Q
We are looking for a frequency synthesis circuit that would allow us to inject
accurate frequencies into the RF amplifier
stage of a 173MHz transceiver unit. These frequencies need to be varied (FSK)
in accordance with a digital data stream. The
frequencies would need to be varied by up to 2400 times a second (2400 baud
data stream). My question is : would your AD9850 family
of DDS chips be suitable for such a purpose ? i.e. can the frequency generator
be accurately "modulated" at such a rate ?
A
What frequencies do you require the AD9851 to generate. The maximum output
frequency of a DDS is limited to approximately 1/3 of the master clock
frequency, so the AD9851 can generate frequencies up to about 60MHz before the
SFDR starts to become fairly horrendous.
Assuming that you want to generate frequencies up to 60MHz and mix them with a
175MHz carrier, then yes, I believe the AD9851 is suitable for your
application. If you require the AD9851 to generate the 175MHz carrier and hop
between two frequencies in this range, it's still possible to use the AD9851
but you would need to use an intermediate frequency and a PLL to mix up to the
required carrier.
I think your question really refers to the speed at which settles when you
switch between two frequencies. When you are implementing FSK in the analog
domain, the settling time (or lock time) is a key consideration since you have
to tune the analog circuit to the new frequency. In a DDS however, this is not
really an issue. You are not tuning the circuit, instead, you are adjusting the
step size of the phase accumulator. As soon as you update the frequency word,
in the DDS the phase accumulator immediately starts to output the new frequency
( there is a latency of a few master clock cycles while the register is updates
but nothing significant).
The limitation is how fast you can update the serial or parallel interface with
the required tuning word. It should have no trouble responding to an update of
2400baud.
The AD9835 has Phase a Frequency registers on chip. You can program these
registers with the two frequencies in your FSK modulation Scheme and then use a
single pin to switch between the two output frequencies. The AD9835 is ,
however, limited to a maximum clock frequency of 50MHz.