VFO and LO Board

This board contains the Variable Frequency Oscillator (VFO) and Local Oscillator (LO).

The VFO drives the 1st Mixer which converts RF signals from the nine HF amateur bands to a fixed 1st IF frequency of 70.455-MHz. The LO drives the 2nd Mixer which converts the 1st IF frequency to the 2nd IF frequency of 9.000-MHz.

The VFO and LO are implemented using a Silicon Labs Si570 I2C Programmable XO/DSPLL . There are several versions of the Si570 available, the version used in the Express Receiver is model no. CCC000107G:

C – 3.3V CMOS, Output Enable Polarity High

C – Temperature stability +/-7ppm, total stability +/-20ppm

C – 10- to 160-MHz

0001070 – 10.000-MHz start up frequency

G – Operating temperature range -40 to +85 deg C

I have found the performance of the Si570 in this application to be excellent in terms of stability and close in phase noise.

Craig Johnson, AA0ZZ, wrote an excellent QEX article describing a VFO based on the Si570. This article is a great source of information on the application and programming of the Si570.

The VFO and LO frequencies are programmed by the Main Display and Control Board using the Express Receiver I2C bus. The VFO frequency is continuously variable in 1000-, 100-, or 10-Hz steps, while the LO operates at a fixed frequency of 79.455-MHz.

In the schematic diagram below, the 1st LO implements the VFO function, and the 2nd LO the LO function. As can be seen, the Si570 requires a minimum number of external components in this application.

Q1 through Q4 convert the bidirectional Express Receiver 5V I2C bus to the Si570 3.3V I2C bus. Similar circuits use 1KΩ resistors for R1 through R8; however, I found that 1KΩ resistors could not pull the 3.3V side of the bus below the 0.5V VIL threshold of the Si570. This problem was solved by increasing R1, R3, R5 and R7 to 4.7KΩ.

R9 and R10 should be adjusted to ensure VFO and LO drive to the AD831 mixers (500Ω input impedance) does not exceed 1Vp-p.

VFO and LO Schematic, Page 1
VFO and LO Schematic, Page 1

Unless special ordered, the Si570 will have an I2C address that is fixed at 0x55. For programming purposes, this becomes 0xAA when shifted left 1 bit to the 7 MSBs of the 8-bit read/write I2C address byte. Therefore, when there are two or more Si570 devices on an I2C bus, external device selection logic must be provided. U3 is a CD4052 CMOS analog multiplexer that is used to switch the I2C bus lines, SDA and SCL, between the two Si570 devices using a single I/O line from the microcontroller on the Main Display and Control Board.

U4 and U5 provide +5V and +3.3V regulated power rails, respectively.

Express Receiver 1st-2nd Local Oscillators Page 2
VFO and LO Schematic, Page 2
VFO & LO Board Installed in the Express Receiver
VFO & LO Board Installed in the Express Receiver

The ExpressPCB .pcb file for the board is here: 1st-2nd LO version Final.

J1 & J2 are female machined pin breakaway headers from Sparkfun, PN PRT-00743. Refer to the RF Band Pass Filter page for additional information.

Refer to the RF Band Pass Filter page for information on the connectors J3 & J4.

Ferrite beads FB1 & FB2 are Laird Technologies EMI Filter Bead, Mouser PN 875-HI1806N910R-10 or Digikey PN 240-2541-1-ND in 1806 SMD packages.

All 0.1uF capacitors are SMD 1206 style chip capacitors.

C9 & C11 are 33uF 20%, 10V tantalum capacitor in a 1206 SMD package, Mouser PN 80-T491A336K010.

C8 & C10 are 0.47uF 20%, 25V tantalum capacitor in a 1206 SMD package, Mouser PN 80-T491A4746K025.

All resistors are 1/8W 5% SMD 1206 style chip resistors.

U1 & U2 are Silicon Labs Si570 I2C Programmable XO/DSPLL type CCC000107G, per the discussion above. I obtained these devices directly from Silicon Labs.

U3 is a CD4052 CMOS analog multiplexer in a 16-SOIC package, Digikey PN 296-14272-1-ND.

Q1 through Q4 are BS170FTA N-channel MOSFETs in SOT23-3 SMD package, Digikey PN BS170FCT-ND.

U4 is a LM2940IMP-5.0 low drop out, 1 amp, 5 volt regulator in an SOT223 SMD package, Digikey PN LM2940IMP-5.0/N0PBCT-ND.

U5 is a LM3940IMPX-3.3 low drop out, 1 amp, 3.3 volt regulator in an SOT223 SMD package, Digikey PN LM3940IMPX-3.3/N0PBCT-ND.

© 2014 – 2015 Rod Gatehouse AD5GH

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