ignorant question…. Can’t you just buy another crystal with the required frequency??? or may alter it with a variable cap???

> ignorant question…. Can’t you just buy another
> crystal with the required frequency??? or may
> alter it with a variable cap???

Good question:
1. Not every frequency value is available. Amateur frequencies are available from suppliers, but not every frequency, just the most common. And, you rarely find Amateur frequencies in ‘industrial’ crystals (like the usual 4Mhz, 16Mhz, etc. that you might run your Arduino or PIC from).
2. You can custom order the frequency you want. There are number of manufacturers who’ll do this for you. But it’ll cost maybe US$15 to US$35 PER custom valued crystal.
3. You can ‘swing’ a crystal with a variable capacitor, or a trim-cap, just not by much.

Mike Y
KM5Z
Dallas, Texas

In the “Old Days” we lowered freq of xtals by rubbing solder on it, raised it with comet cleanser and a cotton rag.

@Juancubillo: Usually you can’t. Crystals that aren’t mass produced at common frequencies are often orders of magnitude more expensive, if available at all.

Variable capacitors only allow you to bend the frequency so far (from what I’ve read, something on the order of a few kHz). You try and capacitively bend the crystal much more than that, and stability suffers.

You could use this to lower the operating speed of a micro of some sort, so that it would be easier to logic analyze.

basic rule of thumb for this method.

Thicker – slower
Thinner – faster

Many a crystal radio set was moved in frequency by either sanding down (comet works well) or building up (pencil lead or solder work well).

Don’t plan on moving it very far though.

@oren: Harmonic modes are good for larger shifts, but are always going to be a multiple of the primary resonance. you can get the resonance out of the xtal itself or pick it out of an overdriven amplifier. W1GHZ uses this technique in his 902mhz,1.2ghz,2.3ghz and 3.4Ghz transverters to get the local oscillator frequency for example.

As to the ‘get a new xtal’ suggestion, custom frequency crystals can be ordered but often take time to get and are often Not Cheap. iirc, the xtal for my 50mhz beacon (which needed to be @ 50,076,500hz +/- a few hz) was about $30 and took 4 weeks to get. Putting it in a xtal oven keeps it within a few hz, less xtal aging.

Lest you think those last few digits are insignificant, when you start multiplying to get up into ghz range for microwave radios, it adds up. Multiply 50,076,500 by 50. Do the same for 50,076,550. The difference is 2.5Mhz. More than a whole FM radio band channel separation. Makes it hard to hear the other guy.

When I was in college we got 2 radios from the airport that were used to communicate with planes; they accidentally left the original crystals in them for about a month before we accidentally told them about it, afterwhich they flipped out and a bunch of people over at the FCC or the airport or whatever got written-up.

We could have been talking to airplanes; the frequencies of these crystals are not easily reproduced though, since we’re talking about super precise secret frequencies.

To answer Devlin’s question. Even though I am an amateur radio Operator, I don’t know what I’d modify a crystal for, or even if I would even ever do it. I could see myself doing it to bring an old rock bound station to life operating, exclusively on a net frequency. In tat case the process is made easier in that many of the older stuff used crystal sockets for crystal holders that where made to come apart for easy access to the crystal. Otherwise I’d spend the money to have a custom crystal prepared, because it wouldn’t be something I’d need to do often, or rarely. Other than applications where the crystals can be mass produced inexpensively, crystal control is reserve for applications where accuracy, and stability is required over a wide range of changing operating conditions. IMO in such application money spent for a custom crystal, is money well spent, and may be the responsible thing to do, to do all I can to prevent interference to others.

This would be useful for people who have simple crystal-bound transmitters or receivers (i.e., http://www.kenneke.com/~jon/pixie/ ) who want to be able to hop around a radio band more so than can be provided by a variable capacitor alone.

CAN SOMEONE TELL ME in a single sentence why this works? (I don’t want to read a million pages.) It seems like adding/removing electrically-conducting material is just a more dramatic way to alter capacitance, but I’m sure there’s more to it than that…

@Scott very simplified in one line: More weight means slower vibration. Slower vibration equals lower frequency.
In a range, of course. And it doesn’t even has to be a conucting material.

After all, that’s what xtals do, vibrate. Then there’s the piezoelectric effect, electrical models, equivalent impedance, and a lto of stuff about it, but it all comes down to the thing vibrating at it’s resonant frequency.

When I was a novice I used a pencil.

The reason for knowing this, if there is an an emergency, where you have a radio, or other rock bound device ,and need to call for help in a primitive environment.

I live in a sea-side city, and here radio amateurs use to modify crystals to submerge them in sea and communicate over long distances using sea as a medium…

What about DDS? It can be implemented with a cheap micro nowadays.

@Scott Crystals are mechanical not electrical component, think about them as tuning fork

P.S. Crystal control is no more accurate than a PLL digitally tuned radio.

The way you prevent interference is to use the PROPER radio design proper feedline, proper antenna.

If your radio is garbage and has a lot of sideband lobes, then you buy cans to filter them out. if your antenna is not tuned dead on it causes interference. Amplifiers that are designed poorly cause band splatter.

I have a digital rig that will outperform ANY crystal rig out there in signal cleanliness and stability.

That said, I can build 50,000 good crystal QRP rigs for the price of this one Icom…

1:1 match No impedance nodes in your feedline, clean and properly tuned amp, clean and properly designed transmitter exciter. THAT is how you prevent interference.

Xtals have nothing to do with it.

@fartface – “Crystal control is no more accurate than a PLL digitally tuned radio”

“Phase jitter” is a problem that arises in the PLL (divider) itself, not from its tach or reference signal which is usually (Ham, CB, Avionics) sourced from a crystal.

A PLL can only be as good as the signal it locks to.

In the days when crystals were square and supported between two plates with raised lands at each corner, in openable carriers, it was possible to run a screw through the side to change the clamping pressure on the plates and produce a VXO – Variable Crystal Oscillator, but still only over a very limited range.

I remember using a similar crystal to overclock a 386 sx. I cant remeber the frequency I achieved over the one it was already working, but I remeber feeling pretty awesome about it. I may still have this motherboard around :)

@xrazorwirex

“since we’re talking about super precise secret frequencies.”

Oh noes – the secret is out – and dirt cheap

http://www.mypilotstore.com/mypilotstore/secd/12

@ people who responded

Yeah, I’m not a huge aircraft OR radio person, but I’m sure it’s not such a big deal these days. All I know is that there is equipment out there that uses extremely precise crystals that you can’t easily obtain; whether it’s because of the cost making it or if it’s a frequency that the FCC thugs don’t want you chatting on is something I never looked into that much.

Hi!!I bought 2 rc cars with same freq.the controls interfere with each other.pliz help me.how should i change the freq. In a step by step method.thank you in advance