Test Equipment Solutions Datasheet Test Equipment Solutions Ltd specialise in the second user sale, rental and distribution of quality test & measurement (T&M) equipment. We stock all major equipment types such as spectrum analyzers, signal generators, oscilloscopes, power meters, logic analysers etc from all the major suppliers such as Agilent, Tektronix, Anritsu and Rohde & Schwarz. We are focused at the professional end of the marketplace, primarily working with customers for whom high performance, quality and service are key, whilst realising the cost savings that second user equipment offers. As such, we fully test & refurbish equipment in our in-house, traceable Lab. Items are supplied with manuals, accessories and typically a full no-quibble 2 year warranty. Our staff haveextensive backgrounds in T&M, totalling over 150 years of combined experience,which enables us to deliver industry-leading service and support. We endeavour to be customer focused in every way right down to the detail, such as offering free delivery on sales, covering the cost of warranty returns BOTH ways (plus supplying a loan unit, if available) and supplying a free business tool with every order. As well as the headline benefit of cost saving, second user offers shorter lead times, higher reliability and multivendor solutions. Rental, of course, is ideal for shorter term needs and offers fast delivery, flexibility, try-before-you-buy, zero capital expenditure, lower risk and off balance sheet accounting. Both second user and rental improve the key business measure of Return On Capital Employed. We are based near Heathrow Airport in the UK from where we supply test equipment worldwide. Our facility incorporates Sales, Support, Admin, Logistics and our own in-house Lab. All products supplied by Test Equipment Solutions include: - No-quibble parts & labour warranty (we provide transport for UK mainland addresses). - Free loan equipment during warranty repair, if available. - Full electrical, mechanical and safety refurbishment in our in-house Lab. - Certificate of Conformance (calibration available on request). - Manuals and accessories required for normal operation. - Free insured delivery to your UK mainland address (sales). - Support from our team of seasoned Test & Measurement engineers. - ISO9001 quality assurance. Test equipment Solutions Ltd Unit 8 Elder Way Waterside Drive Langley Berkshire SL3 6EP T: +44 (0)1753 596000 F: +44 (0)1753 596001 Email: [email protected] Web: www.TestEquipmentHQ.com RF DIRECTIONAL THRULINE® WATTMETER MODEL APM–16 Introduction The Bird APM-16 is an insertion wattmeter designed to measure RF power and load match in 50 ohm transmission lines. It has a maximum VSWR of 1.05 for frequencies up to 1000 MHz. The meter provides direct readings in watts with an expanded linear scale for easy reading. The scale is graduated for 25, 50, and 100 full scale. Elements are available in a variety of power and frequency ranges (See the Bird Electronic Corporation Catalog for details). Description The Bird APM-16 is portable and rugged, with an aluminum housing. For addi- tional protection, the microammeter is specially shock mounted. Bumpers on the base and back allow the meter to stand or lie flat. Refer to Figure 1 to identify components. At each end of the line section are Bird Quick-Change RF connectors that may be interchanged with any other Bird “QC” connector. The wattmeter housing does not interfere with connector changes. Figure 1 Outline Drawing 1 Theory of Operation Traveling Wave Viewpoint The easiest way to visualize Thruline operation is from a travelling wave view- point. In transmission lines the voltages, currents, standing waves, etc., on any uniform line section result from the interaction of two travelling waves:  The forward wave (and its power) travels from the source to the load. It has RF voltage E and current I in phase, with E / I = Z . f f f f o  The reflected wave (and its power) originates by reflection at the load and travels from the load back to the source. It has an RF voltage E r and current I in phase, with E / I = Z . r r r o Each wave is mathematically simple and has a constant power: W = Watts Forward = E2 / Z = I2 Z = E I f f o f o f f W = Watts Reflected = E2 / Z = I2 Z = E I r r o r o r r Z is the characteristic impedance of a uniform line section. For useful lines it is o usually a pure resistance of 50 ohms. The RF circuit of the Bird APM-16 is a length of uniform air line with Z = 50 ohms. o Coupling Circuit The Bird Plug-In Element contains a coupling circuit that samples the travelling waves. The element circuitry and its relationship to the rest of the Bird APM-16 are illustrated in Figure 5. Figure 2 Schematic 3 Current is produced in the coupling circuit by the travelling waves in the line section. Both inductive and capacitive coupling contribute to this. Inductive cur- rent flows in the direction of the travelling wave, while the capacitive current is independent of the direction of the travelling wave. Therefore, the inductive current produced by one travelling wave will add in phase with the correspond- ing capacitive current, while that produced by the wave travelling in the oppo- site direction will subtract. The additive or “arrow” direction is assigned to the forward wave. The electrical characteristics of the element are carefully adjusted so that, for the reverse travelling wave, the inductive current will completely cancel the capacitive current. The result is directivity greater than 25 dB. Thus, the element is sensitive at either of its settings, but to only one of the two travelling waves. Thruline Wattmeter measurements are also independent of position along the transmission line. Like similar diode devices, the Bird APM-16 indicates the carrier component of amplitude modulation, with very little response to side band components added by modulation. Load Power For loads with a VSWR of 1.2 or less, the power dissipated in a load (W) is equal l (with less than one percent error) to the forward power (W). When appreciable f power is reflected, as with an antenna, it is necessary to use the exact load power, given by: W = Watts into Load = W – W l f r Good load resistors, such as Bird Termaline Loads, will give negligible reflected power. Standing vs. Travelling Waves As mentioned previously, the Thruline Wattmeter reacts to forward and reverse travelling waves to measure power in a transmission line. The standing wave viewpoint, also widely used, is highly developed both in theory and in practice. This viewpoint can be traced to the early use of slotted transmission lines. The slotted line measures the standing wave ratio by mechanically positioning a voltage detector at peaks and nulls along a length of line section. Its drawbacks are that it is usually too long, too expensive for good accuracy, not portable, and too slow. These problems grow rapidly as the measurement frequency drops below 1000 MHz. The Thruline Wattmeter by comparison is fast, convenient, and accurate. It provides the same information as a slotted line except for the phase angle of the reflection coefficient (distance, load to minimum). 4  vs.  The simple relationships: 1+  –1 2 Where  = VSWR  = ---------------- and  = ------------ 1–  +1 and  = Wr / Wf can be used to convert between the standing wave ratio ( and the reflected/ forward power ratio , which can be directly read from the Thruline Wattme- ter. The relationship between  and  is graphed in Figure 3 and Figure 4. Note: Attenuation, measured in dB, can be derived from the power ratio by the equation N = 10 log . db VSWR scales and their attendant controls for setting the reference point have been intentionally omitted from the Bird APM-16. Experience using the Thruline Wattmeter for transmitter tune-up, antenna matching, etc. will show that the power ratio measurement is as useful as the standing wave ratio. A trial is suggested – forget about VSWR for a few days and think in terms of  = W / W. The meter readings, W and W, give a useful, approximate picture of r f r f the results without bothering to calculate the power ratio exactly. Consider that, for an antenna matching problem, the main objective usually is to mini- mize W. Anything done experimentally to this end will be seen when the ele- r ment is in the reflected power position. 5 Figure 3 Percent Reflected Power vs. VSWR (1.0 – 8.0) 6 Figure 4 Percent Reflected Power vs. VSWR (1.0 – 1.3 7 Low Reflection  = 10% ( = 2) is the typical limit of antenna match. Further effort is frequently not worthwhile because below this level reflected power is hard to measure, and W can not be significantly increased. TV and VHF transmitters are examples l of systems requiring lower reflected power but for reasons other than maximiz- ing power transmission. CAUTION For low reflection measurements, do not rotate the reflected power element to read forward power. Damage to the element or wattmeter could result. When the same element is used to measure both forward and reflected power, meaningful readings are possible down to about  = 5% ( = 1.5). For accurate measurement of very low levels of reflected power, i.e.  = 0.6% ( = 1.17), use a second element rated at one tenth of the full scale power of the forward ele- ment. This method should not be used with element ranges differing by more than 10:1. For example, consider an 80 watt transmitter and a Bird APM-16 with 100 and 10 watt elements. Measure W with the 100 W element. Measure W with the f r 10 W element (make sure the arrow points towards the transmitter). W can be r measured down to at least 0.5 W, so that  = 0.5 / 80 or about 0.6%, corre- sponding to  = 1.17. Transmitter Monitoring The Thruline Wattmeter can be used for continuous monitoring of transmitter output or reflected power, for instance while checking intermittent antenna or line faults. Component Testing The Bird APM-16 is very helpful in component testing, and may be employed in several ways: 1. Insertion VSWR or  can be measured by placing the component between the wattmeter and a good load resistor. 2. Attenuation (power lost by heat in a line) as well as insertion VSWR or  can be measured by inserting the unknown line between two Thruline Wattmeters, or between a Thruline Wattmeter and a Termaline Absorption Wattmeter. Note: Very small attenuations require allowance for normal instru- ment errors. To correct for this without any calculations, connect the wattmeters directly, with no line between them, and adjust their zero settings. 8 3. Line loss using open circuit calibration: The high directivity of elements can be exploited in line loss measurements, because of the equality of forward and reflected power with the load connector open or short circuited. In this state the forward and reflected waves have equal power, so that f = 100% and r = ¥. Open circuit testing is preferred to short circuit, because a high quality open circuit is easier to create than a high quality short. To measure insertion loss, use a high quality open circuit to check forward and reverse power equality, then connect an open-circuited, unknown line to the wattmeter. The measured f is the attenuation for two passes along the line (down and back). The attenuation can then be compared with pub- lished data for line type and length (remember to halve Ndb or double the line length to account for the measurement technique). This measurement should be supplemented by either time domain reflec- tometry or DC continuity and leakage checks, since the attenuation mea- surement alone cannot account for faults such as open or short circuits partway down the line. Note: Very small attenuations require allowance for normal instru- ment errors. Make sure to note exact readings, or their difference, on the initial equality check, and correct for this. Frequency Response Bird Plug-In Elements have a flat frequency response over their specified oper- ating range. A sample set of curves is shown in Figure 5. Notice that for the low power element, the rolloff outside its frequency band is more pronounced than for the high power elements. For example, at 40 MHz the 10C element will have a loss of 4 dB, giving a reading of about 40% of the true value For the 100C, the loss will only be about 1 dB, for a reading at 80% of the true value, and the 500C should be within the normal 5% of full scale tolerance. Figure 5 Representative Frequency Response 9