Frequency modulation transmitter design need help

[ericyeoh]

Hi all,

I'm doing a FM tranceiver project. The FM TX is required to produce output frequency of 40.x Mhz which is ISA band. The TX is also required to stick with VCO design. I;m stuck with the design.

Any 1 have ideas about it?

 

[tony_lth]

Your question is not clear.
For FM, you can feed in FM directly to the Vt pin of VCO.

 

[RCinFLA]

You did not give much details of what modulation index you are looking for.

For narrow band FM, a common method of modulation on a synthesizer is called two port modulation. A varactor on the RF VCO and varactor in the crystal reference oscillator is modulated in exact proportions to result in net zero frequency delta as seen by synthesizer phase detector. This avoids the issue of synthesizer correcting out modulation only placed on RF VCO.

Newer Frac-N synthesizers do this directly on synthesizer by digitially modulating the dividers as well as the RF VCO. Same objective, to keep net zero frequency shift on the phase detector.

 

[tony_lth]

Some DDS may implement this function.

 

[ericyeoh]

Hi all,
I;m new to FM design. In fact, i;m still fresh to learn. My supervisor has told me to try achieve the modulation index, B ~ 5.0 (if possible).

My project is basically design a FM transceiver.
Input : microphone (human voice)
Output (TX) : able to produce the (human speech)

Design requirement
TX :1.Frequency must be ISM band ~ 40.6Mhz.
2.The design must stick with VCO + PLL.
3.Intergrated circuit is restricted for designing the VCO.

RX : The design could use IC to implement and must be able to receive the signal.

 

[tony_lth]

Select VCO about 40M, and select analog devices PLL chip, use ADIsimPLL design PLL circuits.

 

[ericyeoh]

Thx Tony. Do u hv any suggest on VCO? i've searched through the google, there were lots of oscillator design, colpitts (series or paralle) , Hartley. I've no idea which 1 i should choose.

Is there any simulation could be used before going to actual design?
I;m currently using ADS software, i was wondering how can i use it to identify whether the design could work or not.

 

[kalyanasv]

You can look up SA605 it is an FM IC which you can use fro the receiver. In the case of transmission you can use a LC circuit to generate the oscillations. Then you create an antenna well if you use copper your looking at an antenna length (full length ) (0.951)*c/freq = wavelength antenna. an omni- directional could also be salvaged from an old radio.

 

[ericyeoh]

How about the VCO? any suggestion?

 

[kalyanasv]

You can look at a typical Xtal VCO and a PLL CX9725B or similar PLL's are available..but the SA605 is an IC which does the FM reception.

 

[ericyeoh]

Hei guys, do i need to build frequency divider circuit or frequency amplification circuit in my design?

 

[tony_lth]

**broken link removed**
you can use this VCO and don't need freq divider or multiplier.
Freq amp maybe needed according to your designed system chain.