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





Tuesday, 28 June 2016

Yet more IC7300

I have taken delivery of an Alpha Delta 4 way coax switch to direct the output from the 7300 to the 4/6m antenna, HF antenna, dummy load or to the Anglian 2m transverter.
I have opted for the simple approach go interfacing to the Anglian and Gemini 2 PA.
I use just a 20dB power attenuator after the switch into the Anglian. I have the 7300 28MHz band power set to 8W. The reasoning is that if I was to accidently forget to turn the 28MHz power down (it is set per band) then the max that would hit the Anglian would be 1W and transverters tend to be more tolerant of overlod due to the usual input attenuator before the mixer. The 20dB attenuator is a25W unit which will stand a degree of overload before burn out.
My tests using the spectrum analyser on max hold etc indicated no significant spiking of output power when set to low power. I have seen some reports of ALC overshoot on SSB and CW. I have not seen this.

I had an interesting problem when set up. The Gemini 2 would fail to SWR when driving bove 100W output. This was eventually traced to the bulkhead lightning arrester on the antenna feed starting to trigger at this

level. The arrester has been replaced by an N bulkhead adapter. All is well now.

This evening saw several new locators added to my 6m total in the NAC contest. Now to try and leave  the system as it is now instead of yet another rearrangement.........
WTS

Sam






Saturday, 18 June 2016

More IC7300 tests

Since I want to try my 2m system with the IC7300 I was interested to know how well the power control on the new Icom works. In particular, does it spike on switching between receive and transmit?
I set the rig to 28.2MHz and (carefully) connected it through a 150W/40dB attenuator to my HP8592 spectrum analyser. The analyser was set to 20mS sweep and 5MHz span. With the IC7300 set for full output, the reference level was set to 0dB on screen. The mode was SSB. The analyser was then set to Max Hold in order to capture any spikes. The IC7300 mike gain was set to zero.
Keying the IC7300 transmitter on and off produced a small spike, caught by the Max Hold on the analyser. The level of the spike was around -55dBc
The IC7300 was keyed in different modes with similar results.
Winding the power down to '0%' resulted in the spike amplitude being similarly reduced.
Power output at 0% was 22dB down on maximum output, or about 700mW. This is below the  2W claimed.

It would appear that the IC7300 does not power spike to any significant level.

Connecting the IC7300 to my 2m Anglian transverter with DG8 mast head preamp, though the 40dB attenuator, produced a barely audible increase in noise.  No surprise there!
Reducing the in-line attenuator value to 20dB result in a greatly increased noise output and lots of beacons appearing on the 7300 spectrm display.
16dB is about optimum for my system and so it was with an MCL 15dB 2W attenuator that the system seems to work well and achieve about as good a dynamic range as I could expect with this set up.

Using a 15/20dB power attenuator is required to further reduce the power into my Anglian. I don't like to dissipate more 28MHz power in the transverter than absolutely necessary.

Although the IC7300 does not appear to spike, this is still not proof against forgetting to turn the power down.............

A 20dB, several watt, IF attenuator will give some protection and then fuse if severely overdriven. Hopefully you will have noticed a problem by then and there could even be time for an overdrive protection circuit to operate. I do not advocate this approach in order to drive an LDMOS SSPA as it would probably fail a whole lot faster than any protection circuit unless carefully designed to prevent any drive reaching the PA before checking its level.

Sam



Monday, 30 May 2016

Cold Bank Holiday

So far it has been a very dull and chilly Bank Holiday Monday, so I've been using the time to catch up on some writing. I have two Conference papers to write with both of their deadlines looming in the next week.

In addition to those I have an equipment review to write for Radcom, so what am I doing writing this blog as well?  Good question, but whilst the words were flowing seemed like a good time to put something onto the blog about the new rig.

The new IC7300 has been working out well, with a number of new locators and DXCC worked on both 6 amd 4m since Saturday. I've also been thinking about how to add a sutable transverter interface to the 7300 so that I can use it to access other bands.

The touch screen spectrum display is a pleasure to use. That doesn't mean it's perfect, by any means, but it is a big improvement over some other implementations and I think in ICOM's next SDR transceiver it will be even better.

One of the shortcomings of the touch display is that when you select the signal you want to listen to it brings up a static, magnified, view of the spectrum around the signal. You then need to touch the screen again to select the signal. I find I cannot touch the right spot to tune accurately to the signal without then having to use the tuning knob to tune it in accurately. I am sure that with the power of modern DSP processors it should be possible to do this automatically. Of course you would then probably need a 'defeat' facility in the event you wanted to select a weak signal next to a strong one!
One other observation is that there is no 70MHz band 'button' necessitating a frequency input, which means several button presss to go to, e.g. 70.2 input
You quickly get used to this, but a 4m band button would be nice, please, Mr ICOM
More anon.
Sam


Sunday, 29 May 2016

IC7300

I was able to pick up my new IC7300 at yesterday's Waters and Stanton open day. I have been in two minds about the wisdom of buying one, but Dayton finally decided me. I already have a several rigs, but I had several reasons for going ahead with the 7300. Not least is the desire to develop a suitable interface to an external transverter for 2m and/or 70cm. With all those 7300s out there, just asking for use on higher bands, I just need to do this.  A complement to the Anglian transverter, maybe?

My first impressions of the 7300 on 6m and 4m are very positive. The multiview touch screen is easy to use and a very much easier to use implementation of a spectrum display than the P3 I use with my K3.

On 40m I initially thought I had a problem. It appeared that the receiver was deaf. But it turned out that the receiver was just very quiet. Signals really stand out from the noise when you tune to one.
Selecting signals to listen to is very easy using the touch screen spectrum scope. It's certainly better than winding an encoder knob to bring a marker onto a signal and then pressing the encoder knob to select (in the process pushing the P3 backwards!).

It is also nice to have up to a full 1MHz display. You rarely need more than this as the resolution is likely to be poor on most screens. The display was especially nice on 4m where it was possible to see the many OIRT FM broadcast signals, at once, as they popped up and disappeared.
I have been sold on the future of SDR architecture for both receivers and transmitters for some time. This is one of the best implementations I have seen yet. 
I have already increased my 4m and 6m locator and DXCC scores with the 7300.

More as I get to know the rig better. And maybe something about the rather nice IC31E D Star handheld I won in the raffle at W&S. Thank you W&S and thank you ICOM!

Sam




Sunday, 15 May 2016

A bit more 13cm EME and Upcoming Dayton visit

I set up again today to check out some changes I had made in the shack. I've repaired my MFJ memory keyer and can now run either a keyboard or straight key. I may buy a decent paddle at Dayton. My 'peers' keep telling me I need to use CW more if I want to get my speed back up!
Anyway, whilst testing I noticed that although the moon is currently at apogee and therefore the degradation is high, I could occasionally hear as well as see my own echoes with the 200W PA in the shack and with an unacceptable amount of transmit cable loss. My plan is to move the PA out to the dish. The 27V PSU is already out there in a weatherproof case with 110V AC supply and all ready to go.

Dayton
I am off to Dayton on Thursday and back early the following week. Whilst in Dayton I will be attending the new VHF/UHF weak signal dinner in Miamisburg, Dayton on Friday evening. This new event replaces the excellent event previously run by Tony Emmanuel, WA8RJF, but sadly no longer organised by him.

I will be in the Dayton fleamarket much of the time, on stands 2766-2769 with WA5VJB.
It looks like it will rain (nothing new there, then!) Friday afternoon through Saturday, so I'll need to take the wayerproofs!
I'll report back on Dayton on my return.

Sam