Re: LF: MSK with a Class D transmitter...?

[Andy Talbot <andy.g4jnt@gmail.com>]

I was half asleep last night when I first replied - hence the confusion over upconversion.  As you're on 9kHz, converting doesn't apply and the PC generates direct Tx drive.  The quadrature bit is not needed for direct output, so you only use one audio channel.

 

As mentioned by several writers  now, you need to double the frequency first to pass into your divide by 2, or bypass that and drive the transmitter directly - after making sure the waveform is very close to a 50% duty cycle square wave.   The drive will almost certainly have to be push-pull, so you'll need to make a squarer / comparator and generate Q/Q-bar drive signals

 

Andy  (doesn't do late-at-night :-)  G4JNT   

 

On 21 April 2011 01:28, Scott Tilley <sthed475@telus.net> wrote:

Hi Andy

Holy mind bogglingness batman...

Here's the parameters to make this real simple:
- I want to generate a MSK signal out of the transmitter at 8970Hz.
- I'm using Spectrum Lab, which allows me to create a MSK drive and at a baud rate I want at any frequency my sound card can generate.
- SL allows me to output quadrature outputs.

So, If I understand correctly pump said FET driver with the quadrature outputs from the sound card audio (squaring up the sine waves of course) and it should work?

Thanks,
Scott

On 4/20/2011 2:09 PM, Andy Talbot wrote:

Bin too quick to type..

NOt thinking straight...

 

You need to the upconvert either the resulting doubled audio tone by twice the RF carrier, or upconvert first and double the final RF.  Same ideas for frequency doubling can be used.

 

See http://www.g4jnt.com/LFUpconv.pdf  for a direct from audio upconverter.

 

Must fully check before hitting SEND !

 

Andy

G4JNT

On 20 April 2011 21:50, Andy Talbot <andy.g4jnt@gmail.com> wrote:

I assume you are genrating the MSK from soundcard output, in which case two options come to mind:

 

1)  Double the frequency.  Full wave rectify the audio sinusoid, pass the AC part of the result into a comparator and into the divider.  Alternatively, as the signal bandwidth is probably quite low, square up the audio to start with, feed into one inpiut of an XOR gate.  Delay the waveform by 90 deg and feed this into the othe side of the XOR.    Output of XOR to divider as before

 

2)   Remove the  divide by two, and replace with an inverter / buffer pair to generate the out of phase drive signals.   The  input now needs to be close to a 1:1 squarwave at output frequency.   Pass the soundcard audio into a comparator and make sure it gives a 50% duty cycles.

 

 

Andy

www.g4jnt.com

 

ps.

or 3)    Convince the software author the S/W needs modifying to give a 2.F output

 

 

On 20 April 2011 21:32, Scott Tilley <sthed475@telus.net> wrote:

Hi All

I wonder if someone can explain to me how to feed an MSK signal into a typical Class D transmitter?

I mean by typical that the TX has a 4013 type /2 system feeding two FET drivers to drive FETs in a push-pull configuration.

If I understand correctly due to the fact MSK does not have amplitude component to its signal that a non-linear transmitter can be used.

So, how do you do this with a Class-D TX as often used in LF/VLF?

Thanks,
Scott
VE7TIL