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Old 17th Dec 2018, 10:08 pm   #1
G4_Pete
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Default Xtal digital replacement

My local Top band net is on 1985 Khz with xtals quoted at £25 per Xtal plus postage I was briefly considering a frequency synthesiser replacement in this service? Has anyone done this ? If so any further details, type supplier ect.
Pete
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Old 17th Dec 2018, 10:55 pm   #2
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Default Re: Xtal digital replacement

Although I haven't used the project as a signal source for a transmitter, I did construct this PLL-based, PIC-controlled signal generator that covers that range in 1kHz steps to use as a signal generator.

It requires the SAA1057 PLL to work with the HEX code source given and I sourced a pack of 5 for 'pennies' from the usual auction site (on a BIN basis) - the SA1057 circuit I used was one published in Elektor magazine that was, in its own right, used in their signal generator project.

http://home.deds.nl/~radio102/freddos/default.htm

I suppose the modern way would be to use the DSS modules now readily available - which I have since progressed to - but the 'good old' SAA1057 design is very good all the same....
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Old 17th Dec 2018, 11:36 pm   #3
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Default Re: Xtal digital replacement

Si5351A module + arduino mini and a low pass filter is probably best bet these days.

I'd get the first two on Aliexpress. Less than $5 each.

Typical set up: https://vk3hn.wordpress.com/2017/02/...51-dds-vfobfo/
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Old 18th Dec 2018, 3:59 am   #4
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Default Re: Xtal digital replacement

Or Si5351 Xtal replacement

Don't let the Picastar title bother you, it can be used for almost any xtal replacement function.

If you want VFO like variable frequency Simple DDS VFO

So short answer, yes, it's been done.
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Old 18th Dec 2018, 7:02 am   #5
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Default Re: Xtal digital replacement

We experimented with a sealed crystal oscillator module (Readily available) and a freq. divider. A bit of buffering, a cheap freq counter, and it did work.
Also we tried a NE555 osc, with precision resistors and caps with a buffer xistor to good result also.
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Old 18th Dec 2018, 10:44 am   #6
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Default Re: Xtal digital replacement

Quote:
Originally Posted by G4_Pete View Post
My local Top band net is on 1985 Khz with xtals quoted at £25 per Xtal plus postage I was briefly considering a frequency synthesiser replacement in this service? Has anyone done this ? If so any further details, type supplier ect.
Pete
Have you tried the chap who advertises xtals in Radcom? He's a lot cheaper.

voyjako@gmail.com

73

Roger
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Old 18th Dec 2018, 11:44 am   #7
Steve G4WCS
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Default Re: Xtal digital replacement

in the spirit of vintage radio, how about taking apart a vintage quartz crystal unit and regrinding to the frequency you require

http://www.af4k.com/crystal_holders.htm

https://www.electronics-tutorials.co...l-grinding.htm

I have a milk powder tin full of these and its quite possible I have one thats there or thereabouts for the frequency you need
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Old 18th Dec 2018, 4:22 pm   #8
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Default Re: Xtal digital replacement

All thanks for the info,

Steve , you are right yes this has to stay vintage.

pm sent
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Old 18th Dec 2018, 6:08 pm   #9
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Default Re: Xtal digital replacement

1.8432Mhz is a common 'baud rate' crystal used with older UART / Modem circuits, etc. I assume if modifying to frequency you have to start with one which is a bit lower than the required frequency?
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Old 18th Dec 2018, 6:26 pm   #10
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Default Re: Xtal digital replacement

Disconnect the 20pF capacitor from cathode to grid, then inject a suitable external signal-source; the xtal osc valve then becomes a simple buffer-amplifier.

I'd build a simple L-C VFO for this. Remember that a typical medium-wave valve radio tuned to 1.5MHz so its LO would have been running near 2MHz [assuming 450-470KHz as the IF]. You should be able to repurpose a MW local-oscillator coil [throwing away the slug and adding some extra capacitance] along with something like a 25pF airspaced variable-capacitor to do the tuning, and an EF91 as the valve, in an 'electron-coupled' oscillator config.

Stabilise the HT though: in the past a purple-glow valve would have been compulsory; these days a cheap Zener and series-pass transistor make a lot more sense.
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Old 18th Dec 2018, 6:39 pm   #11
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Default Re: Xtal digital replacement

I used a programable oscillator in my prc320 for the lsb cio,pretty sure it was made by epson,was a lot cheaper than an xtal,maybe an option,73 m3vuv.
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Old 18th Dec 2018, 7:24 pm   #12
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Default Re: Xtal digital replacement

Ive probably still got 1985 crystals laying around here - it used to be the local AM TB net in this area as well
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Old 18th Dec 2018, 7:34 pm   #13
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Default Re: Xtal digital replacement

Perhaps, as a variation of Tanuki's idea, rather than use a valve external oscillator, use a FET. No heat is generated so temperature compensation is less of an issue. Get the DC to drive it from the valve LT supply and put it through a chip to stabilise. Many years ago, I did this for the receiver side of a Pye Vanguard, multiplying the signal (and any drift) by 12, and it was quite good. I think it used something like a BC108 transistor as a buffer, all built on a small pcb in a diecast box, with a standard air-space cap for tuning.

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Old 18th Dec 2018, 10:39 pm   #14
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Default Re: Xtal digital replacement

Pics of crystals
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Old 19th Dec 2018, 1:06 am   #15
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Default Re: Xtal digital replacement

I use one of Cumbria Designs SI570 VFO modules in one of my homebrew transceivers. They are super stable and have a nice LCD readout and IF offsets if necessary.
My main topband transceiver uses a standard LC VFO with a bolt on homebrew HuffPuff stabiliser. Also super stable, but a frequency readout requires a frequency counter module of the kind you can get from rallies.
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Old 19th Dec 2018, 9:48 am   #16
G4_Pete
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Default Re: Xtal digital replacement

Wow thanks for all the replies lots to think about here.

Steve , thanks for the pictures and offer of the crystals to grind. I did a 4 year engineering apprenticeship and was taught by an old hand how to figure of 8 lap slide valves. Interesting how those formative years really fix memories so from what I read this is the same principles but far more delicate.

So yes sounds a good challenge.

Pete
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Old 19th Dec 2018, 10:56 am   #17
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Default Re: Xtal digital replacement

Quote:
Originally Posted by steve.collier2 View Post
Pics of crystals
Do you have any around 1807 kHz. I'm after a 1807.5 to double to 3615 in my TCS transmitter.

Loads of crystals here but never the one I want!

One thing occurs to me about using crystals is that several years ago I modified a Marconi Commandant TX to work on LSB for Sea Cadets use. It used, IIRC, 100kHz as the carrier frequency and I moved the crystal by making a CMOS divider down from 10MHz and buffering it with a transistor. It might be possible to find a crystal around a multiple of 1985kHz and divide it down?

Just a thought.

73 for '19,

Roger
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Old 19th Dec 2018, 4:12 pm   #18
GW3OQK Andrew
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Default Re: Xtal digital replacement

Pete, I'd keep it vintage and convert that oscillator to a VFO, with bypassed cathode resistor R5 taken to a tap on the coil. That's what I have in an 80m two valve rig and it's quite stable enough for break-in CW. It would sound vintage with a little pulling on tune-up and a bit of FM if you put the BFO on.
73, Andrew
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Old 20th Dec 2018, 8:10 am   #19
Steve G4WCS
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Default Re: Xtal digital replacement

Quote:
Originally Posted by G3VKM_Roger View Post
Do you have any around 1807 kHz. I'm after a 1807.5 to double to 3615 in my TCS transmitter.
I havent, Im afraid, sorry
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Old 28th Dec 2018, 8:07 pm   #20
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Default Re: Xtal digital replacement

Steve,
Crystals arrived today many thanks.

I have had an exploratory go at shifting one of them using wet and dry paper and a diamond sharpening stone. I have also got some of the cerium oxide for polishing windscreens but I have not tried that yet. A well trodden path by some but all new to me.

As found the crystal was 1595 Khz and with about half hour of very gentle grinding and frequent testing it has shifted up 15 Khz. The amount of material removed was immeasurable on my vernier caliper remaining at 1.03mm thickness.

The target value will need a shift of 380Khz, over 12 hrs at current progress, so I may have to be more aggressive whilst keeping the wafer thickness parallel and in one piece but nothing ventured nothing gained.

The test setup uses a dip meter connected to a frequency counter with the crystal placed back in its holder for testing. It is very easy with this holder design to slide the wafer back in for the test. Using a dip meter allows the oscillator bias to be adjusted giving some indication of crystal activity. The FT243 holder in the picture is just used for the pins to plug into the dip meter.


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