Crystal Clear

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XK50
Posts: 13
Joined: 25 Aug 2020, 17:57
Location: West Midlands, UK

Crystal Clear

Post by XK50 »

Elsewhere on this forum, I describe how I am building a Remcon 12 Versatile “Reed” Tx, from scratch. My current task is acquiring the crystals. The hunt has created some questions and I wonder if anyone can educate me?

The 27MHz crystals to be used are specified as “3rd Overtone” and various third-party ads in RCM&E show that this is quite common.

Q1 Why the 3rd overtone? What was special about it?

Further, I’m guessing that the signal is attenuated with ever higher overtones.

Q2 Does this mean that, with these crystals, the “best” signal is going out on 9MHz, the primary tone?

Next, as early as Feb 1962, RCM&E published a newly-adopted crystal “protocol”. The adoption of this (Brown through Blue) enabled those with superhet receivers to fly 6-up, and no “waiting your turn”.

Essentially, the Tx “colours” were each separated by 50/60Khz, with the corresponding Rx using a crystal 455KHz lower, to produce the intermediate frequency.

Now, at least as late as 1966, Remcon released the “12 Versatile” with a designed tx/rx IF separation of 465KHz. I’m guessing this tied kit builders to Remcon crystals and might have provided a modest reduction in the chance of interference for Remcon receivers, but it would create a problem when “splits” (further dividing up the 27MHz band) were introduced, a few years later. (Indeed, my recently purchased “spare” Remcon Superhet receiver shows sign of its crystal being changed to the “standard” 455KHz separation, at some time).

So,

Q3 Is this 10KHz difference significant? Can it be “tuned out”, easily, in the IF coils?

I realise this historical information is a bit arcane, these days, but someone might know, and any answers will help me understand what is going on.

Q4 Am I in danger of taking all this a bit too seriously?

Many thanks,

John
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Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: Crystal Clear

Post by Pchristy »

Quite a lot of questions there! I'll try and answer as best I can, and I'm sure someone will be along to fill in any gaps I leave!

3rd overtone: It is very difficult to make fundamental crystals above around 20 MHz. They get very thin and brittle (easily crash damaged!). However, it is possible to cut crystals in such a way as to enhance the odd numbered harmonics. For 20-60 MHz, this means a 3rd overtone. A 3rd overtone crystal will try and resonate at 3 times its fundamental frequency, and it is not easy to get it to go at its fundamental. For 27 MHz, a 3rd overtone will have a fundamental frequency around 9 MHz, so its quite thick and robust, but will tend to resonate at 27 MHz.

Higher overtones are also available, typically 5th overtone, which will take you up to 100MHz or so, but these will always try and resonate at a lower harmonic if they can (3rd), so care has to be taken with the oscillator design to ensure that it can only go at the wanted overtone. American sets operating in the 72MHz band tend to use 5th overtones.

Most modern single conversion superhet receivers use 455 KHz as the IF frequency. This is largely because that's what the Jaapanese manufacturers used, and their crystals were readily and cheaply available! Early British superhets tended to use 470KHz, and some European manufacturers used 465 KHz.

Trying to retune a set designed for 465KHz to 455KHz is a bit of an ask. It might work, but will probably be on the limit. You might do better with 470KHz crystals - only 5 KHz away instead of 10!

You could always use a Futaba crystal in the transmitter, and get a receiver crystal made. There are lots of places still around making crystals to order. A quick Google brings up http://www.quartslab.com/ or https://www.euroquartz.co.uk/manufacturing/custom-build .Don't expect them to be as cheap as mass produced items, but they shouldn't be extortionately expensive!

Otherwise, you could replace the IF cans with ones designed for 455 KHz, although these are harder to get hold of these days than they were only a few years ago (no-one building their own anymore!).

The channel spacing was not simply a question of the receiver selectivity, but also of transmitter cleanliness! The 50 KHz channel spacing arrived because of the American CB situation. In the USA, 40-channel CB was introduced on 27 MHz, with certain channels being reserved for model control. These channels could not be selected on a stock CB set. The US RC channels were 50 or 60 KHz spacing, so crystals were readily available for these frequencies and we followed suit. However, at 50 KHz channel spacing, you could get away with quite sloppy filtering on the transmitter, without causing adjacent channel interference! Many of us remember certain outfits being unable to cope with even 20/30 "split" channels, due to sideband splatter from some very well known transmitters!

Hope this answers some of your questions! If I've missed anything, shout!

--
Pete
XK50
Posts: 13
Joined: 25 Aug 2020, 17:57
Location: West Midlands, UK

Re: Crystal Clear

Post by XK50 »

Thanks Pete for your very comprehensive reply. I knew from reading this forum, and others, that THE man to answer my questions was yourself. I am delighted that you should reply.

Meanwhile, I have established contact with Dave Hayes at Quartslab. He needed to know a bit more than name, rank and serial number but I’m about to get some truly bespoke crystals.

Thanks, again, for your kind reply.

Best wishes,

John
Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: Crystal Clear

Post by Pchristy »

Your welcome, and let me know how you get on with your crystals!

--
Pete
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