yeah as i think
if the system channel spacing is about 5Mhz and the bandwidth is about 60 Mhz from 2110 MHz to 2170 MHz
u can design an integer N type and use the reffernce oscillator to be 5MHz
this also will be the bandwidth of the loop is good enough to give u the settling time u want
the loop bamdwith is 1/20 of reffernce osc , so loop bandwidth could be 250Kz or more
yeah as i think
if the system channel spacing is about 5Mhz and the bandwidth is about 60 Mhz from 2110 MHz to 2170 MHz
u can design an integer N type and use the reffernce oscillator to be 5MHz
this also will be the bandwidth of the loop is good enough to give u the settling time u want
the loop bamdwith is 1/20 of reffernce osc , so loop bandwidth could be 250Kz or more
the main problem in GSM system that the channle spacing is very small so if u use integr N loop the bandwidth of the loop will be very narrow but i think the integr N loop can do the job
Well, GSM channel spacing is 200k, so bandwidth is about 20k, and integer-N setting time is about 200us, it is not meet most GSM requirement, so, I think switch to fractioanl-N may be necessary
In GSM it also depends on how you do the frequency plan. If the synthesizer has to hop between TX - RX - MAHO - then a frac N is needed for the switching speed.
Well, GSM channel spacing is 200k, so bandwidth is about 20k, and integer-N setting time is about 200us, it is not meet most GSM requirement, so, I think switch to fractioanl-N may be necessary
An Integer PLL can certainly do the job for GSM. As you will realize if you open the main part of the cellular phones on the market today.
The problems are as already stated that the channel spacing is 200kHz so you bandwidth should be below 20kHz but to this adds another problem; As the reference frequency is 200kHz you can expect spurs only 200kHz from the carrier. To suppress these you could attempt to decrease the bw further but you simply can't afford the increase in settling time.
So again if you do have a fractional-N pll that might help you (but be aware that hardly no commercial Frac-N pll meets the spec. in their own datasheets.. so go for one that can use a high ref.frequency)
So again if you do have a fractional-N pll that might help you (but be aware that hardly no commercial Frac-N pll meets the spec. in their own datasheets.. so go for one that can use a high ref.frequency) quote]
Well, it makes me confusing again. you said, "use a high ref. frequency", sure, I can use 5Mhz as refrence, however, question is how do I get 200K spacing from 5MHz?
Yes. For example Nokia WCDMA phones on the market use 2 integer-N
PLLs. One for RX and one for TX frequencies.
Well, GSM channel spacing is 200k, so bandwidth is about 20k, and integer-N setting time is about 200us, it is not meet most GSM requirement, so, I think switch to fractioanl-N may be necessary
Siemens and Nokia (perhaps others) run the GSM PLL at twice the wanted
frequency. This allows easy generation of quadrature signals for the
IQ modulators (flip-flops) and also a 400kHz reference frequency at the
integer-N PLL. This means that the loop bandwidth can be up to 40kHz
leading to acceptable settling time at least to class 10 GPRS (if I
remember correctly..)
So again if you do have a fractional-N pll that might help you (but be aware that hardly no commercial Frac-N pll meets the spec. in their own datasheets.. so go for one that can use a high ref.frequency) quote]
Well, it makes me confusing again. you said, "use a high ref. frequency", sure, I can use 5Mhz as refrence, however, question is how do I get 200K spacing from 5MHz?
I said "use a high reference frequency if you have a Frac-N pll". If you use a Frac-N you can generate any frequency spacing from a higher one (that is what the name 'Fractional-N' stands for. Ie a fraction of the reference frequency(*N) in contrast to an integer-N pll.)
Yes it is. I have designed integer-n PLLs for GSM handsets with
25kHz loop bandwidth with a 400khz reference. This was sufficient
to meet class 6/10 GPRS settling time requirements. Typical settling
time was <200us. Fractional-N is not necessary for GSM.