I would like some advice for a new design using the LMX2592. This is a fixed frequency synthesizer, with a small variation about the nominal frequency. The nominal output frequency is 6.834682GHz. The maximum tuning range is +/- 1kHz. Also required is square wave FM at a modulation frequency of 120Hz, achieved by writing to the fractional register every 4.16ms. The deviation will be up to 200Hz peak.
I have used this method of generating FM on other synthesizers, usually with success. The method depends on an "on the fly" change of the fractional register without any reset of the integer register, or reset of the fractional register to the seed value. Frequency calibration must be disabled.
This synthesizer will operate for long periods, potentially several years without a power on reset. Initial setup of the registers on reset can be followed by a frequency calibrate, however during operation this is not possible due to the FM requirement. I note that a temperature change of 125degC is allowed without the VCO drifting out of lock. As the operating temperature in this design will be between +40 and +85degC this requirement is fulfilled. However please confirm that there is no long term drift mechanism that could cause unlock.
With a 20MHz PD frequency, the divider value is 341.7341. The fractional value of 0.7341 allows adequate room to implement the FM and tuning requirement without changing the N divider.
Unfortunately I have to use a 10MHz reference frequency. I can use the ref doubler, but please confirm that I cannot use the reference multiplier as the values in section 7.3.2 table 1 are not met.
Please comment on this design.
regards
Cosmo Little