August 2014. Boredom is a terrible thing, you end up doing tasks which are completely unnecessary just to stay sane. A couple of weeks ago I needed the Droitwich Mk2 to check a nice HP ovened crystal oscillator, and it occurred to me that although it was working well it was a bit big and scruffy. One of the repairs I had made to the case (an old scrap case from a worn-out Thandar instrument) had failed and it was getting a bit rickety.
As it happened, I'd brought home from a rally a scrap IC290. It had a blown-up PA and a PLL fault, so I paid a pittance for it just for the case. Always handy, and now I had a use for it. It was significantly smaller than the old one, but after a few measurements I decided it was possible as long as I left out the 'fast AGC' board and the mains PSU.
And so work began. With trembling hands (ahem...) I dismantled the old one, liberating the various modules for re-use. I then positioned them neatly on a new PCB 'chassis' and soldered them into position. In a most uncharacteristic manner, I made sure all the pins and sockets were accessible so that I could solder all the leads without difficulty. I even remembered to keep the mounting holes clear! Once I had the main modules mounted and connected, I applied some volts and waited for it to lock. At this stage there were no signal indicator or lock LED's, so I just counted the seconds til something happened, and after about half a minute the pitch of the 10 MHz signal on my HF receiver began to move upwards and finally settled. A quick dose of Racal 1998 counter told me it was on 10.000000 MHz. This counter is a trooper, but it hasn't been well recently and it's looking bad, Jim. But for now it'll do until I can sell my one remaining kidney so I can afford a new (used) one. Below is a pic of the board at this stage.
The modules are (top left) input RF board. This amplifies the incoming 198 KHz signal from the aerial box and narrows the selectivity still further. A crystal filter would be nice here, but it seems OK without. To the right is the large tin can containing the f/99 divider, the f/50,000 divider and the 4046 phase detector. Bottom left is the 10 MHz crystal oscillator and buffer, and bottom right is the board which contains newly-designed op-amp drivers for the 'signal' LED and the 'lock' LED. There is a new one to drive a centre-zero meter which indicates how far the oscillator is being pulled to keep it on frequency.
Below is a shot of the rear compartment which houses the PSU and 10 MHz output amplifier. Not a lot to say about this, except that it delivers 1v p-p into 50 ohms. The 'psu' is just a 7808 regulator which regulates the 12 volt input from a wall-wart.
Now we are into September. The meter drive circuitry kept me amused for hours. I wanted to be able to see the overshoot as the PLL settled onto frequency, and I spent ages optimising circuit values to make it do what I wanted. Eventually I had to stop fiddling and build the chassis into the case, attach the front and rear panels and do the final tidy-up and test. Below are a couple of shots of the more-or-less finished article.
Much smarter now, and with tweaked values in the loop filter the performance is very good indeed. By finding the 93rd harmonic on my Funcube Pro Plus I was able to see a gentle 8 Hz p-p 'sway' on the signal. Scaled down to 10 MHz, this is just 0.08 Hz, a good figure for an off-air standard. I think. The other BNC socket on the front is as yet unassigned, ie it has no wires going to it. You can find the block diagram for the original Droitwich on the Droitwich MK2 page. Now all I have to do is fix my old Racal counter!
October 2014. The recent colder weather has cooled my shack down quite considerably and the 10 MHz xtal oscillator has moved up in frequency, causing the PLL to go rather near the bottom of its range to keep it on frequency. It's still locked but I suspect that if it gets much colder (it will...) it will lose its grip. So a little thermostatically-controlled heater seemed a good idea.
Precise control of the temperature wasn't required, just something to add a little heat to the oscillator module when the shack gets cold. So I knocked up a simple on-off heater consisting of a small rod-type thermistor, an op-amp and a home-made heating element made from six 470 ohm resistors in parallel. In the photo below I've arrowed the new bits. The control board is stuck on the side-panel and the heater itself is on top of the osc module with the sensor nearby. There's a plastic cover that fits over the oscillator /heater to keep the heat in. It's still under test, but so far it looks very promising. That ugly bunch of black wire belongs to my thermometer.
And of course there's a new LED on the front panel to show when things are cooking!
I'm sure there'll be more mods along soon.