Sigrietics NE602 as a Test Instrument
Use the Signetics NE602 doubly balanced active mixer as a down converter to expand the useful bandwidth of an oscilloscope. The demonstration circuit shown has been employed to observe clearly the 13th harmonic of the crystal oscillator at a frequency of 1663.857 MHz. In this example, the 3rd harmonic of the signal generator operating at a fundamental frequency of V3 1663.857 MHz or 554.619 MHz is positioned a few kHz above or below 1663.857 MHz to provide a convenient IF display on the oscilloscope. The 4th harmonic of the crystal oscillator is observable using a fundamental frequency from the generator. Lower frequency signal generator sources may be used to observe other bands of interest, however, as the converted frequency and the upper bandwidth limit of the oscilloscope converge, filters must be employed to obtain meaningful oscillographic displays.
• Distortion will result if either pin 1 or pin 6 is overdriven. Drive voltage from the oscillator shown will be optimum at 127.989 MHz with C1, C2 eliminated and direct coupling made to the 0.01 blocking capacitor.
• Trimmers C1 and C2 provide a "peaking" adjustment at particular harmonics. The effect of these adjustments is clearly displayed on the oscilloscope.
• The signal generator is an ancient, continuously tuned UHF TV tuner. A panel-mounted BNC receptacle brings out a small amount of its local oscillator voltage to the demonstration circuit. Coarse calibration was made with the help of a modern digitally tuned TV set.
• It is probable that almost any oscilloscope can be made to function to some extent in this system. Best results will be obtained when using an oscilloscope with triggered sweep, 5-MHz or greater bandwidth and vertical sensitivity of at least 5 mV per cm.
The crystals used for my oscillator experiments were especially processed to yield fundamental frequencies of 40 to 160 MHz. Processing capability to at least 500 MHz is possible using this technique. Most crystal suppliers are limited to the production of fundamental crystals of not more than 20 MHz, a few will take orders for units up to 25 MHz and some to 30 MHz with relaxed tolerances.
Performance specifications for either low or high fundamental devices are much the same except for package capacitance. Typically, low to high fundamental processing results in a 3 to 1 capacitance reduction. This reduction becomes increasingly important for successful overtone oscillator performance as frequencies increase above 150 MHz, no matter what order overtone is attempted.
Successful 13th overtone operation at approximately 237 MHz has been accomplished with my circuits when using a fundamental crystal of 18.28 MHz. However, for a practical purpose, such performance must be considered as a technical curiosity.
Overtone operation somewhat above 150 MHz is perfectly feasible with conventional crystals but most suppliers are reluctant to assume orders for this range and still guarantee performance.
Initial quantities of high-order fundamental crystals were obtained in February 1989 (from Innovative Frequency Control Products, Inc, PO Box 300, Plainfield, PA 17081). Two oscillator circuits were devised to exploit the unique capability of these crystals. One, for use in a negatively grounded system, the other for a positively grounded application.
To date, completely satisfactory fundamental crystal-controlled oscillation has been accomplished easily at nine different frequencies from 40 to 160 MHz. All of the original samples performed satisfactorily at their 3rd overtone. For example, the 160-MHz device proved to be an excellent oscillator at about 480 MHz.
Four of the samples did well at their 5th overtone. Three at their 7th, and a 57-MHz fundamental sample operated properly at its 9th overtone of about 513 MHz.
None of the samples were characterized for oscillator service. All were fabricated as filters.
These oscillators are relatively insensitive to power-supply voltage change as well as to environmental capacitance and temperature effects. All appear to operate with efficiencies equal to crystal oscillators operating at lower frequencies.
I am convinced serious overtone oscillator experimentation above 100 MHz should not be undertaken without, at least, minimum instrumentation to monitor progress every step of the way. The NE602 down converter/signal generator/oscilloscope combination was found invaluable for this endeavor. A sensitive active wave meter calibrated over the entire spectrum from 1.5 to over 800 MHz was employed to detect spurious responses and double check overtone frequencies.
The high frequency down converting capability of the NE602 was discovered as a most welcomed by-product during development of instrumentation for this crystal-oscillator project. It was found to be usable to at least 3 GHz. My 1988 Signetics catalog specifies 6-GHz junctions for this device, but I feel current production could be specified to be near 9 or 10 GHz, based on my crude evaluation of its performance.
Do not overdrive the NE602, otherwise severe distortion will result. Also, because of its high frequency capability, exposure to short duration spikes in the microwave region will surely zap the device.
The down-conversion circuit functions in a manner similar to the autodyne system found in some early radio sets. The term "direct conversion" I believe is used now.
The direct-conversion arrangement provides operating benefits not generally appreciated: (1) The bandwidth of each event is determined by the bandwidth of the local oscillator; (2) The usable gain is set by the amplitude of the signal plus the amplitude of the local oscillator plus any differential gain supplied by the NE602.
I have constructed a test fixture complete with NE602, BNC connectors and power supply as a companion to my oscilloscope. It has been used to examine the SSB waveform directly from my home-built 2-m transmitter. I am sure it could be used for similar observations at any higher frequency band to at least 2.3 GHz.—Clint Bowman, W9GLW, Box 282, Prospect Heights, IL 60070
Was this article helpful?