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Square to sine

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rocket100

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So, I have a 200 mhz square wave that I would like to convert into a sine wave so it is suitable for RF transmission. I would like it to be a sine because I know that a square would create harmonics, causing unwanted transmissions in other bands. Is there some sort of filter that I need to build? Would it be an RC low pass filter?

Thanks!
 
So, I have a 200 mhz square wave that I would like to convert into a sine wave so it is suitable for RF transmission. I would like it to be a sine because I know that a square would create harmonics, causing unwanted transmissions in other bands. Is there some sort of filter that I need to build? Would it be an RC low pass filter?

Have you tried working out the values for a 200MHz RC filter :D

RC wouldn't be possible, LC filters would be needed - but these would probably be part of the transmitter anyway, you really need to provide FAR more detail of what you're trying to do, and why?.
 
Have you tried working out the values for a 200MHz RC filter :D

RC wouldn't be possible, LC filters would be needed - but these would probably be part of the transmitter anyway, you really need to provide FAR more detail of what you're trying to do, and why?.

Sorry, I am building a high altitude balloon transmitter. Basically I have a crystal oscillator that will be fed into a PLL to output roughly a 160 mhz square wave. This will be ON/OFF Keyed to send binary temperature information. Once I figure out how to convert it to a nice sine wave, I plan on feeding it info a JFET buffer to amplify the RF to about 300mw. It will be received on the ground by an SDR.
 
Wouldn't AM or FM modulation of a 160MHz sinusoidal carrier be easier (fewer harmonics) than on/off keying?
 
Wouldn't AM or FM modulation of a 160MHz sinusoidal carrier be easier (fewer harmonics) than on/off keying?

I guess, maybe I'll do ASK instead. FM would be a little too complicated.

I've decided to use a Chebyshev Pi LC low pass filter to get rid of those pesky harmonics from the square wave. There is only one thing that I need help understanding: what is Ripple and what should it be set to for RF?
 
How did you license it? Have checked into the legalities of what you are doing?
 
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Methinks transmission from a balloon is going to be very obvious and will draw the attention of the authorities. I hope that the frequency you're using is legal in your country! Any interference with aviation or emergency-services frequencies could have unfortunate consequences.
 
Feed the 160MHz squarewave into a 160MHz LC resonant circuit. Sorted.

Yup! I've decided to use this circuit: **broken link removed**

Methinks transmission from a balloon is going to be very obvious and will draw the attention of the authorities. I hope that the frequency you're using is legal in your country! Any interference with aviation or emergency-services frequencies could have unfortunate consequences.

I have made the decision to bump it up to 434 mhz so I can use the ISM band. Just like this transmitter: https://www.sparkfun.com/products/10534
 
The reason I asked is because I am part of ANSR. I use my airplanes to assist in the recovery of the balloon payloads after they come down under parachute using APRS (our last flight) or airborne transmitter hunting. Our last few flights have gone to over 100,000 ft.

ANSR is staffed with hams, so we have been (legally) using the 2meter and 440MHz bands for downlink telemetry, video, and recovery beacons. A 1 Watt VHF transmitter at 100,000ft over Arizona can be received from California to Kansas.

I would encourage you to get to know some hams in your area, and to get a ham license of your own. Write back if I can assist...
 
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The reason I asked is because I am part of ANSR. I use my airplanes to assist in the recovery of the balloon payloads after they come down under parachute using APRS (our last flight) or airborne transmitter hunting. Our last few flights have gone to over 100,000 ft.

ANSR is staffed with hams, so we have been (legally) using the 2meter and 440MHz bands for downlink telemetry, video, and recovery beacons. A 1 Watt VHF transmitter at 100,000ft over Arizona can be received from California to Kansas.

I would encourage you to get to know some hams in your area, and to get a ham license of your own. Write back if I can assist...

Wow! That is so cool! It is absolutely amazing the distance that can be covered by low power VHF transmitters! I would love to get a license of my own. I know of a few testing places in my area, but I'm not sure where to start in preparation for the test.
One quick question: How far up do radio laws apply? I mean, it's not like the moon has laws about transmitters. I've always been curious about this.

Thanks!
 
Regulation of the radio spectrum is managed terrestrially, and includes near space out to where geosynchronous satellites fly (~23000 mi out).

The 432MHz ISM band is very limited in Power Output, and happens to be in the 420MHz to 450MHz amateur band. Increasing the power output of a "legal" module will put you in conflict with the FCC and your local hams.

Where are you? Maybe I could find you a local ham contact that can help you get a license and/or help with equipment for a balloon flight.
 
rocket100 said:
One quick question: How far up do radio laws apply?
The so-called "radio laws" apply to those people/entities actually in control of transmitting the RF, not the RF itself or the transmitting device.

If your transmitter were on the Moon (and you were in the US, say) and transmitting an RF signal back at the earth, you would be subject to the various US laws concerning that transmission as received on Earth.

If you and your transmitter were on the Moon, however, your transmissions would be subject to no laws as there are no laws, as such, on the Moon.

Although some legal eagle government type might argue that, well, "you're still a US (or whatever country) citizen" and therefore subject to US laws.

So, while true enough, might be tough to prosecute... :p
 
Originally Posted by DaveS
Feed the 160MHz squarewave into a 160MHz LC resonant circuit. Sorted.
Originally Posted by rocket100
Yup! I've decided to use this circuit: **broken link removed**
Sorry but I must contradict these statements.
A simple LC resonant circuit will not convert a square wave into a reasonably clean radio signal.
Something like a 7 pole lowpass filter will get you into the right area.

The circuits in your link also are rather simplified compared to a reasonably engineered circuit.

I have made the decision to bump it up to 434 mhz so I can use the ISM band. Just like this transmitter: https://www.sparkfun.com/products/10534 .
A wise choice.
I have just done a quick calculation which shows that the 10mW or so from that transmitter would be easily received over a 30km line of sight path, (ie 100000ft up in space), if you had a good receiver on the ground with a directional antenna with about 10dB gain.
By good receiver, I mean something with 0.1μV sensitivity, the antenna could be something like a 12 element yagi.

JimB
 
Sorry but I must contradict these statements.
A simple LC resonant circuit will not convert a square wave into a reasonably clean radio signal.
Something like a 7 pole lowpass filter will get you into the right area.

The circuits in your link also are rather simplified compared to a reasonably engineered circuit.

JimB

Right, but it would work great for just multiplying the frequency. Then after you have the ugly, but high frequency signal, you could buffer and filter it into a nice sine wave. Right?
 
Then after you have the ugly, but high frequency signal, you could buffer and filter it into a nice sine wave. Right?
Hmmm .... yes, sort of.
In a real world radio transmitter, there are several ways of producing the final output frequency.
The aim of the designer of the transmitter is to minimise outputs on unwanted frequencies. Most often these outputs will be on frequencies which are harmonically related to the required carrier frequency.
To remove the harmonics (and any other unwanted output which is above the carrier frequency), a low pass filter (LPF) is used.
Depending on the amount of harmonic suppression required, the LPF may have many sections of inductors and capacitors often configured in a "pi" arrangement.

Have a look at the attachment which shows such a filter. (Highlighted in red).
This type of filter is usually referred to as an elliptical filter.

JimB
 

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  • MX294 LPF.JPG
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Hmmm .... yes, sort of.
In a real world radio transmitter, there are several ways of producing the final output frequency.
The aim of the designer of the transmitter is to minimise outputs on unwanted frequencies. Most often these outputs will be on frequencies which are harmonically related to the required carrier frequency.
To remove the harmonics (and any other unwanted output which is above the carrier frequency), a low pass filter (LPF) is used.
Depending on the amount of harmonic suppression required, the LPF may have many sections of inductors and capacitors often configured in a "pi" arrangement.

Have a look at the attachment which shows such a filter. (Highlighted in red).
This type of filter is usually referred to as an elliptical filter.

JimB

I do not see the speaker output ???
 
Why would you expect a speaker? It's an RF transmitter under discussion!
 
LOL!

Also, I found another component that make this much easier: **broken link removed**
It would still need some filtering, but it would be a better start.
 
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