If I had a pound for every time I've got myself onto 23cm but then given up after while because of low activity levels, I'd have at least three pounds by now.
Nothing special, but to keep losses down to a minimum I've decided to completely separate the receive and transmit paths. This means two mixers instead of one, but it gets rid of the switching diodes. Of course the local oscillator will be common to both paths.
12 June 2011.
Here's the more-or-less finished LO board. It will go in a tin box soon. A crystal osc at 96 MHz is multiplied up to 384 MHz where it is filtered with a 3-pot helical. Another multiplier and helical gets it to 1152 MHz at about 1mW and a final MMIC amp achieves about 16 mW, enough to split between the RX and TX mixers.
Some of the parts for this project have been a bit hard to come by. TOKO have decided that helical filters are so yesterday and have switched to dielectric types insted, and of course they don't make them for Amateur frequencies. I managed to scoop up a few 1250 MHz helicals for the RF board and some 1190 MHz ones for the LO board, and luckily they were all within twizzing range. Many thanks to Anthony at BONEX (now BEC) for his help and patience.
Thanks also to Chris G8CHW at MODE COMPONENTS who got a few bits in for me and was as usual efficient and friendly throughout.
The crystal (a cheap and none-too-stable one) came from Keytronics, along with a few parts for stock.
Alan Melia G3NYK cheerfully supplied the two tin boxes and wished me luck with the project.
More credits to follow when I remember who they were....
13 June 2011. Making good progress now. The osc/multiplier is in its tin box and delivering 25mW which is easily enough to drive the two mixers via a resistive splitter. I've also cured a tendency for the oscillator to fail to start. If I get a chance I'll box the main RF board tomorrow and take some pictures. Onward and upward etc.
14 June 2011. A very nice day here in north Cheshire, so after almost dozing off in the sun I got to work. Getting everything to stay put while I tack-soldered the boards into position was difficult, but I triumphed in the end.
That's the local oscillator/multiplier board at the top. I haven't soldered it all the way around yet, but it works well. The SMA socket on the left is 1152 MHz out, and the one on the right is for injecting an external 96 MHz source, just in case.
The main RF board is not quite canned yet. The mixers are those little white ceramic things at the left. Installing the GaAsfet in the front end was (as always) a nerve-jangling process, as the gate is extremely static sensitive.
17 June 2011. I've not got the Mitsubishi brick yet, but at least I've built the voltage regulator and PTT board that drives the relays. There are three 9-v regs, one for TX, one for RX and one for the local osc/multiplier. The little relay switches the 12 volt supply between the TX and RX ones. It all works!
21 June 2011. Still waiting for the PA block and the leads, so I got on with making the PCB for the PA and finding a heatsink. There are only four pins - input, bias, 12v supply and output, so cutting the tracks was easy. I'll apply the bias via a 5K pot from a stabilised 6.8v rail dropped from the main 12v supply.
23 June 2011. The SMA leads arrived yesterday so I can now lay the bits out on the chassis. It's a bit bigger than I wanted due to lack of a more suitable case, but at least it'll be easier to fix when it blows up. Here we see the LO and RF boxes mounted, along with the smaller TX driver. No PA yet, but it's heading for that heatsink yonder...
24 June 2011. It's arrived, and here it is ready to be soldered to the PCB. You can see the 6.8v zener and the bias adjustment pot, and note the bonding of the module's heatsink to the board with brass strip. I've found this to be advisable with the UHF modules.
After building it all up and testing it, I've decided that I need more drive power. Looking again at the spec sheet for the module, it appears to get to its full gain only at at maximum idling current, which is AMPS. I don't like running stuff at amps, so I'd rather turn it down a bit and shove more RF into it. So.. I need a sort of mini-PA.
30 June 2011. Cracked it. Ages ago, Roger G8ILD gave me a power device for 23cm because he's nice like that. I put it on the shelf and forgot about it, but now I've re-designed the driver amp and that device was just what the doctor ordered. Driven by a 50mW MMIC, it delivers 300mW to the PA brick and out comes 10 watts. So just the finishing touches - RX/TX LED's, a low power switch and of course a fan to gently keep things from getting too hot. I don't like hot. Pictures to follow if I can find the camera.
Output spectrum at 10 watts. No sign of LO leakage and the -288MHz image is 60dB down. 144 MHz is also 60dB down but I think most of this was due to pickup on the analyser's input lead, as it varied when I moved the leads around.
13 July 2011: I've retired the old FT290 as it was developing new faults faster than I could fix them. In its place we have a nice Icom IC290. Old but good. I've added temperature sensing to the fan (just a thermistor and a 741 driving a TIP31) and generally tidied things up. I've had a number of QSO's on FM and SSB and reports are good. Another job done (not very) cheap!
No. A few weeks ago I came across an old-but-nice Icom 251 2m multimode. Apart from a few minor issues it worked well and I soon began to entertain dark thoughts about cramming my 23cm transverter into its case.
Here's the rig with its mains PSU whipped out. Should be plenty of room in there...
My old transverter was dismantled (one of those occasions when you ask yourself 'what am I doing?') and prepared for installation.
A late-night snap of work in progress. The PA block is mounted against the back wall of the rig. There'll be a fan on the outside later. The LO, TX/RX strip and driver amp are ready to be tack soldered to the PCB platform.
As soon as the wiring was complete, a quick RX test was run. Sensitivity was good (approx -122dBm for 12dB sinad on FM) and no problems reared their heads. TX was a different matter. Any attempt to run the PA block at its full idling current (to get full gain) resulted in a honking session. It's a good thing these Mitsubishi FET blocks are made of stern stuff, as the oscillation was pretty vigorous.
To cut a long story short, a re-dressing of the 13v supply wiring to the PA stopped it dead. I fitted some more SM decoupling just to make sure. We now have just under 10 Watts of RF with no sign of instability.
Lids on, ready for action.
Just look at that snappy Icom styling, wonderful. Shame about the cheesy mic.