A Battery-operated Microwave Counter with an Innovative Sampler Design

A Battery-operated Microwave Counter with an Innovative Sampler Design

Hewlett-Packard Co. (HP)
Santa Clara, CA

The 53150 series microwave counters are easy-to-use instruments that feature all the performance (sensitivity and FM tolerance) and functionality (frequency offset and power measurement) of a laboratory instrument and the durability and ruggedness to cope with a potentially hostile field environment. Several innovations were necessary for this product to meet its design objectives, including a new sampler design and clean VCO, a single-board layout, effective power consumption management, optimization for harsh environments and an easy-to-use yet powerful human interface. Table 1 lists the counter’s performance specifications.

Enabling Technology

The heart of any microwave counter is its sampler. The sampler translates the input microwave frequency to an IF, which is then counted. This translation involves mixing a harmonic of a phase-locked VCO signal (typically in the range of 300 to 500 MHz) with the incoming signal to achieve an IF signal in the range of 20 to 200 MHz. Thus, the counter’s microwave input is limited at the low end to the mixing of the first harmonic (300 to 500 MHz) with the incoming signal. As a result, the lower frequency limit becomes approximately 300 MHz.

The quality of the phase-locked VCO signal also is critical to the performance of the counter. Any noise on this signal is multiplied depending on the harmonic used to mix down the input signal. Thus, at 46 GHz the 100th harmonic of a 460 MHz phase-locked signal is used. This multiplication represents a 40 dB degradation in phase noise and spurs. With a microwave counter, this degradation is reflected in the residual stability of the least-significant digit (LSD). The 53150 series microwave counters have a residual stability specification of less than 0.8 LSD RMS at 46 GHz. Other designs have residual stabilities that are as much as 40 LSD at 46 GHz due to noisy phase-locked signals. This residual stability sets the accuracy limits of any microwave counter.

For some time, microwave counters have had some rudimentary ability to measure the power of the incoming microwave signal. Typically, this power measurement is accomplished by also using the sampling/mixer diodes as detector diodes. This method places several limitations on the power measurement: The same 300 to 500 MHz lower frequency limitation applies to the measurement of power and the dynamic range is limited by the mixing diodes that are not optimized for detection. These limitations have been a nuisance to users primarily in field service applications where the need to switch inputs for frequency measurements or to carry additional equipment (power meters) for power measurements creates problems. The 53150 series counters address both these problems.

Solving the frequency range limitations for the frequency measurement meant a need for additional switching circuitry in the sampler. When the input frequency falls below preset limits, the sampling diodes are switched out and the signal is routed directly to the IF amplifier and the count chain. Since the power measurement does not use the sampling diodes, it is not affected by this routing change, resulting in the unique ability to measure both frequency and power on a single input from as low as 50 MHz to as high as the counter will count (46+ GHz).

The sampler design uses high efficiency, balanced, GaAs sampling diodes, producing performance beyond 50 GHz and, just as importantly, a 3 dB improvement in drive requirements (from +27 dBm to +24 dBm). Assuming 50 percent efficiency (DC to RF), this 3 dB change represents a 0.5 W reduction in power required from the sampler driver, which is important for a battery-powered instrument. The balanced configuration also reduces kickback noise (harmonics of the drive signal that travel through the sampler to the source being measured). As good as these diodes are, their dynamic range is limited to approximately 20 dB when used as detection diodes for power measurement.

To alleviate this problem, a detection configuration was developed (patent pending) involving placing a zero-bias Schottky diode detector in parallel with the sampling diodes. This detector then measures power without frequency range limitations. Temperature compensation is taken care of with a lookup table. Using a Schottky detector in this configuration improves the dynamic range and accuracy of the power measurement when compared to the more traditional approach of using sampling diodes. It also allows the automatic monitoring of the input signal, which, if absent, can be used to initiate a battery-saving sleep mode. Figure 1 shows the power meter’s accuracy vs. frequency at –20 dBm. Figure 2 shows the counter’s input sensitivity.

A Single-board Layout

A single-board design simplifies manufacturing (improved costs), reduces the number of parts and interconnects (improved costs, lower power consumption and better reliability), and reduces size and weight (important for a field-portable product). The challenge of a one-board approach is basically one of maintaining electrical integrity. That is, the mixing of high speed digital circuits and high sensitivity and/or high gain analog circuits can cause unwanted interactions through radiated or conducted interference. The 53150 series counters incorporate all the counter circuitry typically occupying three to five separate PCBs into a single 8" x 11" board with no compromise in performance.

The toughest performance challenge was to maintain counter sensitivity at or better than the existing 5350 series counters. Inevitably, high sensitivity involves high gain in the IF section along with narrow bandwidths. High IF gain makes the counter more vulnerable to radiated and conducted noise while narrow bandwidths increase the complexity and time required to acquire a signal. Clever use of miniature, chemically milled enclosures ensures no degradation in sensitivity due to RF interference (RFI). In most of the ranges, sensitivity is roughly 3 dB better than the 5350 series. In addition, innovative acquisition algorithms were developed that, together with a fast-switching, phase-locked VCO, achieved acquisition times below 80 ms.

This single-board design also required the incorporation of the latest in very large-scale integration (VLSI) digital circuitry. Recent price reductions and technical advances in programmable logic devices and field-programmable gate arrays (FPGA) have allowed the replacement of much of the peripheral digital circuitry (glue logic) and virtually the entire count chain with a single VLSI chip.

For troubleshooting or testing, the FPGA can be configured as a test signal generator instead of a counter, allowing tests to be performed that will isolate and verify the performance of specific portions of the single-board design. Thus, the service technician can locate and isolate the circuit at fault quickly. Additionally, the method of counting (reciprocal or gated) can be changed on the fly (patent pending). This choice is especially useful when measuring noisy signals. By definition, a gated counter is superior to a reciprocal counter when measuring noisy signals due to the impact of trigger error. However, the gated counter does have limitations in the area of resolution per unit time. The ability to switch on the fly permits selection of the most accurate measurement method under any circumstances while maintaining optimal resolution.

In addition, the microcontroller chosen (the 20 MHz version of Intel’s model 80196) is a good example of extended integration. This chip integrates many circuits that normally would have to be added as part of the digital circuitry, including an analog-to-digital converter.

Power Management and Consumption

A number of elements were combined to yield an energy-efficient product that will run continuously on two standard camcorder batteries for typically more than three hours. The power-saving elements include a backlit liquid-crystal display (LCD) whose backlight can be user controlled to save on power; a microwave sampler, which requires half the drive of previous-generation samplers; the extensive use of CMOS and low power TTL circuitry; and a unique sleep mode.

This sleep mode involves the use of the sampler in conjunction with microprocessor power management similar to that available in portable computers. With the 53150 series counter, the sampler design is used to determine if there is sufficient power at the input for the counter to operate correctly. If sufficient power is not present for approximately five minutes, an interrupt is generated to the microcontroller that tells it to go into a sleep mode until such time as there is a sufficiently large signal that can be counted reliably. The sleep mode involves not only powering down the microcontroller to microwatt levels (a standard feature of the microcontroller), but also turning off a significant portion of the internal circuitry (including drive to the sampler).

Optimization for a Field Environment

An instrument that is to be used in the field must have good mechanical protection, a display readable in direct sunlight or darkness, flexibility in operating power, an interface with data-logging equipment and be lightweight. The 53150 counter’s package is mostly metal with the front and rear bumpers made of shock-absorbing elastomers. Metal was chosen over plastic for better mechanical durability and good RFI shielding. This rugged package weighs 8 lb without batteries. The bumpers protrude over the front and rear panels to protect the various connectors from bearing the full shock of a fall. The handle doubles as a multiposition support stand.

The display is a custom LCD type with a user-controlled light-emitting diode panel backlight, which makes it easily readable at a distance of 15 ft under all lighting conditions. The frequency and power readings appear separately on two rows.

The field environment also requires flexibility in operating power. DC power to 18 V may be input directly into the back connector. AC power from 100 to 265 V AC (45 to 440 Hz) can be accommodated with no change to any rear-panel settings.

To accommodate requirements for data logging in the field, the counter has a standard fully programmable RS-232 interface in addition to the standard HP-IB interface. All that is necessary for gathering data unattended at remote sites is a portable PC with an RS-232 interface.

An Easy-to-use Human Interface

The human interface design objectives of the 53150 series counters were somewhat conflicting: an easy-to-understand front panel with full functionality in an 8" x 3" space. The design features only four buttons for function control (frequency, power, offset frequency and offset power) and four buttons for setting the operational parameters (resolution, sample rate, number of averages and HP-IB address). In addition, the display presents important secondary parameters, including the state of battery charge. A peaking indicator is included to extend the utility of the internal power measurement capability.

Conclusion

The 53150 series frequency counter is rugged, durable and inexpensive for use in the field. Three models are available for measurements to 20, 26.5 and 46 GHz. Prices: $6250 (20 GHz), $7450 (26.5 GHz) and $14,500 (46 GHz). Delivery: four weeks (ARO).

Hewlett-Packard Co. (HP), Santa Clara, CA (800) 452-4844.