VCXO by SM6MUY/SM6PGP

VCXO

VCXO with PLL to be locked to any kind of frequeny reference. The first version was built approx 1995. Now several modification have been done. The latest was to remove the eeprom and some HC-logic and to replace it with a PIC 16C84/16F84 that can be programmed on board.

The heart of the design is an emitter coupled harmonic oscillator( BFS520) which have good short term stability. (RF design, Oscillator design handbook) It is easy to add a varactor in this circuit to pull the oscillator an small amount in frequency. The oscillator is followed by a transistor buffer (BFS520). The signal is fed into an attenuator and a small Si MMIC (MSA2111) to further increase isolation towards the oscillator. In some cases I have used the MMIC as a multipler. Then the attenuator is adjusted to make the MMIC go into compression it is followed by a helixfilter(TOKO) to select the wanted harmonic. The resistor in the bias circuit can be optimized to improve conversion loss in the multiplier.

The synthesizer is a Siemens/Infineon PMB2306T wich is loaded with serial data, Data-Enable-Clock. The signals for loading the synt is generated by the PIC processor 16C84/16F84. The PIC can be programmed on board just by removing two 0ohm jumpers. I use MPLAB from Microchip and a programmer attached to the PC parallell port. I will add some examples of code for the PIC processor on this page later.
Programming software: look for PICPROG by Conquest systems (ZL1HIT) a very nice pice of software and a good description of the needed hardwar, or IC-PROG (added 2004-10-26)..

Some possible combinations of oscillator and reference frequencies:

OSC FQ REF FQ N-div R-div(13 MHz) R-div(10 MHz) R-div(5MHz)

90.666666 MHz    333.333 kHz     272      39            30            15    (2320 MHz xverter)
96.000000 MHz    500 kHz         192      26            20            10    (1296 MHz xverter)
106.50000 MHz    500 kHz         213      26            20            10    (10368 MHz xverter)
108.06666 MHz    66.6666 kHz    1621     195           150            75    (SK6MHI 1296.800 MHz)
117.00000 MHz    500 kHz         234      26            20            10    (5760 MHz xverter)
125.50000 MHz    500 kHz         251      26            20            10    (24192 MHz xverter)
128.00000 MHz    500 kHz         256      26            20            10    (1296 MHz xverter)

The PLL loop filter has to be designed for the selected reference frequency. There is no component values in the component placing drawing.
Use PLL loop filter design by National Semiconductor. I have used a 3:rd order filter. Kvco has to be measured (typical 1-2 kHz/V). Loop BW has to be selected (10 -100 Hz gives reasonable component values)

An example of a loopfilter :

fosc = 90.666666 MHz                        C1 = 33nF
fref = 333.333333 kHz                      C2 = 330 nF
Ndiv = 272                                  R2 = 56 k
Rdiv = 15 (5 MHz reference osc)             R3 = 120 k
Kvco = 1 kHz/V                              C3 = 120 pF
Icp = 1 mA
Att = 60 dB
Phase marg = 56 deg
loop BW = 30 Hz

The VCXO/PLL can be built in a standard 55 x 72 mm tinplate box

NEW 2004-10-26
VCXO Phase noise measurement

A phase noise measurment has been performed with the new AGILENT E5052 Signal Source Analyzer. I got this opportunity at an Agilent instrument demonstration.

Thanks to the Agilent guys !

Things, good to know if you are trying to build this VCXO.

The xtall has to be approx 3 - 4 kHz higher in frequency than the wanted fq. So if you plan building it for 106.5 MHz then you have to order an xtall for 106.503 MHz, this is because the wanted pulling range is +/- 3 kHz. I do usually order 5:th overtone series resonant xtalls.
Run the oscillator and connect a trimpot to gnd and +5 V) and the wiper to the cap-diode so you can check that the osc is running ok and that you get the wanted pulling range. Tune the trimcaps in the osc so that you get approx 2.5 V when you are at the centerfq and so that the oscillator starts at power on. The inductor and the type of capdiode affect the pulling range, you may have to change these. The inductor usually ends up in the range 100-390 nH.

In some cases it has been necessary to remove the 2.2 pF cap mounted from base of the osc transistor to ground.

Change from rev F to rev G, I found that there was a capacitor (1 nF) lost at the bias of MSA2111 between the inductor and the resistor. Should be connected from the connecting point of the 39 ohm/220nH and to gnd.

Unfortunately I have found out that the PMB2306 is becomming obsolete !
I have no plans in modifying the vcxo circuit to accomodate a new synthezeiser. 2001-04-13

Schematic in pdf or in postscript format:
schematic in pdf (35k)
schematic in encapsulated postscript (64k)

PCB layout in pdf or in postscript format:

Board layout in pdf (30k)
Board layout in encapsulated postscript (124k)

Component placement in pdf or in postscript format:

Component placement in pdf (21k)
Component placement in encapsulated postscript (38k)

Use GSview to view postscript documents or download and print on a postscript printer.

NEW 2001-04-13

PIC asm and hex code examples (16C84), the code is as it is and i have not put much effort in the programming
as I am no programmer and just do it because I have to.
Anyone who likes to create better code may do so and I am interested in a copy.

106.5 MHz (LO for 10 GHz xverter)
ref = 500 kHz
vcxo1065.asm
vcxo1065.hex

125.111111.... MHz (LO for P3D 23/13 cm up/dwn link)
ref = 111.111 111...kHz
vcxo1251.asm
vcxo1251.hex

Info for creating the databytes in the asm-code
Refer to datasheet for the PMB2306

Information on this page may only be used for personal use, for any other use pse contact (s m 6 p g p ( a t ) i l l i p e . s e)
Changed 2014-07-26 by SM6PGP