[time-nuts] Frequency Dividers

Pete peterawson at earthlink.net
Mon Aug 10 00:02:59 UTC 2009


I thought I'd share some recent results for 2 useful frequency dividers.

For frequencies above 50MHz, an A-D 9513; and a 74HC4059 for
40 MHz & below. The A-D 9513 likes higher slew rates & works
best above 100MHz. I cheated and bought an evaluation board for
the A-D 9513; layout is really important.

First, I arranged a best case test to benchmark the system & have a basis
for comparison with more flexible options. This setup yields high slewrate
waveforms throughout. The configuration is: (note- the 74LV4046 PD1
input has 50 ohms to gnd)

DTS-2075 100MHz Ref out (400mVp-p squarewave) to A-D 9513 (set
to divide by 10) 50 ohm input.
A-D 9513 10MHz out (AC coupled, 1.6Vp-p) to 74LV4046 PD1 input.
74LV4046 PD1 out to 74HC4059 (set to divide by 1E4 ) Cp input.
74HC4059 out to 250 ohm(divide by 5) probe to DTS-2075 CH1, period
mode. The CH1 signal is 850mV p-p. The DTS-2075 reads 1ms period;
std dev = 3ps rms (+/-10ps p-p) The DTS jitter floor is 2.3ps rms.
This equates to 1.7ps rms additive jitter from the 100MHz ref out to CH1.

Knowing we often wish to examine sinewave sources, I repeated this test
with a BLP-10.7 filter placed between the A-D 8513 and the 74LV4046.
This provides a very low distortion 10MHz, 1.76V p-p sinewave into the
74LV4046 PD1 input. The DTS-2075 jitter changed to: std dev =3.7ps
rms (+/-13.3ps p-p) with period unchanged. This equates to 2.9ps rms
additive jitter.

Finally, measuring the jitter degradation resulting from decreasing sinewave
amplitude helps establish the useful input range for the 74LV4046 PD1.

Input V rms   RMS Jitter Measured     Computed RMS Jitter(2.3ps floor)
612mV            3.7ps                                2.9ps
500mV            3.8ps                                3.0ps
316mV            4.7ps                                4.1ps
200mV            6.1ps                                5.6ps
100mV            9.4ps                                9.1ps
50mV            14.5ps                              14.3ps
25mV            19.5ps                              19.4ps

It appears that inputs above 200mV rms are enough to "overdrive" the
PD1 amplifier since the measured jitter response is so small. The
200-600mV input range is a "sweet-spot" for low jitter.

Pete Rawson 





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