In a Lab, a lot of devices need clocks. And they must all be locked to a 10 MHz Reference. At the same time, they still must produce a useful output, in case those 10 MHz are not available (in case the device is taken to the office for software development e.a.).

Block Diagram of the NB3N502 - Drawing Courtesy of ON Semiconductor

Starting at the left side, the chip contains a crystal oscillator. This is build around an inverter circuit, which has a feedback resistor (internal) to bring the inverter gate into a linear operation. Application Note 118 from Fairchild Semiconductor (1974) mayst be helpful to understand this circuit in detail. This oscillator is equipped with a 10 MHz fundamental crystal. It does oscillate and therefore supplies the PLL circuit with a stable Reference.

In case we connect another 10 MHz Source (external), the feedback fom the crystal is overdriven and the inverter gate acts as a Buffer-Amplifier. A Diode Network (BAV99) limits the amplitude of an external Signal to approx. 1.4 Vpp. A series-resistor as well as a coupling capacitor ensure a very light coupling and also act as a dc-block. It shall avoid, that if a coaxial cable (with no Reference) shall load the oscillator cicuit too much, so that oscillation could stop.

Following the Reference Oscillator/Buffer is a VCO/PLL equipped with a Reference Divider and a Feedback Divider. The VCO consists of a differential voltage controlled ring oscillator design using NMOS gate oxide capacitors in the loop filter to provide lowest leakage.

The Division ratios can be set via two jumpers, which are connected to the S0/S1 inputs of the Chip. It shall be noted, that those two inputs have three valid voltage Ranges. If no Jumper is set, Pin S1 defaults to M and Pin S0 defaults to H. M in this case is something like "middle" as an internal voltage divider pulls this input to approx. 1.65 V when powered by a 3.3 V source.

When used with a 10 MHz input signal, a maximum output frequency of 50 MHz is obtainable. For higher frequencies at the input, a maximum output frequency of 120 MHz is the limit. By design.

The supply voltage is stabilised by a MIC5209 from Microchip Technology Inc. It's datasheet says : "Key features include reversed-battery protection, current limiting, overtemperature shutdown, ultra-low-noise capability". It turned out, that this is a nasty little thing. If your bypass capacitors are too good, it will oscillate !


Possible Multiplication Factors :

MULTIPLICATION	JUMPER S1	JUMPER S0
2.00 X	GND	GND
2.50 X	VCC	VCC
3.00 X	OPEN	GND
3.33 X	OPEN	VCC
4.00 X	VCC	GND
5.00 X	GND	VCC

Even so, the datasheet of the NB3N502 mentions only "typically" values for the Jitter, we know from a trustworthy source, that the worst case jitter occurs when using a 6 MHz crystal. Using a 16 MHz or 26 MHz crystal, the rms jitter is well below 15 ps. And it looks like it goes down when the multiplication factor goes up.

Symbol	Characteristic	Min	Typ	Max	Unit
tjitter	Period Jitter (RMS, 1 σ)		15		ps
tjitter	Total Period Jitter, (peak−to−peak)		±40		ps

From that, we would expect a Phase Noise somewhere near that pictured below (left). And yes, we used this nice tool to create the graph. We made the following assumptions : VCO is like an LC-oscillator. Jitter was measured at 25 MHz. Top level -60 dBm, 10 dB/DIV.

Comparing the 'Guesswork' with the 'Real Life' :



The Cursor readings are : 25 MHz .:. + 9.67 dBm, 25.1 MHz .:. - 54.92 dBm. RBW was 10 kHz. To normalise the offset reading to 1 Hz, we have to subtract 10 * LOG(10 kHz) = 40. This will then give us - 94.92 dBm/Hz. Setting this in relation to the carrier, we get - 104.59 dBc/Hz @ 100 kHz Offset.

Due to the digital nature of this thing, a lot of harmonics are generated. Depending on the multiplication factor, the phase comparator frequency and or the input frequency is visible more or less close to the carrier. So frequency planning is a must, when this is your application !

The webmaster does not read these comments regularely. Urgent questions should be send via email. Ads or links to completely uncorrelated things will be removed.