Saturday, 22 February 2014

VLNA23 revisited

Recently there has been much mention on the VHF/UHF reflector about strong signal 23cm performance.

Being a 2-stage preamp, that was designed primarily for EME work where high gain and low noise is a positive advantage, using the standard VLNA23 for terrestrial work can be an problem. I've always acknowledged this and recommend that a 10 to 16 dB attenuator is inserted after the VLNA23 to reduce system gain. However, the second stage in the VLNA may still be subject to strong out of band signals, enhanced by the gain of the first stage.

A few years ago I offered a single stage VLNA option, where the same PCB is used but the ATF54143 stage was omitted. This wasn't too popular and few were sold. I stopped offering it.

However, I continued to use this single stage version in my own 23cm system with great success until I started to remote the outside shack from the house shack. For this arrangement the signal had to go a total of 65m over FSJ4/50. I usually added back-in a second stage amplifier to overcome the extra cable loss!

Recent transmit problems with my 23cm system meant I had to remove the masthead amplifier. On test it still measured very close to the numbers marked previously on the preamp box.

But, on closer examination I noticed that this single stage VLNA23 did not have the later 'FET on stilts' modification. So, I modified it and ended up with a VLNA23 that exhibits 0.32dB noise figure with 18dB of gain and 6.5dB input return loss. No wonder I needed to add in the external amplifier........

18dB is a far more practical gain for terrestrial use ( 2dB better than the unmodified single stage version) and having eliminated the second stage should have a better dynamic range. I will perform a proper 2 tone IMD measurement in the next few days to determine the actual IIP3.

With a single stage it is possible to add an external band pass filter after the VLNA, but before the radio/transverter. If more gain is needed you can add one of the SPF Amps or PGA Amps after the filter to obtain more gain for more critical (long cable) situations. Less than 0.35dB noise figure with 18dB gain may allow contest stations to leave out an add-on second stage.

I can supply details of what to change for anyone who wants to 'de-sense' their two stage VLNA23.

Sam







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Tuesday, 18 February 2014

End of an era?


Since my neighbour erected an eight foot wall along the edge of our properties I have lost my westerly moon window. I had decided to move the dish and mount to the bottom of the garden where the take off to both east and westerly moonrise/set is much improved.
Taking advantage of the good weather today I managed to dismantle the dish from the mount and start to take the 4 foot square pallet mount apart. After more than ten years the bolts were all easy to undo thanks to my foresight of regularly greasing all bolts and the effectiveness of the galvanising on the large pallet bolts.


A lean-to greenhouse will be fitted against the wall in order to take advantage of the early-morning sun trap.
The big disadvantage of this move is the much longer cable runs that will be needed. I'll probably need to fit a cabinet near to the new dish location. The amplifiers will be located in the cabinets hen they are required.

Sm


Location:Falkenham Road,Ipswich,United Kingdom

Monday, 17 February 2014

A turn-up for the books!

After loaning a soldering reflow oven to see if I liked this technique ( and deciding I did) I bought my own Beta Reflow oven and controller, as previously blogged.

The person who kindly loaned me his system has now borrowed my Beta Reflow system to see if he can get better results with a batch of boards he has to produce!

There is little doubt that the controller makes life easy compared with the previous manual control method and the oven supplied by BR seems to produce a much better quality of reflow.

A couple of days ago I connected up the various air tubes to my new air compressor and dispenser controller and tried it all out ( without solder paste). It seems to work well so the plans are now to try producing a re flowed Nacton transverter board. That will be some test!

Today I ordered the PCBs for the Nacton 7W PA module. They should be here by the end of next week. Next I have to order the new transverter PCBs. The new transverter has been named the 'Anglian' to avoid confusion with the old board design.

Sam






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Saturday, 15 February 2014

New soldering reflow system

My reflow oven and air compressor turned up during the week. Yesterday the dispenser controller also turned up.
Unable to curtail my enthusiasm I installed the oven this afternoon and connected up the Beta reflow controller. After a few initial hitches I had the controller 'learn' the oven characteristics.
It was then a straightforward job to manually apply the leaded solder paste to one of my VLNA boards. The air compressor and dispenser controller are not yet hooked up.
It was relatively easy to position the small 0603 size parts onto the PCB with the plastic(ish) tipped tweezers.
The board was then carefully placed into the centre of the oven, next to the identical board carrying the K type thermocouple.
The oven was then connected up, turned on and the 'solder' button on the controlled pressed.
10-12 minutes or so later I had a beautifully reflow soldered PCB.
Now to get the air compressor connected up so that I can apply the paste just by pressing the foot switch..........

Sam



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Wednesday, 12 February 2014

Birthdays

Lunch in Framlingham today. We were celebrating two birthdays.
G3LQR became an octogenarian at the weekend and G4HUP is now officially one of the grey haired Anglian brigade!
We were joined by Peter, G3LTF and his XYL as well as G4HUP's XYL and Graham, G4FSG.
It was a meeting to reminisce, talk about EME and our future plans. The raging storm outside the Railway Inn in Fram. reminded us all about the problems of weatherproofing our external radio equipment and how to stop large EME dishes getting damaged.
Happy Birthday to both Simon and Dave!





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Sunday, 9 February 2014

Reflow soldering

After my initial experience using a loaned reflow solder system I have now placed my order for one of the Beta reflow systems. Now to wait a few days for it to arrive.......

Having played with the loaned system I am totally convinced this is the way forward. Although it may never be economic for assembling the odd prototype board, many of the interesting chips becoming available cannot be effectively hand soldered. e.g. those chips with a ground pad underneath the chip such as many of the newer PA chips, power detectors, some synthesisers etc.
It also may allow me to offer assembled VLNA and transverter boards. The 'builder' still needs to put it in a box and make other connections, but for those people who cannot handle 0603 or even 0805 size SMD parts there may now be another way!

More info in due course.

Sam





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Monday, 3 February 2014

SMT soldering

As a result of an interesting discussion over coffee at our local Tesco store on Sunday, I have been loaned a toaster oven and solder paste application system.

This is the first time I have really considered mass producing assembled PCB boards such as my VLNA or the Nacton transverter.
My initial attempts with the compressed air powered paste application system were quite successful, but I can't say the same of the oven part.

Without an automatic profile temperature controller it is very difficult to adhere to anything like the correct temperature profile, resulting in some poor joints.
I am assured it can be done, so I will persevere.

One other thing I want to try is soldering with a hot plate. I have a rather nice Jenner hot plate, obtained from a previous employer when they downsized in about 2004. It has remained virtually unused since then.
Such a hot plate is not suitable for simultaneously soldering multiple boards ( unless they are small), but does have the advantage of being able to see when the solder actually flows. The temperature profilers I have seen look suitable for use with the hot plates, as an added incentive to try this approach.

There will be a demo and discussion session featuring Reflow soldering at the forthcoming Martlesham Microwave Round Table (MMRT). There can be little doubt that with components getting smaller and more complex and the need to cope with heat transfer pads underneath some of the more interesting devices, this is something we all may want to get to know more about in the near future.

Needless to say, I won't be doing the MMRT demo!

Sam



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