioned in my last posting on the forum. We are now partially able to map the bits on to the respective waveforms but when we tx the signal, at say 100Mhz, we are not able to demodulate the same the settings on the ni5660 Demodulation FM.vi example proted with labview 7.1, were

Carrier freq = 100.0015Mhz

BW = 10k

Time Duration = 1 Sec

Ref Level = 0 dBm 

I am attaching that vi which we developed yesterday and also a snap shot of the FM demod signal for more clarity.

thanks,

sunil