Clocks are integral to most electronic systems. Timing and communications systems need extremely accurate clocks. When I started working with electronics, in the heyday of Citizen’s Band (CB) radio, quartz crystals were on sale everywhere, even at Radio Shack. The CB crystals congregated around 27 and 28 MHz, but many more frequencies were on offer. As I started designing circuits as an engineer, I used many specific crystal frequencies: 32.768 kHz for real-time clock circuits, 3.57954 MHz for the TV color burst, 1.8432 MHz for the Motorola MC14411 bit-rate generator, 4.9152 MHz for the bit-rate generator in the Signetics 2661 enhanced communications chip, and frequencies such as 4.00, 8.00, 10.00, 20.00, 33.33, and 50.00 MHz for clocking early microprocessors of the 1970s and 1980s. In my first engineering job at HP’s Loveland, Colorado facility, I could get any crystal frequency I needed in just a few days from Colorado Crystal, which was located five minutes away from the HP plant. Colorado Crystal, founded in 1968, is still there in the same Loveland location on 8th Street.
As long as I’ve been an engineer, it’s always been a crystal. Quartz crystals have been used for precise frequency control of electronic circuits for the last 100 years. So, I never gave much thought to the use of crystals for precision frequency control. They were simply the standard component for generating precise frequencies. My colleague Max Maxfield recently wrote about some new oscillators from SiTime (see “It’s Time to Learn More about Timing”) based on MEMS (micro-electro-mechanical systems) structures. Max’s article renewed my interest in frequency generation for digital circuits. I visited SiTime in Santa Clara California several years ago when the company first started making MEMS-based oscillators. Max’s article shows that they’ve considerably evolved and improved their product. SiTime’s entirely different approach uses vibrating, MEMS-based, silicon structures to create precise oscillators that could put the industry on a path to eliminating crystals from the EE vocabulary. Crystals could join selenium rectifiers, wax-and-paper capacitors, tubes, and germanium transistors as components that have largely or entirely been replaced by a superior technology.
Pierre and Paul-Jacques Curie, working at the Sorbonne’s Laboratory of Mineralogy in Paris, discovered the piezoelectric effect in 1880. It’s that fundamental effect that allows quartz crystals to resonate at precise frequencies. Quartz crystals were first used to stabilize the frequency of an electronic oscillator in 1917 and 1918, respectively, by Alexander Nicholson at the Bell Telephone Laboratories and Walter Guyton Cady, a physics professor at Wesleyan University. In 1923, August E. Miller started selling quartz crystal blanks to amateur radio operators (hams), who then used Miller’s blanks to make their own finished crystals for stabilizing the operating frequency of their radio transmitters.
It appears that hams did most of the early practical circuit development for crystal-stabilized radio transmitters. Several circuits for crystal-controlled transmitters appeared in QST magazine during 1924 and 1925. (QST has been published by the ARRL (American Radio Relay League) for hams since December 1915.) By August 1925, General Radio was offering finished quartz crystals for specific frequencies at $35 to $50 each, depending on the desired accuracy. In 1926, AT&T’s WEAF became the first commercial radio broadcasting station to set its transmitter frequency with a quartz crystal. Finally, in 1939, the US military adopted crystal control for its radios, just in time for World War II. After the war, this military technology helped to kick off the CB and walkie-talkie craze. (Max Maxfield covered some of this history in his previous article titled “MEMS Oscillators Address Precision Timing Problems”.)
Over the next 97 years, quartz crystals and crystal oscillators became the pervasive component of choice for precise frequency control in every facet of electronic circuit design. Now, SiTime is ready to change that. SiTime’s oscillators do not use the piezoelectric effect. Their timing comes from precisely machined (etched) MEMS structures with resonant mechanical frequencies. Max’s article described a new OXCO (oven-controlled crystal oscillator) from SiTime based on the company’s Epoch Platform, which uses two MEMS structures. The first structure is a mechanical resonator designed to have a flat frequency response curve with respect to temperature. The second MEMS structure is another resonator that’s designed to act as a temperature sensor. (SiTime describes it modestly as “the world’s most accurate temperature sensor”.)
The two MEMS structures are etched into one piece of silicon, and their output signals feed a second CMOS chip, which contains the oscillator’s active circuitry. Oscillation frequency is based on feature sizes defined by precise silicon photolithography and anisotropic etching. SiTime markets these devices as either Super-TXCOs (temperature compensated crystal oscillators) based on its Elite platform or, with the addition of a heater, as OXCOs based on the company’s Epoch platform. Note that SiTime is reusing quartz-based oscillator terminology to avoid confusion as to what the company’s parts do, but there are no quartz crystals inside of SiTime’s parts.
SiTime’s latest TXCOs and OXCOs are based on the Epoch Platform, which incorporates two MEMS structures: a mechanical resonator designed to have a flat frequency response curve with respect to temperature and another resonator that the company describes as “the world’s most accurate temperature sensor.” A second CMOS chip in the package contains the oscillator’s active circuitry. Image credit: SiTime
I wanted to experiment with this once-in-a-century revolution in frequency control, so I asked SiTime for sample parts. Initially, I asked for OXCOs, having a specific project in mind. However, the new SiTime OXCOs appear to be wildly popular, and no OXCO samples were available, so I agreed to try out some SiT5503 Super-TXCOs instead. I’ve always been fascinated by frequency counters, which precisely measure signal frequencies, yet they incorporate only one precision component: the timing crystal. Usually, these timing crystals operate at 10 MHz, so I requested 10MHz devices from SiTime, and the company sent me two sample devices with the part number: SiT5503AI-WW-33E0-10.000000. Using the decoder on the SiT5503 data sheet:
- “SiT5503A” – Device family
- “I” – Industrial temperature range, -40 to 85°C
- “–“ – LVCMOS output
- “W” – Package size (7.0 x 5.0 mm)
- “W” – Frequency stability, ±5 ppb
- “33” – 3.3V power supply
- “E” – Output Enable pin
- “0” – Fixed frequency output
- “10.000000” – Operating frequency in MHz
Note that SiTime programs the operating frequency of these devices at the factory.
My target instrument for using these SiT5503 Super-TXCOs was a factory-built Heathkit SM-2420 frequency counter purchased from eBay for $40. That 40-year-old instrument is based on a Heathkit-designed OXCO operating at 10.000 MHz. The Heathkit OXCO is a large Styrofoam box containing a heating oven. Inside the oven is a 10MHz crystal. The SiTime Super-TXCO exhibits nearly two orders of magnitude better temperature stability than the frequency counter’s OXCO and needs thousands of times less volume and significantly less power to operate. When the Heathkit counter arrived, it looked like it had been stored in a barn for the last few decades. The circuit board was covered with dirt, and the rotary switch shafts were heavily corroded. The power cord had been cut off. As a backup, I paid $45 for an HP 5314 universal counter, which is based on a now-obsolete Intersil 7226 frequency counter/timer on a chip. The HP 5314 also uses an internal 10MHz reference clock, and HP offered the instrument with a TXCO option, but my copy does not have that option. So my HP 5314 also makes a good candidate for a Super-TXCO clock transplant.
When the SiTime Super-TXCOs arrived, the FedEx package contained two SMT parts. Clearly, I needed a circuit board. For that, I turned to my good friend and frequent collaborator Ron Sartore. I’ve known Ron since 1988 when I engaged him in another hands-on electronics project. Back then, Ron’s company, Cheetah International, made the fastest IBM PC motherboards on the market. These days, Ron’s semi-retired, but his company, Altimeter Motives, makes realistic instrument panels for use with flight simulators such as Microsoft Flight Simulator and X-Plane. Ron has a license for the Eagle pcb layout software and an established relationship with JLCPCB, a fast turn pcb manufacturer in China.
I laid out a simple schematic for the Super-TXCO board:
Schematic for SiTime Super-TXCO board. Image credit: Steve Leibson
Because my target application was a 40-year-old Heathkit frequency counter, the supplied operating voltage would be 5V, but my SiTime SuperTXCOs needed 3.3V, so I used an LDO regulator module from Amazon to convert the 5V to 3.3V. Fifteen of these modules cost $9 and have a TO220 footprint, so they’re similar to using 7800 series regulators, but note that the pinout is different. (Please do not ask me how I know. Just look for the magic smoke above my workbench.)
Next, I needed to convert the Super-TXCO’s 3.3V output to something more TTL compatible, so I reused a passive level-shifter circuit from an HP 5314 frequency counter. Ron kindly created a pad stack for the SiTime Super-TXCO’s unique package, laid out the pcb in Eagle, and sent it to JLCPCB for fabrication. I ordered 10 blank pcbs for just under $25, delivered, so each double-sided board cost me about $2.50. When they arrived from China, Ron took the two Super-TXCO samples provided by SiTime and built up two boards, one with the Super-TXCO and all the components shown in the above schematic and one without the passive level-shifting network. Ron’s lab has equipment for SMT assembly. Mine does not.
The Super-TXCO board with the passive level shifter worked as designed, but the shifted output level was barely enough for the Intersil 7226 counter chip’s clock input, and it was sensitive to the 5V level of the power supply, so I took the second built-up Super-TXCO board and added a 74HCT00 quad NAND gate directly to the Super-TXCO’s output, dead-bug style, using the board space previously occupied by the passive level-shifting network. I bought the 74HCT00 from my favorite Silicon Valley electronics surplus store, Anchor Electronics. They cost $0.22 each in unit quantity. It turns out that 74HCT devices are ideal for converting 3.3V signal levels to 5V logic levels. If I were designing this board for production, I’d use a single-gate SMT device like a 74AHCT1G08 from Texas Instruments, NXP, or Diodes Incorporated for level translation. The modified board also worked well. Both boards were oscillating bang on at 10.000000 MHz, based on the instrumentation available to me. The two built-up boards appear in the figure below. Ron Sartore labeled the board “Steve’s TCO,” but it’s his layout.
Ron Sartore and I built up two boards based on SiTime’s Super-TXCOs. The Super-TXCOs, measuring just 7.0 x 5.0 mm, are mounted at the lower left of each board and the 3.3V LDO regulator modules appear at the upper left. The board on the left uses a 74HCT00, mounted dead-bug style, to shift the Super-TXCO’s 3.3V levels to 5V. The board on the right uses a passive level-shifting component network. Image credit: Steve Leibson
SiTime also sent me their standard evaluation board for the SiT5503 Super-TXCO. The company offers some Super-TXCOs and OXCOs with an I2C interface that can be used to pull the operating frequency a few ppm either way. The eval board that SiTime sent to me incorporates a full-featured Super-TXCO and has a series terminated clock output with an SMA connector. The board also provides access to the I2C frequency control through additional connectors. I can easily envision using a microcontroller with a timer/counter and an I2C interface, a GPS receiver’s 1pps output, and one of SiTime’s Super-TXCOs or OXCOs to create a GPS-disciplined oscillator (GPSDO).
The SiTime evaluation board for the SiT5503 Super-TXCO breaks out all the pins on the oscillator, which is located at the board’s center. Image credit: Steve Leibson
All three oscillator boards produce precise 10MHz clock waveforms. I have two candidate frequency counters for oscillator transplants: an HP 5813 universal counter in near-mint condition and the severely weathered Heathkit SM-2420 counter. It remains to be seen whether the Heathkit counter can be revived, so that’s a story for another article. Meanwhile, check out SiTime’s MEMS-based oscillators next time you’re thinking of using a quartz crystal.
Patrick R. J. Brown N7KRG, “The Influence of Amateur Radio on the Development of the Commercial Market for Quartz Piezoelectric Resonators in the United States,” Proceedings of the 1996 IEEE International Frequency Control Symposium (pp. 58 – 65)