Difference between revisions of "Talk:Radio Crystal Identification"

Latest revision as of 14:27, 2 August 2017

identifying scanner crystals

If you have a crystal scanner available, even if the frequencies in question are not in use in your area, you can at least determine which type of scanner they are for, using the local oscillator. Plug in a crystal (VHF high-band), then look for a signal 10.7 or 10.8 below the marked frequency. For 154.430, listen on or around 143.73 (-10.7) and 143.63 (-10.8) with a second scanner.

I'm not too sure about the lowband crystals. They are probably fundamental, high-side injection. For 46.12, you'd check 56.82 (+10.7) and 56.92 (+10.8) if you have a receiver to cover that range.

references

• Police Scanner Crystal Interchangability Information Reference from Ken's Electronics

decipher these

I'm hoping that it's a 48Mhz crystal. Or, is it bad news and I've got a 98Mhz rock?

You never know how a crystal will be marked. Sometimes with the overtone frequency, more often with the final multiplied frequency, or, if designed for a receiver, the receive frequency (which is offset from the crystal frequency by the IF, either up or down.)

I have an untuned crystal oscillator that drives an LED (which indicates that the crystal is actually oscillating) and I hook up my frequency counter to find where it is oscillating.

A 48 MHz overtone crystal will oscillate on the fundamental, which should be around 16 MHz. (To get it to oscillate on the overtone frequency, it needs to be used in a circuit that provides suitable feedback only at that frequency.)

The upper limit for crystals is about 20 MHz or so (occasionally to 25 MHz.) Above that the quartz is too fragile, etc. So for the higher frequencies a lower frequency crystal is run in overtone mode - the actually overtone frequency is a little higher than what you would get by multiplying up the fundamental.

If you want to pull the crystal to get it on your favorite 2m channel, it will be easier if you run it on the fundamental than in overtone mode: a 16 MHz VXO followed by a tripler will give far more pull range than a 48 MHz overtone oscillator.

A 460 MHz radio will probably have both a transmit and a receive crystal for each channel. I suspect they would be designed for an overtone frequency in the 48 - 54 MHz range that is then multiplied by 9 to reach the final transmit / receive injection frequency. So for a frequency of 460 MHz, the transmit crystal overtone frequency would be 460/9 = 51.11 MHz (fundamental 17 MHz) and for receive (460 +/- 10.7)/9 = 49.92 MHz (fundamental 16.6 MHz). However, another IF sometimes used is 21.4 MHz.

And there are always some sets that use a different scheme. You may be able to use the final amp on 433 MHz, but probably not at full ratings (power and gain). Overtone frequencies are always odd multiples - they are a mechanical resonance mode, not a generated harmonic. Frequency multipliers can generate either even or odd harmonics: generally in a single-ended circuit the odd harmonics are stronger (due to the causes of the distortion that generate the harmonics). A push-push multiplier generates even harmonics more efficiently.

Most commercial equipment will use doublers and triplers as the most dependable circuits (and easiest to adjust) so 2m crystals usually operate around 8, 12, 16 or 18 MHz. Note that if the transmitter uses direct FM, the transmiter will usually be designed to operate on the crystal fundamental, which is then multiplied up. This gives better FM deviation characteristics than if the crystal was used in an overtone oscillator. But this isn't important on receive, so the overtone oscillator is commonly used to reduce the number of stages needed.

terminology

LO = local oscillator

XO = Quartz crystal oscillator - When voltage is applied to a small thin piece of quartz crystal, it begins to change shape producing a characteristic known as the Piezo-electric effect so that an electric charge produces a mechanical force by changing the shape of the crystal. When the shape of the crystal changes it then produces an electrical charge.

Transducer = converts energy, such as mechanical energy into electrical and visa versa.

ICOM = Integrated Circuit Oscillator Module - The ICOM is a metal package that consists of a single quartz crystal and associated circuitry that generates a pure sine wave at the desired frequency.

Oscillator = the combined crystal and amplifier circuit. The amp is used to maintain oscillations by overcoming internal losses leading to decay.

Multiplier circuit = because the resonate freq of the crystal is typically lower than the desired LO (local oscillator) frequency, a multiplier circuit is used to raise the crystal frequency to the LO frequency.

OCXO = Oven-controlled oscillators - Crystal stability can be improved through the use of thermostatically controlled ovens.

TCXO = Temperature-controlled oscillators - rather than using an temp controlled oven, electronic compensation circuitry whose temperature characteristics are selected to be exactly opposite of those of the crystal can be used to increase stability.

Ground-to-frequency crystal =

LC tank circuit = a tuned circuit as in either a tank circuit or LC circuit.

Quartz crystal resonator and quartz crystal oscillator

The difference between a quartz crystal resonator and a quartz crystal oscillator