08-04-2016, 11:20 AM
(This post was last modified: 08-04-2016, 11:24 AM by pamphonica.)
Following a nudge from Mark Hennessy, here are some details of a GPSDO I put together recently.
For those as yet unfamiliar with these devices, they take a GPS signal from a suitably placed antenna and use it to lock a 10MHz ovened crystal oscillator to a high degree of accuracy (parts in 10^12). More than most of us need but I was interested in playing with this technique to set up other oscillators.
You can do all the complicated electronics from scratch but I just bought a working Trimble Thunderbolt module and antenna from a US seller (a bargain at about £120). Tech details can be found here.
This device is controlled via a 9600 baud serial port and there are lots of bits of suitable software available (Trimble's own monitor, or the wonderfully named Lady Heather program) but I didn't want to have to use a PC so I bought a very neat display kit from ZL1CVD.
His TSIP display, also available via ebay (search for "TSIP display"), is very compact. A small PIC board bolted onto the back of a standard 2x16 line LCD display makes an easy monitor display for the Thunderbolt. It shows warm-up, how many satellites etc, and gives warnings if all is not well.
All I had to do was produce a decent front and rear panel for the case, order a bunch of ready-made cables from China and sort out the psu and a simple distribution amp to drive multiple outputs.
The main power has to be 24V for the Thunderbolt, so I used a Chinese 24V/2A brick. That was lowered to 5V for the display and buffer amp using a buck-mode switching regulator from Ebay based on the LM2956S. I tried a simple 7805 but the heat was way too high in an un-ventilated case. These little regulator modules are as little as 5 for £2 delivered from HK! They give out some noise but suitable filter caps deal with that.
The buffer amp is straightforward and based on what I had to hand. It uses a 74AS04 hex buffer fed from the Thunderbolt 10MHz output via a 33pF cap. The first input gate has its input biased to mid-rail using 56K resistors to +5V and to 0V. Two more stages of 74AS04 buffers are followed by a 74AS541 octal buffer with all inputs in parallel and its outputs used in pairs to drive into 50 ohm loads. Each output has a 91R resistor in series and these are then joined in pairs. This gives four outputs with a reasonable match to 50R.
The better photo of the buffer board just about shows the original 7805 regulator in place, with a dropper resistor to share the heat load. The final photo attempts to show the tiny LM2956S regulator board in its place. No heat at all!
Other than making the wiring tidy that was it. The longest bit was the metal-bashing. Huge thanks to ValveBloke for cutting out the large holes for the display and sockets with his wonderful computerised milling machine.
Probably a complete overkill but satisfyingly neat and the same case as my existing Rubidium 10MHz standard which I also boxed and buffered, so everything looks neat.
Quite a change from the valve radios and KW valve transceivers I have been working on!
I hope this helps to explain what is involved for a relatively simple GPSDO.
Jeremy
For those as yet unfamiliar with these devices, they take a GPS signal from a suitably placed antenna and use it to lock a 10MHz ovened crystal oscillator to a high degree of accuracy (parts in 10^12). More than most of us need but I was interested in playing with this technique to set up other oscillators.
You can do all the complicated electronics from scratch but I just bought a working Trimble Thunderbolt module and antenna from a US seller (a bargain at about £120). Tech details can be found here.
This device is controlled via a 9600 baud serial port and there are lots of bits of suitable software available (Trimble's own monitor, or the wonderfully named Lady Heather program) but I didn't want to have to use a PC so I bought a very neat display kit from ZL1CVD.
His TSIP display, also available via ebay (search for "TSIP display"), is very compact. A small PIC board bolted onto the back of a standard 2x16 line LCD display makes an easy monitor display for the Thunderbolt. It shows warm-up, how many satellites etc, and gives warnings if all is not well.
All I had to do was produce a decent front and rear panel for the case, order a bunch of ready-made cables from China and sort out the psu and a simple distribution amp to drive multiple outputs.
The main power has to be 24V for the Thunderbolt, so I used a Chinese 24V/2A brick. That was lowered to 5V for the display and buffer amp using a buck-mode switching regulator from Ebay based on the LM2956S. I tried a simple 7805 but the heat was way too high in an un-ventilated case. These little regulator modules are as little as 5 for £2 delivered from HK! They give out some noise but suitable filter caps deal with that.
The buffer amp is straightforward and based on what I had to hand. It uses a 74AS04 hex buffer fed from the Thunderbolt 10MHz output via a 33pF cap. The first input gate has its input biased to mid-rail using 56K resistors to +5V and to 0V. Two more stages of 74AS04 buffers are followed by a 74AS541 octal buffer with all inputs in parallel and its outputs used in pairs to drive into 50 ohm loads. Each output has a 91R resistor in series and these are then joined in pairs. This gives four outputs with a reasonable match to 50R.
The better photo of the buffer board just about shows the original 7805 regulator in place, with a dropper resistor to share the heat load. The final photo attempts to show the tiny LM2956S regulator board in its place. No heat at all!
Other than making the wiring tidy that was it. The longest bit was the metal-bashing. Huge thanks to ValveBloke for cutting out the large holes for the display and sockets with his wonderful computerised milling machine.
Probably a complete overkill but satisfyingly neat and the same case as my existing Rubidium 10MHz standard which I also boxed and buffered, so everything looks neat.
Quite a change from the valve radios and KW valve transceivers I have been working on!
I hope this helps to explain what is involved for a relatively simple GPSDO.
Jeremy
Jeremy. G8MLK. BVWS Secretary. British Vintage Wireless and TV Museum Friend.
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Pamphonic Website ______________ BTTT OB Truck Restoration