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A Stable, Low-Noise Crystal Oscillator for Microwave and Millimeter-Wave Transverters

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A Stable, Low-Noise Crystal Oscillator for Microwave and Millimeter-Wave Transverters A Stable, Low-Noise Crystal Oscillator for Microwave and Millimeter-Wave Transverters Would you like to try narrow-band modes in the gigahertz bands? If so, you’ll need a very stable and ultra-clean local oscillator. This project fills that need. By John Stephensen, KD6OZH mateurs are using narrow- than at lower microwave frequencies. commercial equipment on nearby fre- band modulation—including On SSB, the indicated frequency quencies or amateur beacons operating ACW, SSB and NBFM—on ever- should be within 500 Hz at both the at the same site. higher frequencies. In the US, SSB is transmitter and receiver, or you may I decided to replace the LO in my commonplace on all microwave bands not hear the station calling you. Dur- 24-GHz transverter and solve both the through 10 GHz and is spreading to ing a microwave contest, you don’t phase-noise and stability problems. the 24- and 47-GHz millimeter-wave want to adjust both the antenna and This article describes the crystal oscil- bands. In Europe, narrow-band opera- the frequency while trying to make a lator and multiplier designs that re- tion has taken place as high as contact. I wasted many hours during sulted. They work nicely with existing 411 GHz.1 the last 10-GHz-and-Up contest be- transverters using the KK7B LO The local oscillator (LO) used at cause the LO in my transverter was design2 with a few modifications and these higher, millimeter-wave fre- 85 kHz off frequency at 24.192 GHz. can be adapted to others. The KK7B quencies must be much more stable A lesser-known problem is phase LO was originally designed for a noise. When extended to millimeter- 2304-MHz transverter and was ex- wave bands, most published LO designs tended by N1BWT (now W1GHZ) with 1Notes appear on page 17. for amateur microwave transverters an additional ×5 multiplier for his have excessive phase noise that limits 10-GHz transverter.3 Replacing the 153 S Gretna Green Way the dynamic range of the transverter. original LO with the circuit described Los Angeles, CA 90049 Many contesters on mountaintops have here and modifying the first multiplier [email protected] experienced this desensitization from board make this technology much Nov/Dec 1999 11 more usable on the millimeter-wave perature sensitivity that can vary the put resistance of the 2N5179 emitter is ± Ω bands at 24 GHz and above. resonant frequency by 10 ppm very low (26/Ie ). With the transistor from 0 to +70°C, which amounts to biased for 5 mA emitter current, as The Phase Noise ±100 kHz at 10 GHz. Temperature- shown, the input impedance is approxi- Problem Explained compensated crystal oscillators and mately 5 Ω. At resonance, the fifth- The LO for a microwave or millime- those in ovens do better at ±0.3 ppm or overtone crystals used in this circuit ter-wave transverter is usually a ±3kHz at 10 GHz. This is totally unac- have a series resistance of about 60 Ω. crystal oscillator operating around ceptable at millimeter-wave frequen- In this type of oscillator, the peak cur- 100 MHz, which is then multiplied up cies, where the drift is multiplied to rent through the crystal is approxi- to the amateur band in use. It is critical ±7 kHz at 24 GHz or ±75 kHz at mately equal to the standing current; that this oscillator have a very low noise 250 GHz. the RMS current through the crystal is floor because each stage of multiplica- To provide a rock-stable LO, the 3.5 mA, and the power dissipated in the tion adds noise to the signal at a rate of only solution is to phase lock the crys- crystal is I2R or 0.735 mW. 6 dB each time the frequency doubles. tal oscillator to something more The trouble is that the amount of In addition, the frequency multiplica- stable. Small rubidium frequency power appearing at the 2N5179 emitter tion process is lossy; signal levels can standards are now available at mod- is only (0.0035)2(5) = 0.061 mW or decrease rapidly with high-order mul- erate cost on the surplus market. Typi- –12 dBm, and the noise figure of the tiplication, and the noise inherent in cally, they are removed from obsolete 2N5179 is probably about 10 dB in this low-level amplifiers can degrade the radio-navigation equipment.4 The configuration. The noise floor can be noise floor even faster. Multiplying long-term accuracy over temperature calculated by taking the noise power the crystal frequency by 100 to reach is 1 part in 109 or 0.001 ppm. This caused by circuit resistance, –177 dBm, 10 GHz increases the noise floor by at results in an accuracy of ±250 Hz after adding the noise figure and subtracting least 40 dB. At 250 GHz, the added multiplication to the highest amateur input power level. This yields –177 + 10 noise would be 68 dB, or more. band at 241-250 GHz. – (–12) = –155 dBc. This is only a first- Many VHF crystal oscillator circuits order estimate, and the actual oscilla- have a noise floor no better than Improving Phase Noise tor could be several decibels worse. –155 dBc/Hz. This means that noise 155 The phase-noise problem is solved by The power level in the crystal can- decibels below the carrier power will building a crystal oscillator with a not be raised, as it would cause exces- appear in each hertz of bandwidth at lower noise floor. The ratio of the noise sive aging and instability. We must the oscillator output. Multiplying to at the input of the transistor to the sig- provide more input to the transistor by 245 GHz increases this to at least nal arriving from the crystal ultimately increasing its input impedance to pro- –87 dBc/Hz. This is the best case, and determines this noise-floor level. vide a better match to the crystal. With the degradation will always be several The traditional common-base Butler a bipolar transistor amplifier, this decibels worse due to the noise figure of oscillator described in The ARRL UHF/ means reducing the emitter current; components within the multiplier Microwave Experimenter’s Manual but this would also reduce the power chain. (see Fig 1) shows the problem. The in- available, exacerbating the problem. The problem with a high LO noise floor is that signals outside the re- ceiver passband mix with this noise and appear as increased noise within the passband, reducing sensitivity. If NBFM is being used, the noise that appears in the 16-kHz bandwidth will be –87 + 42, only 45 dB below the level of the interfering signal: The receiver dynamic range is 45 dB. Therefore, any signal appearing within the pass- band of the RF circuitry (several giga- 1 hertz wide) that is 45 dB (about 7 /2 S-units) stronger than the desired sig- nal will mask it completely. When transmitting, broadband noise will be radiated that is only 45 dB below your signal. The problem is smaller at lower frequencies but still results in a re- ceiver that is easily overloaded. The same example at 10 GHz results in a 71-dB dynamic range, which is more acceptable, but still 15 dB worse than most VHF/UHF transceivers and more than 30 dB worse than many HF transceivers. Improving Stability The first issue is frequency stability. Quartz crystals have inherent tem- Fig 1—Conventional Butler Oscillator 12 QEX The input impedance of a common- (1,600,000/60) and the loaded Q is amplifier providing a high-impedance gate FET is the inverse of the trans- about 8700 in the worst case input for the signal from the crystal conductance and is independent of the (1,600,000/(60+125)). This results in a (Y1). A 2N5179 bipolar transistor (Q2) standing current. The transconduc- bandwidth of about 11 kHz, which is is an emitter follower providing low- tance for a J310 FET is about 8,000- fine for SSB and CW operation. To impedance drive to the crystal. Y1 is a 18,000 µS; its input impedance in a ensure that there is no additional deg- fifth-overtone, AT-cut crystal ground common-gate configuration is, there- radation in Q, the crystal must be for operation in the series-resonant fore between 56 and 125 Ω. It also has driven from a low-impedance source. mode with a load capacitance of 30 pF. a noise figure of less than 2 dB at Once we have a low-phase-noise os- The feedback path is completed 100 MHz. Replacing the 2N5179 with a cillator, we need to make sure that it through a resonant circuit consisting of J310 and keeping the same crystal dis- is not degraded by succeeding stages C1, C2 and L1 that selects the desired sipation results in an input power of in the LO chain. The amplifier(s) im- overtone. R14 loads the drain of Q1 to (0.0035)2(56) = 0.686 mW or –2 dBm in mediately following the VCXO must ensure linear operation. It also sets the the worst case (lowest input imped- contribute very little noise, and the loop gain of the oscillator. The dual ance). We can also assume that the J310 initial frequency multiplication must varactor diode, D1, in series with the noise figure may be degraded some- be done in small increments to mini- crystal, provides for pulling of the crys- what, to 3 dB, by noise in later stages. mize reduction of the LO level at any tal frequency by ±500 Hz.
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