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Replacing LC with a crystal

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zeller

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What do you mean by rx modulation plan? I want to put a 27MHZ crystal because the transmitter is also 27 MHZ.The receiving end is sound.

---------- Post added at 14:31 ---------- Previous post was at 14:26 ----------

The reason I want to use a crystal rather than LC is because it is hard to obtain exact frequency on LC i.e need varicap, on top of the fact that the transmitter is an exactly 27MHZ.
 

buckaroo

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Hi zeller,
There is always a loading cap requirement for crystal, if the loading conditions is different from that specified by the datasheet, there will be frequency offset, in your case, several hundreds ppm(or even larger) frequency offset will be expected. If you can tolerate the frequency offset, I dont think that replace the LC tank with crystal will be a problem.

Rds
Buckaroo
 

zeller

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RDS Buckaroo,

I dont get what you say, would i need a varicap together with crystal for it to work? is i need a loading cap should this be in parrallel with the crystal and what value should it be?
 

chuckey

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Crystals are a very high Q series resonant circuit, i.e. they posses an ESR of 10 ohms at their resonant frequency. If you shunt them by a small capacitor 50pF or so, they have a very high impedance, like a parallel tuned circuit, with a Q of 5,000, but you lower their frequency by several hundred hertz. So straight away you can see that the bandwidth of the circuit will be 27MHz/5000 ~ 5KHz, but to achieve this you must very loosely couple your aerial into the circuit, say with a 10pF capacitor. You have to make sure that both the RX and TX crystals are for the same frequency.
Frank
 

FvM

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Strictly spoken you can't simply "replace the LC part" by a crystal:
- the transistor would miss the DC bias
- you don't have a means to couple the aerial with suitable impedance matching.
Furthermore, each LC circuit has a characteristic impedance and Q according to it's purpose. It's not clear, if the crystal will meet this requirements.
 

zeller

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Crystals are a very high Q series resonant circuit, i.e. they posses an ESR of 10 ohms at their resonant frequency. If you shunt them by a small capacitor 50pF or so, they have a very high impedance, like a parallel tuned circuit, with a Q of 5,000, but you lower their frequency by several hundred hertz. So straight away you can see that the bandwidth of the circuit will be 27MHz/5000 ~ 5KHz, but to achieve this you must very loosely couple your aerial into the circuit, say with a 10pF capacitor. You have to make sure that both the RX and TX crystals are for the same frequency.
Frank
I just want the receiver to received a fixed frequency of 27MHZ do i still need a paralle capacitor?

---------- Post added at 22:35 ---------- Previous post was at 22:23 ----------

Strictly spoken you can't simply "replace the LC part" by a crystal:
- the transistor would miss the DC bias
- you don't have a means to couple the aerial with suitable impedance matching.
Furthermore, each LC circuit has a characteristic impedance and Q according to it's purpose. It's not clear, if the crystal will meet this requirements.
okay so its quite complicated using a crystal, will stick to LC. Thanks,
 

FvM

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I think, it's easy to tune the LC circuit to the transmitter. You would in fact need a varicap or variable inductor, but you get a resonable bandwidth for a simple receiver. A crystal would tend to have an unsuitable small bandwith for audio transmission.
 

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