ISM Transceiver help needed

[krishnaprasadsristy]

Hi,

We are designing a wireless link for command guidance at X-Band. We are planning to use an ISM 2.4GHz transceiver and use frequency multiplication to translate the RF out to X-Band during transmission. For reception, we are planning to down convert using mixer to 2.4GHz and allow the same transceiver to demodulate.

Is this a correct way doing? Is this approach having flaws? Pl. comment.

Best,

 

[betwixt]

It should work but there is a better way. Don't multiply the 2.4GHz, up convert it with a fixed local oscillator. You can use the same oscillator to down convert when receiving. The problem with multiplying the frequency is you also multiply the drift and multiply the deviation. Up and down conversion using a mixer preserves the modulation and can be more frequency stable.

Brian.

 

[biff44]

what modulation are you using in the 2.4 ghz module? There are many nuances to multiplying a signal in frequency.

 

[krishnaprasadsristy]

Allow me correct the question friends. For practical reasons, we have moved to 900 MHz Transceiver and the modulation is GFSK. Does the upconversion and downconversion work to X-Band and Down to 900 MHz?

 

[betwixt]

Yes, in fact up-conversion and down-conversion work in most circumstances. The only time you need to be specially careful is when the fundamental and local oscillator frequencies are very close to each other because it makes filtering the products more difficult. As you are delaing with frequencies spaced well apart it shouldn't be a problem.

Brian.

 

[krishnaprasadsristy]

Brian,

Thanks for your help. My only worry is whether the frequency deviation of GFSK at 900 MHz is harldy 100 KHz. Will that be preserved on an X-Band carrier and there on till the down conversion to demodulator?

- - - Updated - - -

Brian,

Thanks for your help. My only worry is whether the frequency deviation of GFSK at 900 MHz is harldy 100 KHz. Will that be preserved on an X-Band carrier and there on till the down conversion to demodulator?

 

[vfone]

Multiplying a signal about x10 (from 900MHz to X-band) for example the frequency deviation of a 500kHz GFSK signal become 5MHz, which definitely will change your system specifications.

 

[FvM]

Multiplying a signal about x10 (from 900MHz to X-band) for example the frequency deviation of a 500kHz GFSK signal become 5MHz, which definitely will change your system specifications.

I think the OP wants to use up/downconversion according to the suggestion of post #2, which won't multiply the transceiver frequency error, but add the drift of the local oscillator.

 

[krishnaprasadsristy]

Yeah, FvM is right. I have decided to up/down convert rather than multiplication thanks to Brian. Is it alright if I use up/down conversion?

 

[biff44]

yes up/down conversion is very common. But you might want to increase the frequency deviation at 900 MHz band. That way you are more tolerant of the other frequency errors. IF you can not, use a much better clock crystal source for the 900 MHz transceiver chip (which I assume has an internal PLL, otherwise it will probably not work!).