NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, West Virginia Electronics Division Internal Report No. 82 NRAO UNIVERSAL LOCAL OSCILLATOR

Transcription

1 NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, West Virginia Electronics Division Internal Report No. 8 NRAO UNIVERSAL LOCAL OSCILLATOR M. Balister OCTOBER 1968 NUMBER OF COPIES: 100

2 NRAO UNIVERSAL LOCAL OSCILLATOR M. Balister TABLE OF CONTENTS Page LGeneral * 0 O 0... Q0 1. Synthesizer-Frequency Controller OOOOO 3. Multiplier Unit * W , 3 4. Universal Local Oscillator Installation Operation of the Universal Local Oscillator.. 0 Go o o Goo see o 5 6. Noise Performance. 0 o o e m0000eee0000tooe 7 LIST OF FIGURES Figure 1 Photograph of the Universal Local Oscillator Figure Block Diagram of the Universal Local Oscillator.. 9 Figure 3 Control Unit for Hewlett Packard Synthesizer Figure 4 DP 3000 Circuit Board O OOOOO n Figure 5 Switch Position Digital Readout to Computer Figure 6 Sweeper Circuit O OOOOOOO Figure 7 Multiplier Unit Circuit Diagram o o w aeoe 14 Figure 8 Multiplier Unit Printed Circuit Board Figure 9 Spectrum of RF Output woo... O

3 NRAO UNIVERSAL LOCAL OSCILLATOR M. Balister General The NRAO Universal Local Oscillator (ULO) is designed to give any output frequency in the range 1- GHz with an accuracy of better than 1 part in The system is built around a Hewlett Packard 500 MHz synthesizer which can be remotely controlled to switch between signal and comparison frequencies. A power amplifier with a frequency range MHz drives a frequency multiplier, the output cavity of which can be tuned to any frequency in the range 1- GHz. The actual multiplication used depends on the output frequency desired; the following table demonstrates this. Synthesizer Output Frequency - MHz r Multiplier Ratio Multiplier Output Frequency - MHz The multiplier generally gives at least 100 mw at the output terminal on the rack at all of these multiplication ratios. This is sufficient to drive most of the NRAO front ends which operate in the 1- GHz frequency range. A 1 watt TWT is incorporated in the LO and may be used to drive higher frequency receivers that require more power than is available from the multiplier. A photograph of the system is shown in Figure 1; the block diagram is shown in Figure. Three of these LO systems have been built. Two will be permanently installed at the 140-foot and 300-foot telescopes and the third is for laboratory and emergency use.

4 . Synthesizer and Frequency Controller The Hewlett Packard 5105A Frequency Synthesizer and 5110B driver form the basis of the local oscillator. The driver wait contains a temperature stabilized 1 MHz quartz crystal with an aging rate of less than d 3 parts per 10 9 per 4 hours. The frequency selection is via front panel push-buttons or by remote switch closure and may be in steps of 100 MHz, 10 MHz down to 0. 1 Hz. A search oscillator provides continuous variable frequency selection with an incremental range of L 0 Hz through 10 MHz. Manual or external voltage control is possible. The remote frequency controller has three adjustable thumbwheel dials for setting the required signal and comparison frequencies. These are designated F19 F and F F is the signal frequency and F and F are the comparison frequencies. o o 1 Normally, F i = F but it may be sometimes desirable to switch the comparison frequency alternately above and below F o, for example, F i - F o - F - F 0 - F i Fo, etc. The thumbwheel switch positions may be read and recorded by the DDP-116 computer and the 140-foot and 300-foot telescopes. The input reference signal (0 V signal and -6 V reference) drives three DI-0 digital modules. (See Figure 3.) Each circuit consists of a -input gate, followed by a transistor inverter amplifier. When all inputs are -6 V the gate turns the transistor on, and the output is clamped through the transistor to zero volts. When any input goes to 0 V the transistor is turned off, and the output falls to the clamp voltage of -6 volts. The center (F 0 line) DI-0 circuit drives another which in turn drives a FA-0 flip-flop circuit whose outputs are connected to the alternate DI-0 ihputs (in F i F lines). This ensures that F i and F are selected alternately when the reference input switches to -6 V. A wafer switch is included so tha the local oscillator can be switched manually to F i, F o and F ; this is useful for system testing of the radiometer. An LD-30 followed by a DP-3000 (homemade, Figure 4) applies -1.6 volts to the digits selected by the thumbwheel switches. Three more LD-30 circuits drive display lights which show which frequency is selected. The synthesizer is capable of changing frequency in 0 As; the external control circuits will increase this figure somewhat. However, the switching speed is considerably faster than necessary.

5 3 The thumbwheel switches have, in addition to the decade switches which are used to control the synthesizer, digital readouts which are used to record frequency with the computer (Figure 5). Since the computer can read a maximum of 16 bits, the switches have to be read sequentially in a total of six steps. All 10 digits of Fo are read but only six are read for F 1 F since these are of less importance. The position of the mode switch is also read. The input connections to the first DI circuit are shown in full the other 15 are not shown to avoid confusion. The inputs are listed in Figure 5. A sweeper is incorporated on a separate panel to enable one or more of the synthesizer decades to be continuously swept at a rate 3-30 Hz. The circuit and output waveforms are shown in Figure 6. The supply voltages are obtained from the synthesizer control unit. 3. Multi lier Unit The multiplier unit (Figure 7) accepts an input in the frequency range MHz and converts this to a frequency in the range 1- GHz. The conversion is performed by a wide-range tunable multiplier (Resdel Engineering Corp., XV-900/ 500). This multiplier is designed to take a maximum input of 5 watts in the frequency range MHz. Harmonic outputs in the frequency range MHz can be selected using a 3-cavity 'tunable filter. The output power is in the range up to 500 mw when driven at the maximum input level of 5 watts. The multiplier is driven by a broadband transistor power amplifier (Microwave Power Devices, M) which covers the desired MHz range of input frequencies. The amplifier requires approximately 0 dbm input for 5 watts output. The input power to the multiplier is monitored by means of a 50 db directional coupler and HP 43 detector. An operational amplifier circuit (Figure 8) drives a A meter which is scaled to read 0-10 watts. The variable resistor in series with the input is adjusted to get the best square law response (reading proportional to power) and the adjustable feedback resistor sets the correct gain. The RF output is monitored in a similar manner. The output of the LO system may be adjusted or levelled using an HP 3505 variable diode attenuator. The output power level can be boosted to greater than

6 4 1 watt by switching in via RF switches a HP 489A Microwave Amplifier (TWT). A tunable filter with 5 percent bandwidth follows the TWT in order to remove noise and spurious signals that may be present in the output. A 0 db coupler is incorporated in the output line to provide a convenient point to monitor the RF output. Two circulators are incorporated, one between the multiplier and diode attenuator and the other is between the tunable filter and TWT (if used). The front panel has a 3-position wafer switch; the output power is unleveled in position 1. The power level is varied in this position by adjusting the power control potentiometer (maximum output = 10 V); the leveller amplifier (Figure 7) is connected as a unity gain amplifier by S1A. The maximum diode current available is 5 ma (5 db) and is sufficient provided the multiplier output is maintained below about 100 mw when used in conjunction with the TWT amplifier. Position is the local level position. The output power monitor circuit provides a voltage (10 V F_-' 1 watt) which feeds the summing junction of the leveller amplifier (connected as integrator) shown in Figure 7. A back-off is supplied to the same point by the power control potentiometer. The output power can be read off the power control potentiometer 100 E 1 watt. Position 3 uses an external voltage usually mixer crystal current monitor for levelling the output power. The mixer current monitors (max. 10 V) are brought into the top of the LO rack, and can be read off a front panel meter on the multiplier unit. One of them drives a unity gain differential amplifier, the output of which drives the leveller circuit. A n levelled ' i light indicates when the output power is levelled. The driving circuit consists of a RCA CA 3000 integrated DC amplifier which senses when the HP 3505 diode is conducting (i. e., attenuating). When the current drops to zero, the levelled light is extinguished. Two Lambda LH 15 FM power supplies power the multiplier unit. 4. Universal Local Oscillator Installation The ULO is expected to become part of the permanent telescope equipment and should remain in position when receivers are changed. There are various top panel connectors which should remain permanently connected to the rest of the receiving system. These connectors are listed below.

7 5 P output. Ref _pu in. L. This is connected via a low loss telescope cable to the LO input on the front-end box. This signal drives the frequency controller and is normally derived from the autocorrelation receiver or computer if a multichannel filter receiver is to be used (0 V signal, -6 V reference). Mixer inputs. These are connected to the front-end equipment rack and enable the mixer crystal currents to be monitored. The input signal should be 5 V/mA crystal current. VLB Standard. For programs, e. g., VLB, that require a better frequency standard than the HP 5110B driver, the required input frequency is 5 MHz. 5. Operation, of the Universal Local Oscillator It should be remembered that the ULO is capable of greater than 1 W output power it is possible under some circumstances that this could damage the mixer crystals if care is not taken. The 10 db line loss which exists at the 140-foot and 300-foot telescopes will protect most receivers. It is suggested that a 10 db fixed ttenuator be used between ULO and front-end in the laboratory. In order to tune up the ULO, the following steps should be followed: 1) Ensure that there is 115 V applied to rack. Turn on both synthesizer units and the two Lambda power supplies. The TWT should be off and the TWT switch on the Multiplier Unit should be OUT. Set F 1, F and F on the Frequency Controller Unit o to be the required frequencies, and turn selector switch to F position. The REMOTE/LOCAL switch o on the synthesizer should be in REMOTE. The RF output control on the synthesizer should be adjusted

8 so that the multiplier input power meter reads 4-5 watts. Set switch on Multiplier Unit to OPEN LOOP position and power control fully clockwise. ) Set the multiplier micrometer to within 5 MHz of the required LO frequency. (See chart with ULO.) The tunable filter should be set within 10 MHz of the required frequency. Tune the MATCH and TUNE knobs close to the synthesizer output frequency. An output should be observed on the mw range of the output power meter. When this occurs the six tuning adjustments should be optimized, taking care to avoid applying an excessive power to the receiver mixer. It play be necessary to use the power control to decrease the output. 3) When tuning is accomplished, the output may be levelled internally or externally. The power control should be set to the approximate required power (100 = 1 watt) and then the switch can be tuned to the required levelled position. The mixer current can then be set to the required level with the power control. When frequency switching, it is important to ensure if either of the "levelled" modes is used that the output power is levelled at F 1, F and F o. If not, some further tuning of the multiplier may be required. The bandwidth is sufficient to allow switching ± 5 MHz in the range 1- GHz, unless the output power is marginal. If this is the case, the TWT will have to be used. 4) It is important to note that tuning the multiplier in the levelled configuration can only be done by noting the attenuator current, i. e., measured power out is compensated by more current (attenuation). This can only be satisfactorily done if there is a reasonable current? 0.5 ma.

9 7 A final test which should be done is to confirm that there is sufficient power margin when running levelled. This can be checked by increasing the power control by 10-0 percent; the levelled light should not go out. 6. Noise Performance The power output has exhibited no spurious output signals when the multiplier is correctly tuned, i. e., all six adjustments optimized for maximum power output. Harmonically related frequencies are at least 80 db down on the desired output. The noise sidebands close in to the carrier are at least 60 db down on an HP 8551B spectrum analyzer (Figure 9). No deterioration in mixer noise figure has been observed in the 1- GHz frequency range. When the output power is multiplied by 6 to X-band ( GHz), a considerable proportion of the output power is in the sidebands (10 percent or more). The highest frequencies used for VLB and line work from the ULO (with extra multiplication in receiver) have been 5000 MHz and 9000 MHz. The noise sidebands did not interfere seriously with the experiments at either of these frequencies. The noise sidebands which become more obvious on multiplication result from the noise sidebands on the synthesizer output. For higher frequency requirements where the noise may well be troublesome, one solution could be to phase lock a VCO cavity oscillator operating in the frequency range MHz to the synthesizer output. One test which was made to verify this consisted of replacing the synthesizer with a 300B oscillator. The output spectrum at GHz showed no evidence of noise sidebands using the HP spectrum analyzer.

10 8 Maate!.:1. ear PIP , ' = sesr - Figure 1

11 5MHz REF REF R F INPUT OUTPUT A A I-GHz 9 MIXER XTAL CURRENT vv H-P MICROWAVE AMPLIFIER MODEL 489A MULTIPLIER UNIT El P +1V SYNTHESIZER FREQUENCY CONTROLLER MHz SWEEPER H-P FREQUENCY SYNTHESIZER MODEL 5105A H-P SYNTHESIZER DRIVER MODEL 5110B 1--- AA LAMBDA PS. LH 15 FM LAMBDA PS. LH 15 FM +8V -8V BLOCK DIAGRAM OF UNIVERSAL LOCAL OSCILLATOR FIG.

12 0 + REFERENCE INPUT BNC FA 0 D 1 I REMOTE v---4 (37)1 J9 9 L LOCAL H713.7-E6-7- -SYNTHESIZER IN D I 0 1 A IN400 DI 8 31 DI N D I IK -6V DI 1 9 MOD 1 8 F I \F 1 4 MOD FO F CARD REQUIREMENT FOR UNIT 1 PB- 30 = I EA. POWER SUPPLY FA - 0 = I EA. CONTROL D I - 1 EA. LD -30 = 1 EA. DP =1EA. HOMEMADE DI FO 35- FF-. MOD FO L D ( L D 1 5 ( 6 35 C 35 E L D I 8 F. P. SW. 35 F ID ID F. P. I 0 W F. P DP3000 (1)371 PN -30 = EA. DIGITAL XD-30 = 3 EA. DI -30 = EA. F. P. = FRONT PANEL 8 DP 3000 F. P. -I8V - I.6V CONTROL UNIT FOR H.P. SYNTHESIZER FIG. 3 FRONT PANEL THUMBWHEEL SWITCHES DECADE Fl 0 0 O O O o o 0 o o o o o o o o O o o o O O 0 o o o o o I.6V f f J7 J7 J7 J7 J8 J8 J8 to e le FO 0 o I o 014 o7 o39 o fffff ft' 6 38 I J8 J9 J I0 0 10' I 0 I O O O 0 0 I : 0 O o 6 O F I 0 0 o 3-1.6V fffff o o to 1 oo oo o o o O o o o I o o J7 J8 J9 THUMBWHEEL SWITCH TO SYNTH. J7,J8,J9 PIN CONNECTIONS

13 (6) (10) (14).5K 1. 5K (1)>--PsNse--o-- (16) 1.5K 4Vti 41, ALL TRANSISTORS N K 1.5K I.5K 1V D, w 1W I8V 500/5 T V A 1V 18V PIN(5) (18) () <75 ma RIPPLE 150mA DP 3000 CIRCUIT BOARD FIG. 4 1N977B

14 ELCO SELECT pr. LINE Mil INPUT FROM COMPUTER P0 M pf THUMBWHEEL SWITCHS DIGITAL (TYPICAL WIRING) *--01 F F F O SW 1 * F-3--- Fl Fo Fi F SW MODE F0 F1 F SW * F 3 0 F 3 F 1 F SW F0 F 1 F sw FoFi F sw., r b PO Fl I A o B1 FO o 1 i Fo F1 F F 1 sw 4 1 C 6 6 -! P I L. I 10 -,D 3 MODE SW XD F GRD <N <rvi F O Fl F SIN NOTE: DIODES IN400's M. = MODE SWITCH POSITION DIGITAL READOUT TO COMPUTER FIG. 5 -;"*NI DI XD 3 10 GRD ELCO PIN OUTPUTS TO COMPUTER <e <f I 1 1

15 SWEEP.=. RATE 43K 3-30Hz CENT. FREQ. 1 K 5K 1K ma --o 18 I M +1 DEVIATION 350K 7K 4p.F GOD B TI" 18 B K V PIN I 18V PIN GRD. PIN FET Ti 0K V IK 4-- fsan K OUT NEG. TRIG. 3-30Hz 0 V 5V I.7K 0.8V FRONT PANEL I SWEEPER CIRCUIT FIG Hz C,4

16 MIXER CURRENTS T RF INPUT MHz OdBm HF- F8 I I I I 15 p.h MULTIPLIER DRIV /I. A 4.9K/I% 4.9K/1% RF AMP + 8 5W 50 dj El HP K MIXER -8 CURRENTS RESDEL MULTIPLIER II 1 13 * _ K 3 1 / SIM 0-PRINTED CIRCUIT BOARD CONNECTOR ATTR. CURRENT TRANSCO SWITCHES TWT IN/OUT I +8-8 IK 3 SIB IK /LA POWER OUTPUT TWT TO FROM T 3 1 \sic IK MA, RC 8 1 o 10K A3000 i 91K +8 sa iv AE IN4735 IN IN4735 IK E _ -8 N N3904 SWITCH POSITION I. - OPEN LOOP.-LOCAL LEVEL 3.-REMOTE LEVEL +8.7K TUNABLE FILTER I- GHz R F RF OUTPUT MONITOR I - GHz (t) I >< LEVELED INDICATOR 1 r-p _ -8 6V db POWER CONNECTOR A - GRD. B E -8 F8 II I I /LH d8 T ><3 HP 43 I MULTIPLIER UNIT CIRCUIT DIAGRAM FIG. 7

17 K 10 v At 9 4V%A. 100K II 6 0 ': LEVELER AMPLIFIER II 100pF K 105 XTAL CURRENT MONITOR IOOK 100K 15 II K lvv\t 150pF 0K 4.99K OUTPUT POWER MONITOR 0K 4.99K MULTIPLIER INPUT MONITOR I5V IN4744A TRIM 1 +15V IN4744A MULTIPLIER UNIT- PRINTED CIRCUIT BOARD FIG. 8

18 Vertical: 10 db/cm Horizontal: 30 MHz/cm IF Bandwidth: 100 khz FIGURE 9 SPECTRUM OF RF OUTPUT