FM modulation vi

[MVK]

Hi Andy

i want to measure amplitude of FM versus time for FM demodulated  signal,but in labview example vi,acquired value is plotted as FM versus time,what i have to do to calculate the required value. herewith i attached labview example vi.

[MVK]

Hello andy,

I want to find out the message signal amplitude value. In case oF AM demodulation, after the demodulation directly amplitude Vs time signal available. But in case of FM, we are getting Frequency Vs time signal. I want to find out amplitude of the demodulated message signal. Please give me some idea. For your info I am using IQ read measurement then i am doing demodulation. Please make a look over attached vi in the first thread.

[Andy_Hinde]

Hello MVK,
When performaing FM demodulation on an acquired RF signal, the correct, resultant output is the frequency of the RF carrier as a function of time. If I have a 1 kHz sine wave and FM modulate it, the frequency of the RF carrier will oscillate at a rate of 1 kHz. However, scaling is a different issue involving a parameter called FM deviation. I can have a 1 kHz sine wave message signal and transmit the same information using a FM deviation of 1 kHz or 10 kHz for example. With a FM deviation of 1 kHz, my RF carrier will oscillate between +/- 1 kHz offset from the carrier frequency at a rate of 1 kHz (message signal frequency). With a 10 kHz FM deviation, the RF carrier will oscillate between +/- 10 kHz offset from the carrier at the same rate of 1 kHz (message signal frequency). So the amplitude of the message signal is used to scale the frequency offsets of the carrier based on the FM deviation used. That is why the demodulated message signal results in a plot of frequency vs. time.
FM signals have constant signal level amplitude in theory, which is why FM is superior to AM in noise rejection and broadcast difference, as the information is not encoded as a function of signal level, but instead frequency which is not subject to change due to buildings in the way or going under a bridge. The level of the RF carrier is constant with just the frequency changing.

Regards,
Andy Hinde
National Instruments

[MVK]

Hi andy,

You are telling that "FM signals have constant signal level amplitude in theory, which is why FM is superior to AM" .... absolutely correct. But i want to find out the message signal constant amplitude. The situation is some thing like this..... i don't know about message signal. DUT(device under test) delivering modulated signal. I have to acquire that signal using PXI5660, after demodulation i need to find out message signal amplitude. I have tried using IQ read vi but i am not getting proper value. So i found that is not a right way to find out amplitude of message signal. So in what way i can find out message signal amplitude? (but in case of AM signal i am using IQ read vi and then using single tone measurement in to find out message signal amplitude. that is 100% working. then i tried same thing for FM also. but not working for FM)

Message Edited by MVK on 07-25-2005 11:56 PM

[Andy_Hinde]

Hi MVK,
The amplitude of the message signal is the amplitude of the carrier. One way to measure this is to acquire the IQ data of the FM signal and convert it to Magnitude vs time data. Since the I vs Q plot of a FM signal is a circle whose radius is the magnitude of the message signal, acquiring the IQ data and converting to Magnitude vs time (i.e. the radius from the origin to the circle) should do the trick.

Thanks,
Andy

Message Edited by Andy Hinde on 07-26-2005 03:51 PM

[MVK]

Hi andy,

You are saying that "The amplitude of the message signal is the amplitude of the carrier". How it can be andy?. It may equal to carrier or may not. Say for example message signal amplitude 1v and frequency 1KHz, carrier 10v and 100MHz. after Frequency modulation carrier signal frequecy should vary according to the message signal amplitude, but amplitude of the carrier will be remain same. Am i right?.... but as per your vi (which u have attached) it is doing amplitude measurement of carrier. But i want to find out message signal amplitude. Please make me correct if i am wrong. And i have contacted Ni India support. They are saying that there is no specific method to findout Frequency modulated "message signal(modulating signal)". If it is my customer may feel bad. so please give some better solution in this regard.

Thank you

[Nick_Storton]

Hi MVK,

In frequency modulation, the instantaneous frequency of the radio-frequency wave is varied in accordance with the modulating signal. As mentioned by Andy earlier, the amplitude of the carrier is kept constant. The number of times per second that the instantaneous frequency is varied from the average (carrier frequency) is controlled by the frequency of the modulating signal. The amount by which the frequency departs from the average is controlled by the amplitude of the modulating signal. This variation is referred to as the FREQUENCY DEVIATION of the frequency-modulated wave. We can now establish two clear-cut rules for frequency deviation rate and amplitude in frequency modulation:

 

AMOUNT OF FREQUENCY SHIFT IS PROPORTIONAL TO THE AMPLITUDE OF THE MODULATING SIGNAL

This rule simply means that if a 10-volt signal causes a frequency shift of 20 kilohertz, then a 20-volt signal will cause a frequency shift of 40 kilohertz.

RATE OF FREQUENCY SHIFT IS PROPORTIONAL TO THE FREQUENCY OF THE MODULATING SIGNAL

This second rule means that if the carrier is modulated with a 1-kilohertz tone, then the carrier is changing frequency 1,000 times each second.

So in order to calculate the actual Amplitude of the modulating signal you must have prior knowledge of the ratio of amplitude to deviation otherwise you will get a signal amplitude that is directly proportional to the modulating signal.

Hope this helps,

      Nick

[MVK]

Hi Nick,

You have suggested very good idea. Now i will explain why i need to find out modulating signal (message signal) amplitude.

My customer having a DUT (device under test) called Transreceiver. When the DUT under transmitting mode, it will transmit an RF signal. For that i have to generate and feed 250mV, 1KHz audio signal to DUT. Then DUT will modulate (FM) this signal with carrier then DUT will transmit. Now customer wants to acquire that signal and demodulate, then find out message signal amplitude. This audio output value will be the reference(I have to take this value as a reference) Assume V₀

Now i have to change the message frequency (generated by me) from1KHz to 3KHz, feed into DUT; now take the RF signal from DUT. After demodulation find out the amplitude of message signal. Say V₁

Again same procedure for 0.3KHz. Now amplitude of message signal say V₂

Now find out difference with reference i mean... V₀~V₁ ,V₀~V₂

This difference they are call it as " Modulation response"

So for this only i want to find out message signal amplitude. even though i knew the message signal specs( whatever i have generated audio), after the modulation and transmission then again acquisition and then demodulation , recovering original message signal may not be equal to what ever i have generated right. So they want to do this entire modulation response. Please revert back as quick as possible. and thank you for your suggestion

Note: whatever i have mentioned specs all are customer specification. For all above process i am using PXI 5660, DSA PXI-4461

[Nick_Storton]

Hi MVK,

      From what you say I'm assuming that you do not change the amplitude of your modulating signal only the frequency. The test that you are describing is somewhat similar to a frequency response test. In this case if the response of the DUT is flat then the FM deviation and hence the demodulated amplitude will be the same for all 3 frequencies. Any difference in FM deviation / demod amplitude between the 3 frequencies will be a measure of the passband response of the DUT, and because this will be in the form of a ratio between the reference and the 2 other frequencies it can be expressed directly in dB's which is the most likely form your customer requires, therefore you don't need to know or calculate the absolute amplitude of the demodulated output.

Hope this helps,

    Nick

[Engr. Waqar Hussain]

hi every body, I am waqar hussain from Pakistan.

You can generate FM signal from just a DAQ card and LabVIEW (not PXI 5660 or modulation toolkit) and demodulate it using the same DAQ Card and for this you can may have a small antenna on transmitter and reciever side. If you have any interest, please contact me at my email address. waqar [at] au-irc.com or hussain [at] rcm2.co.uk

I will send you a detailed solution.

Regards

Waqar Hussain
Air University, Islamabad
Pakistan

Message Edited by Support on 07-31-2007 02:10 PM