Low distortion Sin wave generator

[jewdai]

I am trying to create an FM transmitter which will eventually be used as a QPSK later.

What is the best way to create a sinusoid using basic circuit components that have low harmonic distortion?

I already know that you should use a Crystal Oscillator, but in colpitt configuration has a lot of distortion.

 

[MikeMl]

Are you talking about the carrier frequency (100Mhz) or a modulating frequency (audio, few Khz)?

 

[jewdai]

Carrier Frequency

 

[Sceadwian]

PLL stabilized Wien Bridge would be my first recommendation. Wien Bridges produce very low harmonics, and making one of it's parameters variable to allow for feedback from the PLL of a crystal controlled clock will provide rock solid stability.

Wien bridge oscillator - Wikipedia, the free encyclopedia

 

[MikeMl]

For the carrier freq, nobody worries about the spectral purity of the oscillator. The "distortion" in the carrier manifests as harmonics at 2X,3X,4X,... of the carrier, which is usually taken care of with tuned LC circuits between the oscillator and the antenna...

 

[jewdai]

is there anyway I can do this with lower level components, transistors? (say if i want a higher carrier frequency, remember the GBP for opamps is not a myth)

how would I do the circuit analysis for a circuit like that, it uses non-linear elements.

 

[MikeMl]

What frequency are you generating?

 

[RCinFLA]

You may be focusing on wrong parameter. For QPSK, (and higher order), close in phase noise performance is of most import.

For example, TCXO's for GPS receivers (BPSK) have sideband noise specs of:

-45 dBC/Hz at 1 Hz
-75 dBC/Hz at 10 Hz
-110 dBC/Hz at 100 Hz
-135 dbC/Hz at 1000 Hz
-145 dBC/Hz at 10 kHz
-150 dBC/Hz at 100 kHz

This is for their fundamental mode TCXO at about 14 to 20 MHz, used for synthesizer reference. To be fair, GPS needs exceptionally good close in noise performance to maintain correlation lock on a very weak CDMA signal. (-145 dbm to -155 dbm at receiver input). The GPS TCXO also has 0.5 ppm frequency accuracy over operational temperature range which is necessary to speed initial time to first fix performance.

High phase noise will result in the phase modulation constellation to be 'fuzzed up' with jitter which will degrade low signal level recovery performance, whether the jitter is created in the transmitter or receiver side.

The parameter of 'fuzz balls' in a nPSK constellation signal is called Error Vector Magnitude (EVM) and is specified as % peak and r.m.s. error over some period of time.

Agilent and Rhode and Schwartz test equipment supplier's web sites have some good application notes on this topic.

 

[jewdai]

my goal is to build this out of the lowest possible level of circuit components, i.e., transistors.

 

[MikeMl]

I say again, what frequency are you generating?

 

[jewdai]

it doesn't matter, I want to be able to configure it myself based on the system equation. Aim for more in the FM range if you must.

 

[mneary]

You already wrote FM or QPSK, what frequency?

 

[jewdai]

it doesnt matter. I want to be able to be able to manipulate the frequency based on the equations that describe the frequency of the system. I want to deal with it in terms of variables.

But if you must aim for in the 88-105 MHZ range, i.e., FM.

 

[xpi0t0s]

It does matter because different components have different frequency characteristics. So a design that works at 1kHz might not work at 1GHz, and possibly vice versa.

 

[MikeMl]

It does matter because different components have different frequency characteristics. So a design that works at 1kHz might not work at 1GHz, and possibly vice versa.

Hell, a design that works at 1MHz will not work at 10Mhz, one for 10Mhz will not work at 100Mhz, and so on.

 

[xpi0t0s]

Hell, a design that works at 1MHz will not work at 10Mhz, one for 10Mhz will not work at 100Mhz, and so on.

yeah, well I'm a bit of a beginner too in some respects which is why I picked the extremes of 1kHz and 1GHz and hedged my bets further by using "might"...

 

[jewdai]

duh.

usually the issue is being able to use higher frequencies and not worry about stray capacitance or inductance bringing the carrier frequency down. That is why I want to work with the lowest possible circuit elements so that these worries are minimized as much as possible.