OK, time for an evaluation of the receiving stations design and build quality.
This is just my opinion based on having worked with Military and specialist Gov't equipment over the years.
The Dish section1. The parabolic prime focus dish is a quality item made from spun aluminium. Good enough to be from Andrews Communications stable IMHO.
2. The dish to receiver chassis coupling is a quality solid aluminium ring with castellation.
3. The feed horn matches into waveguide. A rugged and quality design but with associated losses.
4. The feed horn has a Mylar window to seal it and the whole horn/waveguide assembly is protected inside a plastic radome. Good design.
5. A quality waveguide adapter sits at the end of the waveguide on the rear of the dish chassis coupling. Its output is via an SMA socket.
6. The connection between the waveguide adapter and the receiver module uses rigid coax with SMA plugs at each end. This small diameter coax is a lossy link but is reasonably short. Direct connection to the receiver module input without the rigid coax would have been lower loss.
The Receiver Section1. Quality aluminium housing employed. Milled out of solid Aluminium block. This is expensive to make but offers custom internal and external shapes and/or cavities. This is especially good for small production runs.
2. Professional connectors. TNC and Amphenol used for cable connects. Always a good sign on a piece of equipment. The Amphenol is a high quality and expensive connector type.
3. The Signal strength meter is more often found on Military equipment and is of good quality.
4. The case lid is held in place by 20 socket head recessed machine screws. These apply even pressure to the gasket below.
5. A quality silver wire impregnated EMC gasket sits between the lid and the lower part of the case. Such screening gaskets are often used to reduce the vulnerability to outside RF interference or to reduce emissions from a unit.
6. The interior of the case is milled out into two sections. The main Receiver, and a separate I/O section accessed via an aluminium lid plate.
7. The Receiver is split into two tiers. A lower RF input and down converter deck on its own aluminium chassis, and the upper deck that is formed by the main Receiver PCB.
8. Stainless steel machine screws with anti vibration washers are used throughout the unit.
9. The lower RF deck contains what appear to be high quality modular components. Namely the RF pre-amplifier and RF frequency down-converter. A pre-amplifier can be used to provide low noise amplification and/or to isolate the down converters Local Oscillator from the aerial feed. So preventing radiation of the Local oscillator via the dish. Important in some situations. These modules are likely adapted items from one of the major Microwave module manufacturers, rather than truly custom designs.
10. The down-converter does not appear to contain a VCO as no control line is present. There is a Local Oscillator adjustment screw only. This could be a DRO or LC oscillator. Frequency stability will NOT be that good in such a design.
11. The 1st IF output from the Down converter will be below 2GHz as that is the upper limit of the MAR6 MMIC amplifier. The connection between the down converter and next stage is via quality silver plated wire/Teflon coaxial cable.
12. The next stage sits on the Main PCB within its own metal housing. It contains MMIC amplification of the 1st IF and an active 5GHz capable mixer from M/A Com. The 2nd Local Oscillator that feeds the M/A Com mixer is a VCO design and is also within the same module. It comprises an MAR7 MMIC with Varicap tuning and LP2951 LDO Regulator. The VCO can provide AFC tracking of the signal to offset drift in the Down-Converter Local Oscillator. This is a common design for video receivers that are quite tolerant of frequency drift.
13. The 2nd Mixer module is very obviously adapted from a design that usually has its own phase locked loop controlling the 2nd LO frequency. All the PLL parts have been deleted from the BoM for this application. Another case of using proven modules and adapting them for this low production run product.
14. The output of the 2nd Mixer (2nd IF) is fed into a MAR1 MMIC using just hook up wire so the frequency is likely to be relatively low. Very high frequencies would normally dictate a more elegant inter stage coupling than simple hook up wire. The 2nd IF is below 300MHz as that is the maximum frequency capability of the Baseband Video IF/Demodulator that receives it.
15.The MAR1 amplifies the 2nd IF prior to it passing through a chain of MMIC amplification and filtering before being presented to the MC13155 IF input.
16. There appear to be two MC13155 IF/Demodulator IC's connected in cascade (or parallel?) in this design. I have yet to trace out the circuit. This could be in order to use some parts of the IC for other purposes, such as the IF gain stages, RSSI meter or AFC drive to the 2nd Local Oscillator.
17. The MC13155 provides the demodulation of the FM 2nd IF signal and provides Baseband Video output. This output is then fed to an NE592 Video amp and LM359 Norton Op-Amp.
18. There are two IC's that I failed to identify when the lid was off. They appear to be OP-AMPs, one driving the 2nd Local Oscillator frequency control pin and the other driving the signal strength meter. The MC13155's appear to be driving these two IC's. One for each.
PCB construction comment1. The PCB material used is FR4 This is acceptable for the frequencies involved.
2. The PCB layouts are relatively old fashioned and functional rather than pretty.
3. The re-purposing of the 2nd Mixer/IF module involves the use of hook-up wire. These wires are not secured to the PCB as I would normally expect. This part of the design could be better as loose wiring is not good for reliability.
4. The 2nd Mixer/IF module has bare copper PCB tracks which is a surprise to me. The tracks should really be plated. The PCB is lacquered to avoid corrosion though.
5. The main PCB has plated tracks but it has not been lacquered. Again this is a surprise to me. Lacquer helps to prevent moisture ingress or corrosion.
6. The main PCB contains several wire bridges. Some appear part of the design, whilst others may be error corrections. Not great to see but at lest they are of adequate quality.
So what do I think of the design ?It uses basic technology and well proven RF and IF stages. There is not much to go wrong with such a design. The build is old school and the various stages are easily discriminated along the signal path. This is the sort of kit that I used to design and build. It is very much about making an equipment that serves a specific need rather than trying to get clever or adventurous and risk operational problems. It very much employs the KISS principle and so for that I applaud it. There are areas that I do not like but as they do not effect the operation of the unit I suppose the designer was satisfied that it was 'good enough'.
It may surprise some people, but it is a fact that some very specialist equipment that has very small production runs, sometimes only a single unit, appears more like something a skilled hobbyist put together. This is because people like me were tasked with providing Product 'X' to achieve Objective 'Y' in Timescale 'Z'. It is not unusual to approach manufacturers of complete RF modules and adapt a current proven design. Why re-invent the wheel with the associated cost and development time. I often used Mini-Circuits modules as they did what I needed and at reasonable cost, with fast delivery.
The challenge is often in designing the custom PCB's that control, or interface with, the modules. For very limited production numbers in very tight time frames, it is not unusual to design a prototype PCB run that becomes the production board after a few corrections. If it meets the spec and time is tight, it goes out the door on time and within budget. That is the world of very specialist and sensitive electronics design and build.
I hope this has proved interesting to other forum members.
Now to decide what to do with this unit. I know ATV fans would love it but I may yet find a use for it myself ? I love parabolic dish based designs
Fraser
Home
Help
Search
Login
Register
