Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com
 

Revision History
11 July 2009. Original
12 July 2009. Expanded with more measurement data
13 July 2009. Added power output versus frequency
14 July 2009. Revised discussion related to calibration and added frequency calibration section

Table of Contents
What_is_an_Si570_and_why_do_I_want_one
Controller_and_Operation
Inputs_and_Outputs
Assembly
Assembling_the_Top_Layer
Assembling_the_Bottom_Layer
Post_Build_Checkout
Frequency_Stability_and_Accuracy
Frequency_Calibration
Phase_Noise
Frequency_Spectrum
Output_Power_Change_With_Frequency
Conclusions

John, K5JHF, and Kees, K5BCQ, have developed an integrated manual controller and display for the Si570. John is responsible for the microcontroller firmware and logical hardware design, while Kees is responsible for physical hardware design, board layout, and producing the kit. This page is my review of the kit and assembly process. You may also wish to view my page Canned Osc Phase Noise for more information on the Si570's phase noise.

The kit price, including a CMOS Si570 integrated circuit, and all electronic parts (including the LCD display and the tuning knob) is $40, plus shipping. For more information on the kit see Kees's web page http://www.qsl.net/k5bcq/Kits/Kits.html

My overall conclusions:

• Exceptional bargain for $40.
• Si570 is an amazing chip, with extremely high quality (low phase noise) output
• The major area of improvement needed is in the documentation. As it currently stands, this is not a kit for the inexperienced builder.
• Assembly time: 90 minutes.
• For more conclusions, click here.

Good question. The Si570 is described by its manufacturer, Silicon Labs, as an "ANY-RATE I2C PROGRAMMABLE XO/VCXO." What this means in normal amateur radio language is that the Si570 is a frequency synthesizer, complete on one very small (7mm x 5mm) integrated circuit chip. (That's the any-rate, programmable part of the name.) The XO/VCXO part means that the synthesizer is crystal controlled (XO = crystal oscillator) and that one variant of the chip accepts an analog input voltage that will shift the programmed frequency (voltage controlled crystal oscillator. That version is not used in the kit.) The final perhaps unfamiliar term in the part description is "I2C" which refers to the method by which frequency commands and other data is communicated between the rest of the circuit and the Si570. I2C is a specific serial communications protocol using separate clock and data ports and for the purpose of this review, it isn't necessary to delve further into the specifics of I2C.

The Si570's intended purpose is in microcontroller and computer systems, where it provides very accurate and stable clock pulses with a frequency capable of very rapid changes. Since computer chips expect digital waveforms, the Si570's output is a square wave with a fast rise and fall.

For our amateur radio activities, the Si570 has a rather different use—it can be used as the local oscillator in a superheterodyne receiver, or it can be the local oscillator in a direct conversion quadrature receiver such as the Softrock.  The primary purpose for which John and Kees developed this kit is the Softrock, I believe. A number of other adaptations of the Si570 to the Softrock receivers and transceivers have been made, including integration with the sound card decoding software to yield a complete multi-band receiver but will not be covered in this review.

In addition to its use inside a receiver, the Si570 kit can be used as a stand-alone signal source for test and measurement and for other purposes, such as a VFO for your old Heathkit DX40 for that matter. Just keep in mind that the Si570's output is a square wave and may require additional filtering for some purposes.

A detailed datasheet for the Si570 can be downloaded at https://www.silabs.com/products/clocksoscillators/xo/Pages/default.aspx by selecting the Si570/571 in the "Resources" tab. I've extracted the summary description and block diagram below.
 

The chip supplied with the kit is the 3.3V version, but an on-board voltage regulator allows the kit to work with supply voltages ranging from 5V to 13V.

My particular CMOS Si570 has a lower frequency limit slightly below 3.5 MHz and a maximum frequency limit above 250 MHz, comfortably exceeding the published specifications of 10 and 160 MHz.

The kit includes a microcontroller, an MC9S08QG8 device, and an outboard non-volatile memory chip, a 24AA32, as well as an LCD and a rotary encoder. The microcontroller reads the user's turns and pushes from the rotary encoder and  translates those into I2C commands  to the Si570 (and, for that matter, to the 24AA32 as well, since it is also an I2C device.)

To enter a frequency (1 Hz steps) push the encoder in and hold it in and  rotate. This moves a decimal point indicator left or right, indicating the digit to be modified. When the desired digit is identified, release the encoder and turn it to either increase or decrease the frequency value. The readout is in Hz, without a decimal point. (The decimal point is only used to identify digits for modification.) You may save the frequency into non-volatile memory (retained when power is removed) with 1,000 memory locations. You may also program in a division ratio to be applied to all entries, useful with the Softrock receivers where there is a divide by 4 or divide by 8 factor between the oscillator and the reception frequency. The division ratio changes the display so that a if a division ratio of 4 is entered, setting the frequency  to 10 MHz causes the Si570 to output a frequency of 40 MHz. There is also an "offset" frequency factor, which adds or subtracts a fixed offset to the display, useful if you use the Si570 as the local oscillator in a receiver.

Other settable options include the number of increment steps per shaft rotation of the  rotary encoder and a direction normal/reverse option, useful in receivers with an inverting frequency scheme.

Inputs and Outputs

The kit's input and output ports are:

• DC power. Power supplied may be between 5 and 14V at approximately 80 mA.
   
• RF Output. Depending on options installed during the build, the RF output will be either single ended referenced to ground or balanced output. My kit has an output power of approximately +14 dBm, or 25 mW, corresponding to 1.1V RMS.
   
• Filter Bandswitching. A separate port contains frequency information suitable for controlling switchable bandpass filters when the Si570 is used with a multiband Softrock receiver.
   
• Battery Power. Direct access to the +3.3V internal power bus for battery operation of the kit. Since this port bypasses the voltage regulator, it is possible to damage the integrated circuits by excess voltage.
   
• LCD. The LCD connects to the board through a 10-pin port.

All connectors are 0.1 inch pin header style, single row, except for the LCD, a 2x5 0.1 inch header.

My kit included the following options:

• CMOS Si570
   
• Minicircuits TC1-1T transformer

The kit is supplied with a 6-page "Construction Hints" document and a schematic. Neither a parts list nor a detailed "Heathkit" style instruction set is provided.

Most of the parts are surface mount, with the passive components (resistors and capacitors) being 0805 size. These components can be soldered without too much effort providing the proper tools are used. A temperature controlled soldering station with a small tip is essential, as is the correct solder size. I use a Hakko 936 with the small handpiece and a 2.5 mm tip. For surface mount soldering, I like Radio Shack's 0.015 inch diameter, 2% silver solder. Radio Shack has a lot of schlock but their 2% silver containing solder is a remarkably good product and is available in small quantities so you don't need to spend $40 for a full pound.

The documentation does not include board photographs, so I took a series of images as I built the kit.

First, the top and bottom of the board, before assembly. The board is approximately 1.25 inches x 1.5 inches, and in some areas components are closely spaced.
 

Before starting construction, I believe the builder should first decide how the output options are to be configured and mark  the corresponding paragraph in the builder's notes with a highlighter. The output option area is not silk screened and it's easy to get confused by momentarily looking at a paragraph describing a different output option. My selection is:

• Output transformer (you should order this option when you purchase the kit)
• No output termination
• Transformer installed for 1:1 (center  tap floats, and is installed with the center  tap on the output side.)

In the absence of specific instructions on assembly order, my approach to building a PCB is to work in the order of smallest components first. Accordingly, on the top layer, my assembly order was:

• U2, 24AA32
• MCP 1703, the 3.3V  regulator. (No U number in the schematic)
• U4 jumper between pins 2 and 3 (this may or may not be needed in your kit. With a single ended CMOS chip and the output transformer, U4 is not used and a jumper between pads 2 and 3 is needed.
• C9 0u1 capacitor
• R2 2K ohm
• R3 2K ohm
• R5 (in my configuration, a 0 ohm 0805 jumper)
• C8 0u1 capacitor
• R4 is not used in my configuration
• R7 is not used in my configuration
• R8 is not used in my configuration
• C7 (in my configuration, a 0 ohm 0805 jumper)
• T1 (I mounted the transformer so that the center tap is on the output, thereby providing 1:1 ratio, as the center tap pad is not connected in this orientation.)
• R1 (the through-hole resistor. This is needed only if you plan to operate with a DC supply above 11 volts. Otherwise a jumper wire replaces R1.)

My assembly order of the bottom layer was:

• C1 1u0 capacitor
• C2 1u0 capacitor
• C3 1u0 capacitor
• C4 0u1 capacitor
• C5 0u1 capacitor
• C6 0u1 capacitor
• U3, the Si570. Note that the Si570 has 8 pins. Three on either side with large pads and one on each end with small pads.

The photo below shows the board  bottom at this stage of assembly. (The Si570's two end pads are not soldered yet in this photo.)

One tab fits through the large hole. The other tab must be bent horizontally and soldered to the ground pad near the board's outer edge as illustrated below.
 

As the last steps, I installed 3-place header sockets on the board for DC input and RF output. The kit does not include these parts, and instead provides header pins for the purpose. The builder is instructed to install the pins and bend them 90 degrees to the board. I did not install connectors for the band decoder nor the 3.3V DC input as I don't plan to use these features.

I also installed the provided 5x2 header socket on the LCD. The completed board is shown below.
 

There's no formal post-build checkout process documented, so I did the normal things. Resistance check of power supply to ground, check for shorts on the Si570 output circuit and the like.

When powering the kit up, I used an HP E3610A power supply with current limiting set to 100 mA, with the thought that any major fault would trip the current limiting before damage occurred.

I then applied power and found the LCD functioned and I could change frequency, save frequencies, etc., but the output frequency was frozen at the default initialization value of 56.320 MHz. To make a long story short, I found that one of the Si570's two end pads were not well soldered. I touched up both pads and went through the controller initialization process described in the Builder's Notes, and all worked as it should. I changed the default starting frequency to 10.000 MHz.

The image below shows memory position 20 at 10.000000 MHz.
 

I looked at the frequency stability from startup and found the Si570 quite stable. After a brief warm-up change of about 40 Hz, the frequency stabilized within less than 1 Hz. This is not a particularly stressful test, as the temperature in my basement workshop didn't change more than a couple degrees during this time, but it shows that in a reasonable environment, the output is quite stable. The absolute error from the commanded 10.000000 MHz is about 48 Hz. 

After completing the kit, I wrote in this review that it was not possible to calibrate the Si570's frequency. Thanks to Alan, G4ZFQ, John, K5JHF and Kees, K5BCQ, I've learned that is not the case.

The controller's firmware stores the Si570's nominal startup frequency in memory location 0. As explained by Kees, this is 56.320000 MHz because the Si570 parts sold with the kit have a default of 56.320000Mhz. This frequency was selected because it yields a good center frequency when divided by the 20 meter Softrock's divide by for chain—14.080MHz. (If obtained from a different source, the Si570 might have a different default frequency. Kees goes on to say "if not specified, SiLabs generally supplies 10.000.000MHz default parts. This is not a problem at all for the kit, but location "0" has to be changed to the default frequency of the Si570 part you are using."

After applying the calibration, my frequency error was reduced from around -4.5 PPM to 0.1 PPM. (Still subject to some thermal drift and probably long term aging as well, of course.)

The calibration process is:

Memory location 0 holds the default "true" power up frequency. Since I obtained my SI570 as part of the kit, the nominal value is 56 320 000 Hz matches  the nominal startup of my Si570 oscillator. (I'll add spaces for readability but the display does not have spaces.)

The value stored in memory location 0 should be replaced by the actual measured start up frequency. This measured value will then be applied by the controller as a correction factor to all frequency calculations. The correction factor is calculated as part of the controller's power-on initialization, so changed values will not take effect until the controller power is cycled off and on.

Step by step instructions are:

1. Allow the Si570 to warm up, as well as whatever frequency measuring equipment you plan to use.
2. Select memory position 0. You should see 56 320 000 and the Si570's output frequency should be close to this value
3. Enter the actual measured value into memory position 0. In my case is was 56 319 742 Hz.
4. Save this value to memory by pressing in the rotary encoder knob until saved.
5. Turn the power off and back on. (The controller only reads memory position 0's value upon power on initialization.)

The before and after results are quite impressive:

100 MHz was 99 999 530.0 and is now 100 000 000.6 Hz
13 MHz was 12 999 942.0 and is now 13 000 002.0 Hz
10 MHz was 9 999 955.67 and is now 10 000 001.00 Hz

As seen in the stability plot below, of course, the output frequency moves around bit with temperature variation.

After calibrating the Si570, I again looked at stability, this time for a longer period of 7.5 hours. As the plot below reveals, the frequency calibration did nothing for the start-up transient (and of course it should not have done anything) but after a 15 to 20 minutes for stabilization, the output frequency stays quite stable, with a relatively small frequency change.

I started the run early this morning while my basement shop was cool and the air conditioning was not running. By noon, the outside temperature warmed up to 85°F and the air conditioning was cycling on and off, and the shop temperate varied three or four degrees during the air conditioning cycle. This shows up around 250 minutes into the run, where the oscillator temperature cycles over 1.5 Hz or so and tracks temperature changes in the shop.

As a rough estimate, therefore, the Si570's temperature coefficient looks to be around 0.5 Hz/°F at 10 MHz. Converting to parts per million and the Celsius scale, the temperature coefficient is around 0.1 PPM/°C.

The Si570 shipped with the kit has a temperature stability rating of ±50 PPM over the range -40/+85°C. On a per degree basis, assuming the full plus and minus range is used, the temperature coefficient is 0.8 PPM/°C, considerably more pessimistic than observed. I didn't measure my shop's temperature swing, and I could easily be over-estimating the peak temperature excursion. It's also likely that the Si570's temperature coefficient is non-linear, with greater stability in the normal operating range. Crystals are usually cut with the region of maximum stability in the target temperature range and one would expect this practice to be followed in the crystal embedded within the Si570. Finally, it's possible—and indeed probable—that the Si570 has an on-board temperature sensor and computes and applies a temperature based correction rather than placing all the thermal stability burden upon the crystal.

To be useful in a high performance receiver, the Si570 must have low phase noise. My first measurement is based on John Miles's PN program that collects phase noise data with one of a variety of spectrum analyzers operating under HPIB control. The image below shows the phase noise measurements taken with the PN program and my Advantest R3463 spectrum analyzer.

To see whether the measurements are limited by the R3463, particularly for close-in measurements, I looked at three signal generators, one crystal oscillator and the Si570.

The figure below shows the result of mixing a 10.050 MHz signal from one of two oscillators with a high quality 10.000 HP oven oscillator. Phase noise will appear as broadening of the trace and higher noise level at frequencies further from the carrier. Of course, this image represents the composite noise of the oscillator under test and the crystal oscillator.

Compared with the 8640B, the Si570 compares very favorably close in. (The span is 1 KHz in this image.) Note that the Si570 trace is narrower than that of the 8640B close in to the carrier and that its noise is several dB below the 8640B at you move away from the carrier.

By comparison to the 8640B, therefore, we can say that the Si570 is quite a capable performer. (The 8640B has for many years been the "gold standard" for low phase noise signal generators, although some newer synthesized instruments are at least as good.)

The Si570's output is a square wave, with fast rise/fall times. It is accordingly rich in harmonics, with odd order harmonics predominating. (A perfect square wave has only odd order harmonics.) The two spectrum analyzer images below show the Si570's output with a maximum frequency of 1 GHz with the Si570 set for an output frequency of 100 MHz. (In fact, significant harmonic energy is seen above 2 GHz.) Quite impressive.

The Si570's rated rise and fall time is 1.0 ns, 80%-20% while the TDS-430 performs the more traditional 90%-10% measurement. In addition, the TDS430's internal 90-10 rise time of 850 ps must be considered as the oscilloscope can only measure and display the composite rise time of the input signal and the instrument.

Si570 Output without low pass filter

Jeff, AC0C has asked how the Si570's output holds up with frequency. The plot below shows the output of my Si570 kit from 4 to 260 MHz.

The data is taken with the Si570 kit output connected through 6 feet of RG-316 Teflon coaxial cable to an HP 8482A power sensor, read by an HP 437B digital power meter. Since the Si570 is not designed as a 50 ohm output device, I suspect the wiggles in the measured data are related to mismatches. I have not corrected the data for loss in the RG-316 cable, which is 0.5 dB at 100 MHz and 0.8 dB at 250 MHz.

The linear regression line shows a good fit to a decrease in output power of 0.023 dB/MHz.

The Si570 is a remarkable performing signal source and this kit provides an inexpensive way to build and operate the Si570.

As good as the kit is, it could be improved in a few areas:

• This is not a kit for beginners, in part due to the documentation. Better documentation would allow less skilled kit builders to complete the kit.
• Bottom silk screening would help as well. (In the interest of full disclosure, my Z10000 kit does not have bottom silk screening either. My later kits with bottom mounted components have bottom silk screen, however.)
• If the builder decides to separate the controller board from the LCD display, there's no good way to mount the controller board. One possibility I would like to see is a rotary encoder with a threaded shaft. Or, a pair of mounting holes in the controller board, although there's not a lot of extra room on the current PCB.
• It's not always easy to determine whether you are changing MHz or 10's of MHz or 100's of KHz. The decimal point is presently used as a cursor to indicate the changed digit, thus requiring the user to count zeros to determine which digit is actually being changed. If the LCD supports it, a better arrangement would be useful, such as using the decimal point as indicating MHz and flashing the digits being changed on and off.
• It would be nice to have an RS-232 interface to the microcontroller, with a few simple commands such as set frequency, read frequency, save to memory, recall from memory. The interface could be logic level with a full bipolar RS232 daughter board as an accessory. I see no uncommitted pins in the current microcontroller, however.