Sunday, 24 July 2016

Anglian 3 update

For those of you who don't subscribe to Twitter you will not have seen my progress with the Anglian 3 transverter. I have been tweeting regular updates (for anyone interested!)
The second set of Prototype PCBs were deliver last week and a second unit built up to prove the design works as it should. I'm pleased to report it all worked as planned, although there were still a few very small errors on the PCB. Version 3.1 PCBs have now been ordered and are expected in a few weeks. 
The new Anglian 3 is designed as a direct replacement for the Anglian 2, but with a few small improvements. These include transmit enable delay, better transmit amplifier gain distribution, programmable transmit IF gain (for use with ICOM rigs, with their low transverter output level), better transmit IMD, but retaining the same excellent receive performance, lockable local oscillator, separate IF receive and transmit connections and all SMD construction as the 2.
Unlike the Anglian 2 I have no plans to offer the Anglian 3 as a fully assembled PCB, as this has proven to be uneconomic for small production runs of boards with specialist RF parts.
However, I may offer partially assembled boards, depending on demand.
Also, depending on how Brexit affects component purchase (many parts are sourced from overseas), I anticipate the cost being lower than the Anglian 2.

Sam



Sunday, 17 July 2016

70cm low noise preamp for Contesting and Satellite mode J


I have had a 432MHz version of my PGA144 preamp on the stocks for some time. I never bothered toadvertise  it because I knew it needed more development. I took the time over the last few days to look at the filters again and then put one together. As always some tweaking of values was required compared to what the design software said, die to stray capacity of pads etc.
The results are quire pleasing.
I've not yet measured the input two tone third order (IIP3), but it ought to be pretty good.
Of particular interest is the deep notch I have been able to place on 144-146MHz to suppress signals in this range. Similarly the frequency response is well down in the Upper digital TV band, GSM band and at 23cm.
The noise figure measures under 1dB ( 0.8 to 0.85dB on the prototype) on an open board, which is not the ideal way to measure noise figure due to 433MHz SRD interference.
As the accompanying plots show, the input match is exceptionally good, so that a filter could be placed at the input if required and its frequency response would not be badly affected. try that with many common 432MHz designs!
Noise figure and gain (can be varied if required) at 435MHz

Yes, I know that attenuators is spelt wrong......


Input match (in yellow) marker 1 at 435MHz and 145MHz with frequency response in blue withe same markers

Tuesday, 12 July 2016

Anglian 3 update

As I have run out of the current Anglian 144MHz transverter kits I decided that rather than order more boards to the current design, for reasons that will become obvious as you read the following, I needed to design a replacement. The replacement is the Anglian 3.
It is the same size as the Anglian 2. 

The redesign gave me the opportunity to make improvements, especially on the transmit side. The original Anglian 2 was designed to work with HF rigs like the K3/S or TS590 that have typically 0dBm 28MHz  transvert output. I found that I could get it to work with the typical -20dBm output from some ICOM HF rigs by slightly modifying the transmit IF input attenuator. Whilst the output would not quite reach 100mW (+20dBm) when driven at this level, it has proven adequate to drive a Mitsubishi module to full output. However, I never quite felt comfortable driving the mixer that far below its nominal IF input level where the full IF to RF isolation was achieved. At this drive level the transmit amplifier chain needs the full gain of the two transmit amplifier stages.

Some users have reported a short burst of transmit output when switching from receive to transmit. This appeared from my tests to be dependent on what was connected to the receiver input or transmitter output. The gain from receiver input to Anglian 2 transmitter output could momentarily be 70dB! It didn't take much additional gain in a masthead preamp or PA and an RF burst could appear. The changeover from receive to transmit originally had a small overlap of a few ms where both were on. Reducing regulator reservoir capacitor values speeded up switching since  there was then less charge to keep the receive amplifier working for those few ms.
However, the proper solution was two-fold. Decrease transmit amplifier gain and introduce a delay between the receiver switching off and the transmitter switching on (sequencing).

An improved output amplifier, with lower gain and better IMD, has solved the first problem. A MOSFET switch instead of a bipolar switch improves switching speed and allows the introduction of a simple but effective sequencing delay to solve the second problem.

Of course the reduction in transmit gain now meant that a low IF transmit input could become a real problem. This was solved by the introduction of an IF transmit (28MHz) amplifier stage. If not required it can simply be left out or a link arrangement allows it to be bypassed.
With the new TX IF amplifier the Anglian 3 now easily produces more than 100mW output at 144MHz for less than -20dBm input at 28MHz. Harmonics are less that -60dBc at 100mW output.

I had also been asked if it was possible to include a 28MHz band pass filter in the 28MHz transmit IF input, which I have now done. The TX IF input now passes through a single stage bandpass filter centred on 29MHz.

The LO chain is largely unchanged except that it now works entirely on 5V and runs from a single 5V board voltage regulator. The Anglian 2 had three voltage regulators! The LO still produces +20dBm output and can be locked to an external 116MHz input, if higher stability is required.

I have not changed the receive converter side as it already has about optimum performance for the design and components used.

My plan is for kits to be available later this year. I need to check the revised PCB and if satisfactory, will order a batch of production boards in August. I will probably initially revert fo full kit version with assembled SMD boards coming later. Small runs of assembled RF boards are not economically efficient! 

The aim is to keep the price (component costs permitting) similar to the Anglian 2, or maybe a bit less. We'll see how the pound fairs later this year.

Sam