On 16 Dec 2003 22:14:12 -0800, parth175@yahoo.co.in (Parthasarathy)
wrote:

>Hello 
>     
>     I have to perform QPSK Modulation, on a 5 Mbps data using a 70
>Mhz carrier.
>Is this possible, 

Yes.  It also sounds reasonable, although if your final Tx frequency
is quite high (microwave) and you are using an analog modulator, I
suggest starting at a much higher first IF (in upper UHF) to reduce
costs.

> or do we have any constraints on the carrier
>frequency or date rate?

Yes, the carrier frequency must be greater than half the bandwidth, to
avoid having the spectrum wrap around 0Hz.  Higher carrier frequency
to bandwidth ratios may make the IF filtering easier.

This will not be a problem for the particular rates you mentioned.

>Does this information completely describe my problem?

No.  You also need to specify the following:

- Roll off factor.  This, and the data rate determine the bandwidth.
You have two bits per symbol (because it's QPSK), so the symbol rate
is 2.5 Msym/s.  The bandwidth will be the symbol rate multiplied by (1
+ alpha), where alpha is the roll off factor, between 0 and 1.

- Roll off filter shape.  This will typically (but not necessarily) be
square root raised cosine.

- Differential encoding (or not).


Another approach: consider a completely digital modulator followed by
a DAC.
You could roll your own modulator in an FPGA or ASIC, or buy one (from
e.g. Analog Devices).
This will limit the carrier frequency to HF or VHF.

Regards,
Allan.