Datasheet

LMH1983 Evaluation Kit Users Guide
Page 14 of 25
VCXO
The 27 MHz VCXO (X2) is specified for ±50 ppm APR (min) and ±50 ppm frequency
stability. The calculated VCXO gain is 1000 Hz/V (min). The VCXO gain parameter
(K
VCO
) affects the PLL1 loop response, so it must be considered when designing the
nominal loop bandwidth and damping factor.
The VCXO’s single-ended output clock can be measured at TP26 (XOclk+). The VCXO
clock is received at the XOin+ input of the LMH1983. XOin+ (pin 34) and XOin- (pin
33) are differential inputs for the internal op amp that drives the clock signal to the PLL
blocks. Because XOin is a differential input, the XOin- pin is biased to about 1.25V
using a voltage divider formed by R52 and R56. This input configuration presents a
pseudo-differential signal to the XOin pins.
Alternatively, the single-ended VCXO may be replaced with a different 27 MHz VCXO
with differential clock outputs (e.g. LVDS); in this case, R52, R56, and C48 should be
removed, R54 should be populated with 49.9R to match R55, and R37 should be
populated with a 100R differential load. Using a true differential input signal can provide
more common-mode rejection and reduce noise/crosstalk, and thus reduce sources of
deterministic jitter.
INDEPENDENT AUDIO CLOCK GENERATION
By default, the audio clock generated from PLL4 is phase-locked to the 27 MHz VCXO
clock reference from PLL1, and in turn, is synchronized to the reference input.
It is possible to have the audio clock independently generated from an external 27 MHz
clock source by changing the PLL4 input mux mode via register programming. Once the
PLL4 input mode is properly programmed, pin 17 (normally Fout4) is switched to an
input pin (OSCin) and can be driven with an external 27 MHz clock.
There are two options for an independent 27 MHz clock source. The first option is to use
an on-board 27 MHz VCXO in the footprint X2. The second option is to receive an
external 27 MHz via the SMA clock input, J3. Either one of the options can be used as
long as it’s not being used to drive the Hin reference of the LMH1983. To make use of
this option, populate R21 and R28, then to use the on board oscillator, enable it by
removing the jumper on JP5, and to use the external SMA, disable the oscillator with a
jumper on JP5 and provide the reference on J3.