[Lowfer] Homebrewed SSB filter in the REAL world

[John Andrews]

Andy,

The output of the LC filter will be RF, not AF. Let's say you have a 
carrier generator at 182 kHz (just for example). If you set the computer 
program that generates the audio for a 3 kHz center frequency, your 
balanced modulator will produce signals at 179 and 185 kHz, plus or 
minus whatever the signalling is doing. An LC filter that passes stuff 
up at 185 kHz and rejects a lot of the 179 kHz signal is all you need. 
So the end product of that will be a nice 185 kHz sine wave. If you're 
going to run that through a non linear PA, square that up before driving 
the PA. I used to do that with with a 74AC14:
http://www.w1tag.com/tagtx.htm

You can certainly use op-amps to do the amplifying after the filter. I'd 
suggest an op-amp with at least a 3 MHz BW (leave the LM324's and 741's 
in the drawer). Just get the level up to something that the AC14 can 
cleanly switch on.

John, W1TAG

Andy - KU4XR wrote:
> Hello again All:
> 
> First, I want to thank everyone who responded to my question
> about homebrewing an USB Crystal Filter for LOWfer work.
> Now the real world discovery; the hombrewed crystal filter
> idea will work, however !! it's not monetarily feasible to
> accomplish. to have a crystal cut for the 150 - 500 KHz range
> will cost in excess of $ 100.00. I'll admit; I wasn't expecting
> that at all. I was thinking probably around 25 to 30 dollars,
> so that came as a shock.
> 
> John A's approach is the most feasible. The LC filter. Being 
> passive, it will possibly need a little extra amplification
> in some stages to bring up the drive levels. On the positive
> side, the LC filter can be built around my fixed Oscillator
> for the right passband.
> 
> One more question about the audio. As John D mentions, the 
> sine wave output must be noticably ampilified for a linear PA,
> or converted to an amplified square wave to drive the popular
> K0LR complimentary pair PA.
> Will the conversion ( clipping ) process introduce distortion
> products that might become a problem in the final stages of 
> the system ?? 
> Please elaborate from both the linear and non-linear outputs.
> 
> SIDEBAR !! Common sense engineering would put any audio
> amplification stages " After " the LC filter once you have
> attenuated the LSB component as much as possible, correct ??
> otherwise, wouldn't you defeat some of the attenuation that 
> the filter will do on the LSB audio ??  As John A  said; It's
> going to be so small that it really won't matter from an
> interference standpoint, but could it matter in affecting the
> 1 watt input level for the USB audio. If I'm correct, any 
> LSB component left over will " add to " the USB which will
> slightly lower your desired USB output.
> 
> 73 everybody.
> 
> Andy - KU4XR - EM75xr - Friendsville, TN.
> LOWfer Beacon " XR " @ 184.322 KHz ( QRSS-60 )
> Coordinates:  N:  35* 43' 54" - W:  84* 3' 16"
> http://www.myspace.com/beaconxr
> 
> 
>       
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