Timing

Timing 201 #17: The Case of the Tenacious Legacy Frequency Synthesizer

Author: Kevin G. Smith

Introduction

This post is a tribute of sorts to the oldest legacy device that Skyworks Timing supports, the Si4133 RF/IF Synthesizer. See the functional block diagram below. The Si4133 has been selling for well over 25 years now, since before I joined the company. During that long time period, there has always been sufficient interest in this part to keep it from being obsoleted, hence my use of the term “tenacious”. Why that is the case is the subject of this post: The Case of the Tenacious Legacy Frequency Synthesizer. We will also review how one can write and read registers to and from the old Si4133 EVBs (Evaluation Boards) without requiring a legacy PC.

A Brief History

Silicon Labs released a version of the Si4133 CMOS RF synthesizer chip intended for mobile phones back in 1999. Years later, the Aero mobile phone chip business was sold to NXP in 2007 except for a few select frequency synthesizers. Shortly thereafter, Silicon Labs Timing inherited their support. By 2020, all frequency synthesizers other than the Si4133 and its variants were moved to EOL (End of Life) status. Skyworks then picked up the Si4133 frequency synthesizers as part of its acquisition of Silicon Labs’ Infrastructure and Automotive business, completed on July 26, 2021.

As you can see the Si4133 has a long history rooted in the mobile phone business. That’s why the datasheet lists applications such as GSM, DCS1800, PCS1900, cordless phone, and wireless local loop systems. These were some of the dominant wireless applications of interest in the late 1990s/early 2000s. This heritage explains some of the device’s features which still make it relevant today. First, let’s consider the differences between Frequency Synthesizers and Clock Generators.

Frequency Synthesizers versus Clock Generators

In a general sense, clock PLL devices such as Clock Generators are “frequency synthesizers”. However, in industry, the term “frequency synthesizers” is usually intended for independent frequency devices that can quickly tune communication systems. Originally this meant supplying the LO (Local Oscillator) frequencies to a superheterodyne radio. The highest frequencies are usually sinusoidal. Lower frequencies can be divided down since mixers can be driven by sinusoids or square waves.

The table below compares roughly equivalent powerup settling time specs for the Si4133 versus a couple of different clock generators, the Si5332 and the Si5391 (Grade A). Note that the example clock generators are 1-2 orders of magnitude slower than the Si4133 even given its more rigorous spec. Typically, there isn’t a compelling reason in clock trees to require a clock generator to output clocks much faster than 10s of ms. However, RF Synthesizers such as the Si4133 needed to tune in about 200 us in order to meet GSM time slot requirements.

What Originally Made the Si4133 Special?

Per the block diagram above, the Si4133 integrated not just the PLL phase detectors and programmable dividers but the loop filters, resonators, varactors, and 3 VCOs. It required few additional components and did all of this for 18 mA typical supply current and was available in both a 24-pin TSSOP or 28-lead QFN.

The Si4133 was optimized for a time when radios operated below 2 GHz and often used superheterodyne architectures. The frequency ranges covered are listed below from the datasheet.

Dual-band RF synthesizers
- RF1: 900 MHz to 1.8 GHz
- RF2: 750 MHz to 1.5 GHz
IF synthesizer
- IF: 62.5 to 1000 MHz

These ranges describe the device’s overall frequency coverage. The external inductors establish the VCO center frequencies, around which the PLLs provide a more limited tuning range. More modern radios operate at higher frequencies, use different architectures, and require finer frequency resolution and lower phase noise fractional synthesizers.

Why then is the Si4133 so tenacious?

The Si4133 is tenacious in part because it has just the right amount of Y2K-era integration for certain applications. While it can’t compete against transceiver chips in modern short-range wireless applications markets, it remains suited for custom sub-2 GHz radios, wireless audio, industrial telemetry, and long-life legacy communication systems. At some point, a synthesizer that has been available continuously for over two decades has its own BOM stability appeal.

A few more points in its favor:

The Si4133 is a relatively simple device. There are only 9 registers to write.
The Si4133 supports programmable power-down modes with 1 uA standby current and a modest 18 mA current consumption when active. These features together with the t_PUPspec, and modest phase noise performance, are rooted in the device’s heritage of originally targeting battery operated GSM handsets.
There are a number of Si4133-based variants including the Si4112 which is a popular IF only part with an even lower 8 mA typical current consumption.

Here are some markets where the Si4133 and its variants can still offer “bang for the buck” value:

Wireless audio systems
Legacy industrial communications (railroad communications, remote monitoring, SCADA radios, telemetry)
Low duty cycle battery powered sensing systems such as asset trackers
Defense and aerospace equipment (radios, training, test) where part longevity is a virtue
Education (radio amateurs, university RF labs)

I listed Education as the 5^th market because the Si4133 occupies a sweet spot of integration where a student in an RF lab for example can develop a superhet radio but rely on the Si4133 to provide the LOs. A student can explore many superhet radio topics without having to design and tune multiple VCOs from scratch.

How to interface to the legacy EVB?

The original Si4133-EVB and Si4133M-EVBs were developed in the Windows XP era and the GUI software at the time was based on National Instruments’ LabView. Communication relied on a custom Parallel Port cable. We still have a WXP PC in the cube next to me for supporting legacy devices. (I have a pirate flag sign on it so people will leave it alone and not “upgrade” it 😊.) We looked into a more modern USB version of the EVBs but sustaining the GUI software was a bigger issue than developing the hardware.

It used to be that when customers wanted to buy and evaluate the EVBs we would suggest they buy a used WXP machine with a parallel port as they were cheap. These run between $100 and $200 on eBay as I write this post.

However, there is an alternative to the legacy PC approach: Use a modern SPI host adapter such as the Total Phase Aardvark I2C/SPI Host Adapter shown in the photo below. Many companies have such host adapters on hand already for other development work.

If you only wanted to write to the device, you could simply map the Si4133’s SCLK, SDATA, and SENB to an SPI host’s SCLK, MOSI, and SS configured as active-low. The Si4133 uses 22-bit words but the Aardvark only supports 8-bit SPI words. Therefore, one would have to pad to 24 bits (3 bytes). That’s OK as the datasheet states “The internal shift register ignores leading bits before the 22 required bits.” You could then verify that the Si4133 was configured correctly by measuring the frequency of the outputs.

On the other hand, it is very useful to able to read registers, not just to verify writes, but to read back synthesizer tuning codes as described in AN31: Inductor Design for the Si41XX Synthesizer Family. Reading registers is a bit more complicated as the Si4133 interface is not implemented as a standard SPI mode.

Rather it is better described as a 3-wire synchronous serial interface that is closest to a half-duplex SPI-like bus with a direction pin where reading and writing occur in separate phases. See for example AN31’s Figure 4. Read Timing reproduced below. Note how the Si4133’s 3-wire interface takes SENB high to read data from the part and takes SENB low to write data to the part.

The serial interface is half-duplex because we write data on SDATA and read data on AUXOUT, never both simultaneously. AUXOUT is multiplexed between acting as a functional synthesizer output for LDETB and temporarily acting as a MISO output during read operations. The advantage of this approach is that it saves a pin.

Here are the main points when interfacing the Aardvark I2C/SPI Host Adapter to one of the Si4133 EVBs so as to support both writing and reading registers.

While SENB is low, the read command is only shifted into the input shift register. The Si4133 does not act on it until the word is latched on the SENB rising edge. Only then is the selected value loaded into the AUXOUT output shift register, so AUXOUT does not carry the requested data while the command is being clocked in.
The read-out clocks must therefore arrive after SENB has risen, with SENB held high.
When SS is wired to SENB, every SPI transfer pulls SENB low again. The read-out clocks then arrive with SENB low, so they shift a new word into the input shift register instead of shifting the read data out on AUXOUT.
SENB therefore has to be controlled separately from the SPI transfer. Given the SENB direction requirement, SENB is driven by Aardvark GPIO 0x02 (SDA), and the Aardvark SS line is not connected.
For every read, the Aardvark has to do a dummy transfer of 24 SCLK pulses while SENB is high.The Si4133 ignores SDATA during that time and shifts the value out on AUXOUT, and the Aardvark captures it on MISO.

Below is a table summarizing the Aardvark-to-EVB Wiring using the GPIO-SENB approach just described and which matches the previous photo.

One of my colleagues will be writing an application note with more details on this procedure.

Summary

The venerable and tenacious Si4133 RF/IF Synthesizer continues to find a role in custom sub-2 GHz radios, wireless audio, industrial telemetry, and long-life legacy communication systems. The Aardvark I2C/SPI Host Adapter can be used with the legacy EVBs to evaluate these devices.

I hope this post was interesting and that you find it helpful should you wish to evaluate the Si4133 frequency synthesizer in the future.

As always, if you have topic suggestions or questions appropriate for this blog, please send them to kevin.smith@skyworksinc.com with the words Timing 201 in the subject line. I will give them consideration and see if I can fit them in. Thanks for reading.

Cheers,

Kevin

Timing

By Kevin G. Smith Sr. Principal Applications Engineer

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