• Short preface.

    All new is well forgotten old.

    Over 25 years ago I was working on project of synthesizers in local company, where one day boss ask me if is possible to pick up one day project for single frequency synthesizer with use of VCO from mobiles. In those time Nokia was use Murata MQE series VCO models. I try to use of LMX1511 PLL from National Semiconductors, really simple schematics for experiments and not only.

    The memories of this issue return me to development for multipurpose up/down converter. My version is not for one day project, but reduced one is it. PCB routed for all possibilities RX or TX path, where TX path can up-converting from any radio amateur band’s owners transceiver.

    Basic idea is assembly some casing of standard form and replace only few parts of loop filter and choose one of two types of VCO. Board can working in only one mode – RX or TX at same time.

    Synthesizer is based on LMX1511 PLL with 3rd order loop filter. Calculation was made on XL program from G4JNT (many thanks for a wise investment), compare to datasheet of PLL no mistakes found and time saving. Because this project is single frequency lock purpose, not so strong emphasis on loop filter. Usually is most important part of synthesizer and many important parameters like phase noise and settling time is depend on correct calculation.

    VCO – Maxim company hase producing a line of VCO’s; series MAX2750~2752. Low RF output (and consumption) power devices help to reduce some battle with spurs. Loaded RF level almost -3dBm, and for any purpose in my synth is enough. 2 of 3 devices is good for converting purpose. Working level windows of PLL is run from -10 to +6dBm.

    MAX2750 – freq. range 2250~2600MHz.

    MAX2751 – freq. range 1920~2350MHz.

    Experienced user can say is something wrong here – PLL maximum input from VCO is 1,1GHz, and you are set VCO with twice in frequency and little more. I use of MC12095 frequency divider with two switches – div by 2 and div by 4. Is old, but very stable device.

    Now let’s do some calculations for every range.

    UHF range for converter, there I set TRX on 435,05MHz. For conversion to 2400,05MHz I need VCO freq on 1965MHz. VCO is MAX2751. Division by 2 is meets the requirements of the PLL’s work window – 982,5MHz. This what PLL will see. Loop filter BW=25kHz and phase detector freq is 250kHz. This is big step, but for this case is good because the synth not in jog mode.

    982,5MHz ; 250kHz = 3930. This is N divider’s value. Internal prescaller chosen for 64/65.

    Now I set value of B & A counters. B= 3930 : 64 = 61; Here must be an integer values, so 61 * 64= 3904. A= 3930 – 3904 = 26. A-counter value must be equal or less than B-counter value.

    N=3930; P=64; Fpd=250kHz; B=61; A=26; R=40.

    Reference counter value is depend on frequency REF oscillator. In any case is 2 modes – oscillator or XTAL. If I choose XTAL=10MHz , R-counter = 10MHz : 250kHz= 40. Also, must get integer value of division.

    VHF range for converter, there I set TRX on 144,05MHz. For conversion to 2400,05MHz I need VCO freq on 2256MHz. VCO like in UHF is MAX2751. Division by 2 not suitable here, where I can use div by 4 and final value will 564MHz for PLL’s input on VHF range.

    N=2256; P=64; Fpd=250kHz; B=35; A=16; R=40.

    HF range for converter. Here is must use other VCO - MAX2750, because coverage of 3,5 to 28MHz is close to 2400MHz conversion product. For conversion to 2400,05MHz I need VCO freq on 28,05MHz. Can be chosen other HF range too. Here I’m use in div by 4 also. fVCO = 593MHz.

    N=2372; P=64; Fpd=250kHz; B=37; A=4; R=40.

    If TRX is tuned on freq 14,05MHz, Fvco after div by 4=596,5MHz; N=2386; P=64; Fpd=250kHz; B=37; A=18; R=40.

    All parameters is saved into PIC12F509 compiled and burned HEX file. Why PIC ? – Because to make damage with wrong code or diff voltages very hard, not like Atmel group of micro’s, where wrong code or fuse drop micro and must be restored to factory level of use.

    Frequency converter. Based on MMIC ERA-3 or same amplifier and RF mixer ADE-25MH.

    Required adapt RF level on LO port of mixer, where on PCB is ready to use place for Pi attenuator’s resistors.

    Supply voltages. On board is 3 voltages converters for 5V, 6V (MMIC supply) and one more for use of Si-590 additional pre-programmed source for HF range in RX path.

    Primary was tested one more VCO from Maxim MAX2609, but I have not positive feedback from this device. Better device for RX purpoces is MAX2624, but PCB not routed for. Is possible try to set MAX2624 on place of MAX275X on board. V-tune voltage must up over datasheet values (~4,5V).

    Loop filter calculated with Kv parameter after division. For example, if VCO parameter Kv = 120MHz/V, and VCO’s frequency at working point is divided by 4, Kv also divided by 4 and equal 30.

    PCB actual size is 56X42mm.

    Attached schematics diagrams can find more qualified in PDF format.

  • Hi Armin! thanks for your responce! This proto really have not filters, for only one reason - power level of VCO very low, so frequencies of products is sufficiently spread out. Also, mixer's upper range close to TX & VCO freq. I need to make deeper testing for units, now only lock PLL's and adapt loop filter values. I know divider's out level is much high than VCO and harmonics. Will be continue.

  • Hello everyone !,

    Hi Leonid,

    good old circuits that you can buy cheap :)

    Please write if the LMX1511 can be forced to get the minimum tuning step of 10HZ ??

    I dream of a very simple CW transmitter with QO100 hi.

    It could look like this:

    MAX2750 ==> LMX1511 ==> PIC ...??. ==> encoder

    prescaler / 4

    display not necessarily

    What do you think ?

    Vy73!! de SP8NTP