APR-5A/SPR-2 dokumentationen kommer från www.ibiblio.org och www.aafradio.org.  APR5-A (foto Christer Magnusson) Beskrivningen gäller SPR-2. Det är den amerikanska marinens benämning och är tekniskt samma mottagare som det amerikanska flygvapnets APR-5A som var monterad i DC3an. Avsnitt markerade med rött i dokumentet visar på mottagarens möjligheter upp mot 10GHz.
EXPLANATION OF STEPS AND CONTROLS USED WITH THE SPR-2 1. The SPR-2 search receiver is designed for radar intercept work above 1000 Mc. Its ability to pick up responses from radars depends primarily on the choice of antenna for the frequency range to be searched. Three antennas (two fixed and one rotating DBM-1) are provided for 1000 Mc to 5000 Mc reception. A special waveguide antenna which starts at 3100 Mc and goes to higher frequencies can be used. Its upper limit depends mainly on the way it is installed and the strength of signal. The usual method of changing from one antenna to another is by means of the ANTENNA SWITCH; however, if such a switch is not in use, the SPR-2 lead must be plugged into different antenna outlets on the ANTENNA JACK BOX. 2. After the POWER switch has been snapped to ON, it takes about 30 seconds for the receiver to start operating, and about 10 minutes for it to be fully warmed up. 3. The AUDIO GAIN knob controls the amount of volume in the headphones. Its main value is that it can be used to reduce a loud signal to comfortable headphone volume without changing the presentation on the scopes of the other units. In actual operation it is usually left on full, with gain controlled by the I.F.GAIN. 4. The PULSE STRETCHER switch is left in ON position for all ordinary operations. It makes it possible to hear signals of such short pulse lengths that they would not be heard with the switch off. While there may be some pulses of such long duration that they can be heard better with the switch off, even those can be heard satisfactorily with the switch in ON position. Whether off or on, this switch has little effect on the pulse as seen on the scope of the pulse analyzer. 5. The only phones jack is the one provided on the front panel. 6. The METER SWITCH shifts the one meter into different circuits. To check the operation of the set at the beginning of a search watch, the following should be done: A. Turn to AC LINE 0-150. Meter should read about .77 (115 volts). B. Turn to XTAL 10 MA. Meter should read between .02 and .6 (6 ms). C. Turn to OSC 500 V. Rotate main tuning dial over the entire scale. Readings should be between .2 and .6 (100 to 300 volts) and should never exceed .6 at any point. The technician should adjust the set if it does. D. Turn to OSC 50 MA. The meter should read about .3 (15 ms). E. Turn to IF 100 MA. Then adjust I.F.GAIN knob to get a meter reading of .6, a point which is considered normal for beginning operation. F. Turn to TUN.IND position when searching for a radar signal. It connects the meter into the circuit to show when any radar signal is tuned in. The operator will find that it is better to watch the scopes of the pan adaptor and the pulse analyzer than to watch the response on the SPR-2 meter. 7 & 9. The I.F.GAIN control is the most important means of controlling the presentation of the radar response on the scopes of the pan adaptor, pulse analyzer, and direction finder; of obtaining correct readings from the PRR meter on the pulse analyzer; and for regulating the amount of volume in the headphones. Generally the gain controls on pan adaptor, pulse analyzer, and direction finders are left well advanced, and all gain adjustments made with the I.F.GAIN of the SPR-2. 8. The TUN.IND.ZERO knob needs to be used only once at the start of a search watch. 10. The receiver has one-dial tuning, and a plug-in tuning unit. The dial may or may not have the inside markings of 3030 to 6230. As only one plug-in tuning unit is provided, it should be left permanently in place.
TUNING OVER THE DIAL - FIGURING OUT WHAT IS RECEIVED.
After the SPR-2 (APR-5A) has been turned on according to the instructions, start to search for radar signals by rotating the main tuning control and leave all the other adjustments temporarily as they are. At first glance it would appear that the frequency of anything heard could be read directly from the dial, especially for the 1000 mc to 3100 mc markings. Tuning once over the dial, when a number of radars are operating, is enough to convince anyone that all the responses are not all from different radars, for too many of them sound exactly alike. The problem is one of identifying responses. The following will happen when a radar signal of medium strength on 3100 mc hits the cone antenna, which passes the energy down the antenna lead to the receiver. In tuning from the 3100 mc end of the dial, the radar is found near but not exactly on the 3100 mark. It makes one aware of the fact that the dial markings are approximate. Another response of the same strength and tone is found at about 3040, exactly 60 mc lower in frequency. In tuning down the dial the same radar signal shows up again at about 1540, and at about 1510, 30 mc apart. A similar response appears at about 1020, and again at about 1000, 20 mc apart. This example illustrates the important things to know to operate the SPR-2 1. Responses from the same signal will be found in pairs on the dial, 60 mc, 30 mc, and 20 mc apart, and this fact provides the basis of determining the radar frequency. 2. The upper response of a pair 60 mc apart is the actual radar frequency, and can be read directly from the outer dial. 3. The upper response of a pair 30 mc apart is about half the actual radar frequency. It can be calculated by multiplying the outer dial setting by 2 and adding 30. 4. The upper response of a pair 20 mc apart is about one-third the actual frequency. It can be calculated by multiplying the outer dial setting by 3 and adding 60. 5. Pairs of responses from any one signal might appear at several places on the dial, and any pair can be used to figure the frequency; however, when possible find the 60 mc pair to get direct reading from the dial (and somewhat stronger responses). SPR-21 CHART FOR FIGURING THE FREQUENCY OF A RADAR The following chart will provide a method of finding the frequency of a radar more rapidly after an "upper" response has been located on the dial. For example a response is found on 1100, and the panoramic adaptor proves it is an upper one by the direction it moves across the scope. To use the chart, find 1100 on the top line, and not that 1100 mc, 2230 mc, and 3360 mc are possible frequencies. (The chart is made in 20 mc steps for easy reading). Then turn the tuning control to find a lower response that pairs with the upper one. It should appear at a point 60 mc, 30 mc, or 20 mc lower on the outer dial. The one found will tell the line to read on the chart.
There may be other pairs, usually weak one, such as 15 mc (upper dial response read × 4 + 90 = frequency), but in ordinary situations it pays to concentrate on searching for pairs of responses 60 mc, 30 mc, and 20 mc apart. When all of those have been found and identified, it is time to try to figure out the odd pairs.
Anm. I tabellen ovan framgår att mottagaren hade möjlighet att nå så högt som 15GHz
Teknisk dokumentation om Oscillator och Blandare i APR5-A
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