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XCO-100.php    23396 Bytes    23-04-2024 14:32:12


Micro-Projects • Tools : A universal XCO/PLL Board


Some kind of replacement (obituary) for those valuable crystal oscillators which made clock generation by simply using one of the harmonics that easy.



NB3N502




Motivation




100 MHz crystal oscillator 100 MHz crystal oscillator


As it seems, that the industry switched from producing crystal oscillators with discrete crystals to programmeable pll circuits, a branch of valuable parts for homebrewing high frequency clock sources is dying out. The replacement is very unsuiteable for generating clock sources of high purity due to a fence of spurious garbage.


100 MHz crystal oscillator 200 MHz crystal oscillator



Circuit




The circuit is straightforward. It may be equipped with the NB3N501, NB3N502 or NB3N511 from ONSEMI. As we wanted to use parts not smaller than 1206, the formfactor is 'slightly' larger than the xco in it's DIL-14 case. Therefore assembly can be done without the help of a microscope. With three jumpers, all different frequency options are covered. If necessary, a voltage regulator may be assembled as well. The pcb is single sided, i.e. the bottom layer is ground. One bridge (wired) at the bottom was unavoideable.


NB3N502 inside

NB3N502 inside. Drawing courtesy of ONSEMI.


The table below shows the necessary crystal for a given (desired) frequency.


Desired Frequency
  [MHz]

X
NB3N501
S0
S1
NB3N502
S0
S1
NB3N511
S0
S1
1
no go
 
 
 
 
no go
 
 
2
M
M
L
L
M
M
2.5
no go
 
 
H
H
L
M
3
M
H
L
M
M
H
3.125
H
M
no go
 
 
no go
 
 
3.333
no go
 
 
H
M
H
M
4
L
L
L
H
L
L
5
H
L
H
L
H
L
5.3125
M
L
no go
 
 
no go
 
 
5.333
no go
 
 
no go
 
 
M
L
6
L
H
no go
 
 
L
H
6.25
L
M
no go
 
 
no go
 
 
8
H
H
no go
 
 
H
H


NOTE : The datasheet guarantees operation to approx. 200 MHz. The calculations are therefore limited to 220 MHz. The crystal frequency may not be larger than approx. 27 MHz. Read the datasheet. This tool does stupid calculations to support smart designers.






Performance • Test




NB3N502, xtal : 24 MHz, F : 120 MHz, C7,C8 : 10 pF, not assembled : R1,R4,R5,R6,R9
Supply : 8 V (with 5V regulator onboard), current : 38 mA, output power : +13 dBm


120 MHz PLLXO 120 MHz PLLXO

120 MHz PLLXO 120 MHz PLLXO



NB3N502, xtal : 40 MHz, F : 160 MHz, C7,C8 : 10 pF, not assembled : R1,R4,R5,R7,R8
Supply : 8 V (with 5V regulator onboard), current : 30 mA, (output power : +13.3 dBm)


XCO running at fundamental frequency NOT 160 MHz PLLXO





NB3N502, xtal : 32 MHz, F : 160 MHz, C7,C8 : 10 pF, not assembled : R1,R4,R5,R6,R9
Supply : 8 V (with 5V regulator onboard), current : 40 mA, (output power : +12.6 dBm)


XCO running at fundamental frequency





Fazit




This solution also has spurious emissions which are a multiple of the chosen crystal frequency. Therefore we can control/predict the spurii location. It is advantageous to choose the highest possible frequency for the quartz. This will move the spurii far away from the desired frequency as well as a low multiplication factor offers lower noise (at least theoretically, not yet measured). The output spectrum is rich in harmonics which should make it easy to filter the desired harmonic. No (unwanted) harmonics foul the spectrum, if you use the NB3N502 and select the "REF" output (Pin 4), which reduces the system to a simple crystal oscillator.




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t1 = 6586 d

t2 = 172 ms

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