Table 4-4. Sum and Difference IM Products
CHAPTER 5 Lessons Learned
5.1 Scope
This section describes several lessons learned by the RF systems committee members.
Additions or deletions will take place during subsequent revisions to this handbook.
5.2 Permanently Installed RF Cables Testing 5.2.1 Abstract
The RF cables, which are part of a permanent TM installation, are usually assumed to have nominal loss and no phase distortion. However, RF cables may degrade due to mechanical damage to the cable itself or its connectors. The RF cables may be tested after installation using a variety of techniques.
5.2.2 Driving Event Description
The RF cables installed to route S-band TM signals for prelaunch TM have degraded due to kinking and connector damage. This damage occurred during initial cable installation and later during the normal attachment of other cables and equipment.
5.2.3 Lesson Learned
Degraded cables may pass low-speed signals with little degradation, but may pass high-speed signals with phase distortion, which is significant enough to result in corrupted data.
5.2.4 Recommendation
Periodically inspect and test RF cables to verify that loss, VSWR, and group delay are within acceptable limits using a vector network analyzer, if possible. If the two ends of a cable are physically separated by a great distance, the two RF cables can be connected together at one end and then tested together as one double-length cable; or a scalar network analyzer can be used with a special long-control cable attached to the detector. Another scalar technique is to use a swept RF source or RF noise source at one cable end and a spectrum analyzer at the other. Note that the cable’s insertion loss amplitude will give an indication of group delay bumps without having to measure the group delay directly. Portable analyzers that use Time and Frequency Domain Reflectometry techniques to measure “distance to fault” can be purchased commercially.
These measurements can be made with the cables properly terminated to determine fault locations or with the cables open to determine cable length. Measurements can be stored in a database to track trends in cable degradation.
5.3 Prelaunch TM from Encapsulated Missiles Radiation 5.3.1 Abstract
Radiation of TM from a missile enclosed in an RF-tight canister before launch is sometimes necessary for missile exercises. Various techniques have been developed to couple the signal from the missile/canister, amplify its power to several watts, and radiate it for reception by the TM sites, with good but not 100% success.
5.3.2 Driving Event Description
In many circumstances a missile is enclosed in an RF-tight canister for environmental and logistical reasons and is launched from this same canister. Normal TM operations require that the missile telemeter be energized and its signal received by multiple sites before the missile can be launched. When the missile telemeter is energized, the missile antenna radiates the TM RF signal inside the canister, and it is this signal that must be picked up and retransmitted to the receiving sites.
5.3.3 Lesson Learned
Coupling the S-band RF signal from the missile antenna out of the canister through free-space radiation with no distortion has been the biggest challenge. Some distortion can be tolerated with low-speed PCM/FM streams, but this same distortion will corrupt a high-speed stream. A consistent signal from the canister can also be a problem due to mechanical
positioning and temperature effects, especially when this parameter is not tested in production.
One way to obviate the distortion is to use a coax cable connection between the missile and the canister that pulls away or breaks during launch, but that is not always an option. When multiple TM signals are being amplified during a test exercise, intermodulation products caused by single amplifier saturation will cause interference at other frequencies. The power output from the amplifier and antenna radiating the signal may not guarantee the correct ERP toward the
receiving site. Unwanted feedback caused by the canister not being perfectly RF-tight can cause oscillation of the electronics.
5.3.4 Recommendation
Retransmitting the prelaunch signal can usually be accomplished but each part of the proposed system must be examined in detail to prevent a “weak link” in the system.
If connecting a coax cable from the missile to the canister is not viable, then using a pickup antenna located close to the missile antenna may give the best results. This is because it tends to reduce the “canister multipath” phenomenon caused by the complex paths the RF signal takes in going from the missile antenna to the pickup antenna inside the canister. This applies to one radiator in one canister (enclosure) and does not address multiple radiators in one enclosure.
Once the signal has been coupled from the canister, then it can be combined with other canister signals (at other frequencies) easily using passive RF combiners. The composite signal can be amplified using an S-band amplifier and radiated via an S-band antenna. The amplifier may need to have AGC to provide a constant output power, and the amplifier’s drive level needs to be considered if intermodulation products occur at frequencies of interest. The radiation antenna can be chosen to increase the ERP in a particular direction, but may need to be positioned and
pointed accurately.
5.4 Data Routing using Electronic Switching 5.4.1 Abstract
Data routing using electronic switching is used to replace patch panels. Conventional (frequency response up to 100 MHz) routers only pass low-voltage signals such as IFs. They cannot pass TTL signals without increasing the IF noise floor and decreasing the signal level.
5.4.2 Driving Event Description
The solution is to use TTL data routers to pass Data and Clock. The TTL data routers cannot pass the 70 MHz IF signal. The IF noise level increases by 10 dB and the signal level decreases by the same amount.
5.4.3 Lessons Learned
When using electronic routers, make sure you not only measure the signal level, but the voltage level as well.
5.4.4 Recommendation
The solution is to use conventional switchers that pass IF signals with low-voltage levels and TTL electronic switchers for Data and Clock signals.
5.5 LNA Characteristics 5.5.1 Abstract
The LNAs are usually characterized by gain, frequency response, a 1 dB compression point, and dynamic range. There are other characteristics that must be measured to ascertain the above named parameters to meet not only short-term specifications, but to also meet
specifications under the desired operating temperatures.
5.5.2 Driving Event Description
The LNAs show flat frequency response for the TM bands and meet the gain and noise figure specifications. Testing the LNAs within the RF FAU shows noise floor measurements and autotracking parameters are not stable. Further symptoms show that phasing increases and changes the LNA’s characteristics that create cross-talk problems as temperatures increase.
5.5.3 Lesson Learned
The problem is solved by properly grounding the LNA’s input stage to allow correct amplitude and phasing to improve the tracking modulation and minimize cross-talk.
5.5.4 Recommendation
Measure the LNA input and output impedance at different temperatures and not just ambient temperatures.
5.6 Antenna Feed Dipole Design and Proper Phasing 5.6.1 Abstract
The TM bands cover approximately one octave of frequency (1435 - 2400 MHz).
Antenna pattern measurements meet specifications as well as the axial ratio measurements at lower and upper L-bands. As frequency increases into S-band, the axial ratio and gain measurements do not meet specifications.
5.6.2 Driving Event Description
The cables between the dipoles and the hybrid are hard or impossible to have good phasing. Wrong cable phasing causes excessive crosstalk and will not allow autotracking.
5.6.3 Lesson Learned
The dipoles are found to be connected in such a way that the wire between the dipoles and the tuning elements have capacitance reactance that make it hard to tune for all three bands.
The wire between dipoles is flat and is changed to an “upside down V” bend such that the
capacitance reactance does not change significantly as frequency increases. All axial ratio and gain measurements meet specifications after changing the wire.
5.6.4 Recommendation
When testing a tracking FAU, maintain a record of the crosstalk as frequency increases.
If the crosstalk is erratic, check all input and output impedances.