This board contains the Variable Frequency Oscillator (VFO), Local Oscillator (LO), Beat Frequency Oscillator (BFO).
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 BFO drives the Product Detector which converts the 9-MHz IF signal to audio.
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 Arduino Mega 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.
The BFO is implemented using a Silicon Labs Si5351A Clock Generator on a breakout board from Adafruit.
In the schematic diagram below, the U1 implements the VFO function, and the U2 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. R1 and R6 should be adjusted to ensure VFO and LO drive to the AD831 mixers (500Ω input impedance) does not exceed 1Vp-p.
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.
An ATTiny85 microcontroller is programmed as a slave on the I2C bus with a unique I2C address. Commands sent to the ATTiny85 via I2C are used to drive the A selection input of the CD4052 to select either the VFO or LO Si570. ATTiny85 software is available here on GitHub.
U4 and U5 provide +5V and +3.3V regulated power rails, respectively. U6 lowers the input voltage to 9V for U7, reducing U7 power dissipation.
© 2014 – 2021 Rod Gatehouse AD5GH
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