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Stretchyman 40 W TX Reliability Modifications

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Stretchyman:
Well..They aren't GPS locked or anything fancy just plain DDS which rely on the internal clock being exact. Quite easy to reprogram them to a higher (offset) frequency to negate the error.

If anyone would like such, get in touch!

Who cares anyway I wonder?

More window watchers?

Str.

Brian:

--- Quote from: Charlie_Dont_Surf on February 21, 2022, 2147 UTC ---I don't know how many versions of your synths you have gone through but the ones that I have used have always ended up a bit lower in frequency than I would like - usually about ~100 Hertz low. We're not expecting WWV accuracy here and the consequences of this are not the end of the world but I think that we can agree that they could be closer to target.

I suspect that a number of the loggings on this website indicated as "xxxx.9 KHz" (more or less) come from your synths, but I can't prove that. Perhaps this is yet another way to identify one of your transmitters on the air.  ;D

--- End quote ---
I run 2 rigs here. One Xtal and the other a very cheap Chinesium DDS. The DDS is cheaper than an Xtal (about 20 bucks plus a few more for connectors and the rotary encoder), goes up to 50 MHz and is adjustable in 1 Hertz steps. Less drift than the Xtal rig.
Problem is not getting it to be xxxx.ooo but having the way to measure this accurately.
With the DDS, I'm probably more on channel than many legal stations.

Charlie_Dont_Surf:

--- Quote from: Stretchyman on February 22, 2022, 0735 UTC ---Quite easy to reprogram them to a higher (offset) frequency to negate the error.

--- End quote ---

Someone asked me privately last year if there was a way to fix the offset. I was very bored over Xmas and finally got around to reverse engineering several of your .hex file codes for 43 meters to figure out corrections and get them a lot closer to the intended target (~ +/-20 Hz off, instead of -100 Hz).

The other thing to note is that there are multiple sources of drift or frequency shifts. Most of them are pretty small and not worth worrying about. The largest one is that the synth takes a big jump of ~ -70 Hertz when the final transistor is turned on. That could be due to many reasons but I'm guessing it's due to supply voltage shift. My corrections take this into account but because of self-heating and temperature drift with time, it can't be better than ~ +/- 20 Hertz over the long term, which I feel is acceptable.

Stretchyman:
I've made the offer and no one has asked.

I'll ask again.

If you have a programmable TX and the frequency is offset by xxx Hertz please let me know the frequency +/- XXX and I'll send you the revised .Hex file to put you bang on frequency.

Very simple for me to do.

Please get in touch by email.

Str.

Albert H:
I got really annoyed trying to get true frequency accuracy and stability for Medium Wave from the three or four DDS modules I tried....  I gave up, and built a PLL that's locked to the 198kHz BBC Radio 4 transmitter.  The 198 kHz rig is run from the National Physical Laboratory's frequency standard source, so the Medium Wave boxen I built were rather more accurate than I could show on my frequency counter!

Obviously, there are standard frequency transmitters all over the world - I went for 198 kHz because the rigs were for the UK and Netherlands, and it's easy to derive frequencies in 9 kHz steps from that source with the minimum number of CMOS logic ICs!  Incidentally, I didn't mind adding proper gate driver ICs, rather than trying to cane a logic IC for the drive, and generated twice the frequency I wanted, then used a final bistable to guarantee an absolutely 1:1 M/S ratio.

The advice given on Steve Cloutier's "ClassEradio" website about component ratings is pretty useful, and has helped me avoid the fireworks displays that others have had!  For higher powers, I've been using ex-telecom SMPSUs for high current 48V supplies, and PWM modulators for efficiency.  It's got to the stage that a 200W carrier / 800W peak rig can be built for pretty cheap these days.  The expensive parts it seems are the damned toroid cores and the mechanical stuff.

Now.... back to designing a little (80W PEP) PWM rig for as little cost as possible.....

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