[Hallicrafters] SSB generators: filter vs phasing

[Bill]

I will not speak to what the HT do, but as to filter v phasing
principles:

When Audio mixed in a balanced mixer with an IF signal the output
  is IF - Audio (LSB) and IF + Audio (USB). 
  LSB is selected by placing the filter on the low side of the IF
  USB is selected by placing the filter on the high side of the IF
	General advantages:
	- 1 mixer
	- simpler: can be 1 filter + 2 IF crystals (Lsb, Usb) or 2
filter + 1 IF crystal
	- mechanical or crystal filters make it very practical. 
		Much harder with LC filters to get the necessary
selectivity at IF frequencies
	Con: People don't generally account for the phase linearity of
the filter, they 
		look at filter amplitude ripple but not phase. I suspect
that the complaints 
		about poor audio quality are related to the phase
response of the filter	
		as well as the "narrowness" of it.  For data com's
(rtty) phase is very important.

 Phasing Method.
	Actually quite similar to above, the basic idea is to combine
two 2 mixing systems 
		to cancel out either the LSB or the USB signal.

	By 
		feeding the 1st mixer with Audio and the IF carrier with
a +45 deg phase shift and
 		feeding the 2nd mixer with Audio and the IF carrier with
a -45 deg phase shift
	
	we can generate two mixing systems where the outputs are
effectively:
	1st mixer output =  LSB + IF and IF + USB
	2nd mixer output = -LSB + IF and IF + USB

		(mathematically, its all in the trig (sine/cosine)
identities)

	USB is selected by summing the mixer outputs
	LSB is selected by subtracting the mixer outputs.

	 (if I got the signs wrong, the just flip the
additions/subtraction).

	The signal is bandlimited by Audio filtering with combination of

		high pass (say >300 Hz) and low pass (say < 3300 Hz)

	Cons:
		more complex than basic filtering scheme
		relies on signal cancellation hence amplitudes and
phases need to be matched
			the better the matching the better the
cancellation.
	Pro: 
		potentially better audio response, however it really
depends on the audio filters
		doesn't require sharp IF filters (and expensive?), which
were a technology limitation
		 in the early days.

	
	

bill, ve3jhu


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of
Zengmeiste at aol.com
Sent: Thursday, August 03, 2006 11:40 PM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] SSB generators: filter vs phasing


OK, I'll bite. I'm gonna guess that the operational difference here is
How the sideband is obtained, straight hi-pass / lo-pass filtering
versus negative feedback to exclude one of the sidebands? (I'm guessing,
have not actually played with an xmtr. yet, and 
am still working on theory comprehension)

Thanks & 73, Terry B

ps
 did you notice how well QST mags from the 30's hold up?
they printed it on an incredibly high grade of paper stock. wow! $125
for a Super SkyRider. Kewl! What was that, 4 months pay in 1938?
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