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40MHz OCXO board sold at RAL
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Dave G8KHU
I've been contacted by one person who bought one of the 40MHz OCXO boards from me at RAL. The osillator had no output, this has been resolved to his satisfaction. If anyone lese bought one and the oscillator isn't running please contact me for resolution.
The oscillator can be checked by simply applying 12V DC to the board as purchased and checking for 40 MHz at the coax O/Ps - the OCXO and buffer are always on. The supply is 12V +/- 1v, the supply input is the 4 pin connector directly under the oscillator by the mounting hole. Polarity is 2 pins next to the mounting hole are negative, the two furthest away are positive - IT IS NOT REVERSE PROTECTED. To remove doubt the negative DC input pins have DC connectivity to the oscillator screening can. Any problems let me know, they were sold as functional in good faith. (FYI the best way I've found of cutting the boards to remove the oscillator section is with a slitting blade in an angle grinder.) Dave G8KHU |
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I used a hacksaw, then a diamond tipped circular saw I use for PCB material to trim the edges. There is then a small linear voltage regulator to take input volts on the upper thick track down to the 3.3 the OCXO wants BUT BEWARE. This supply didn't come from the 12V that was sent to the main board, but at around 6v generated via a buck regulator The OCXO takes 1A as it warms up, dropping to 0.5A in operation, so the tiny linear regulator won't be too happy if you put 12V on it - it'll get quite hot. I run it at 5V input. My unit is around 90ppb low in frequency (within spec) at 39.9999964MHz but it has to be left on for a few hours from cold to get there. The stability is then well within the 10PPB specification. It's a pity the frequency isn't trimmable, but at least it's quite stable. The fixed frequency offset error can be taken into account when it is used as a clock for a DDS or Fract-N synthesizer - assuming the latter is one where you can have very fine tuning steps like the LMX2541 or ADF5355. Users of ADF4351 devices will just have to suffer the offset :-( Andy On Thu, 30 Jun 2022 at 15:51, Dave G8KHU <david@...> wrote: I've been contacted by one person who bought one of the 40MHz OCXO boards from me at RAL. The osillator had no output, this has been resolved to his satisfaction. If anyone lese bought one and the oscillator isn't running please contact me for resolution. |
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Dave G8KHU
Circuit diagrams of relavant parts attached.
We tried to get the customer to go for the trimmable option but to no avail. Dave |
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Gareth G4XAT
I bought one of these boards and based on the supplied description fed a suitable voltage the the LDO reg. Yes, it gets warm with too much overhead, but easily solved with a 2A 7805. And it works fine....
Also well worth the purchase price was the 4 sets of driver and PA sections, perusal of the data sheet shows great promise as making perfect pre-drivers for the NEC ex DATV amplifier pallets that have now escaped into the amateur world via Neil and others. The pre-driver just needs 5V, the PA is rated to 7V and includes some sort of bias input. Its not clear from the data sheet (to me) quite how that should be produced (a 10K pot across 5V??), or what its range should be. It looks like there are some bias switching sm transistors on the PCB. Any chance of a circuit for the amp part of the board, or at least a bit more spec on the PA chip (like bias vs gain/linearity/power etc)? I'm particularly hoping for best gain and linearity as if the chain will deliver +30dB of gain, it will allow saturation of one of the NEC pallets from a mere ADALM Pluto driver, so perfect of Langstone/Portsdown usage. With 5 watts it might even drive a pair of them..... Thanks Gareth |
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Dave G8KHU
Full PSU and PA schematics attached :)
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Gareth G4XAT
Very helpful Dave, I have now worked out what does what and where control voltages actually go. Less clear where the RF in and out goes, I'm assuming through some sort of filter arrangement on the output and some sort of signal routing on the input. I suspect the easiest route will be to cut out each PA box in a similar manner to the TXCO and then carefully fit SMA sockets or SMA-hard-line tails to the relevant points for RF in and out. I think its a multilayer board with the RF trace buried in the layers.
Cant find any trace of U200A or U200B, the logic control chips don't tally with the diagram either number wise or pinout usage so I assume U500 and U501 do something else and the bias control was off-board. Was its built for 900MHz or lower? Comparing the actual circuit with the data sheet examples I'm still wondering without measuring the output filter chain component values. I'd guess I could always just couple the output into my own filter via a suitable coupling capacitor. Looks like there are possibly some other handy bits to be salvaged, but some very small and tricky to remove/reuse. But what was it for originally.... :-) Thanks Gareth |
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Colin G4EML
The logic chips are on the opposite side of the board underneath the PA stages. For our application they can probably be removed and the enable signal fed directly into the Q201 MOSFET gate. That saves needing to have the 3.3V supply for the logic.
I have cut my PA stages from the board. It looks like the input can be connected by removing the 0 ohm resistor from the input attenuator and feeding into the pad. Likewise on the output.
Colin G4EML
From: Gareth G4XAT via groups.io
Sent: 03 July 2022 12:04 To: UKMicrowaves@groups.io Subject: Re: [UKMicrowaves] 40MHz OCXO board sold at RAL
Very helpful Dave, I have now worked out what does what and where control voltages actually go. Less clear where the RF in and out goes, I'm assuming through some sort of filter arrangement on the output and some sort of signal routing on the input. I suspect the easiest route will be to cut out each PA box in a similar manner to the TXCO and then carefully fit SMA sockets or SMA-hard-line tails to the relevant points for RF in and out. I think its a multilayer board with the RF trace buried in the layers.
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Dave G8KHU
Here are some screen shots of the PA tracking and placement.
Clin beat me to it :), the logic chips U200 are on the back of the board above the RF input. The input RF track is stripline on layer 2, the output is stripline on layer 7. Colin's suggestion for connection is absolutely correct, alternatively drill out the RF input via just below R205 and the output via to the right of L204 and poke a piece of semi-rigid through from the back. Top and bottom planes are ground. Either way you have to disconnect the remenants of the stripline or you'll get suckouts. The PA design frequency range was 750 - 950 MHz with the exact band defined by the link selectable ceramic resonator filters. There's RF switching in there too - so don't bother with it and just cut the PA block out and use that. Don't forget to heatsink it - all those little via holes under the PA are to get the heat out so the magic smoke stays inside :). Sorry, the original purpose is behind an NDA. You might also want to salvage the DC input swtchers for other uses too. |
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