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GM7HUD > SIX 22.12.05 01:22l 74 Lines 4163 Bytes #999 (0) @ WW
BID : 8C3169GM7HUD
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Subj: CB to 6m conversion 5
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<DK0WUE<I0TVL<CX2SA<GB7YFS<GB7LGS<
GB7ESX
Sent: 051221/2224z 71794@GB7ESX.#31.GBR.EU $:8C3169GM7HUD [Witham, Esx]NNA V3.1
*CB TO SIX METRES* Part5 - Other mods
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above address.*
It is not my intention to provide lengthy and detailed explinations on the
intricacies of SSB or bore those who know this subject well , but to point
out some of the basics with particular reference to this particular
chassis we are concerned with , and hopefully arm you with the neccissary
basic understanding of what we are doing . This will be invaluable when
fault finding needs to be undertaken at any stage .
I mentioned before that "USB and LSB have swapped around and dont seem to
be on frequency " . To understand what has happened here we have to have a
brief look at how SSB gets generated here . The "classic" method of
generating SSB involves a carrier generator spot on the IF frequency
feeding a balanced modulator of sorts . The resulting DSB signal is fed to
switched crystal filters : one for USB , one for LSB , possibly one for CW
and one for AM . As you can deduce from this , the manufacturing cost of
this approach would result in quite high prices on the finished product
being passed on to the consumer . This is probably one of the reasons that
most dedicated Amateur Transcievers have the CW filter fitted as an option,
rather than a standard fixture , in an attempt to keep the final cost to
the consumer low and by implication competitive .
The designers of this (and other rigs) have cleverly sidestepped these
problems with some ingeniuety . How AM is generated should be quite obvious
so we will not dwell on that now but rather just focus on SSB . Inspect the
innards of you rig . You will find only one large crystal filter located
roughly in the middle of the PCB . This filter is about 3-2.7 kc wide and
is a "LSB" filter centered round 10.6935 . (Some rigs are fitted with a
smaller rectangular filter , taking up about half the space . These are
"metal-oxide" filters , but for all intents and purposes can be treated the
same).
Suppose our carrier generator is set to 10.695 (standard 1st IF
frequency) . The 10.695 signal will pass through a balanced modulator chip,
IC4 (AN612) , where it is modulated by the audio signal in such a way that
the original 10.695 is balanced out and you are left with a double sideband
supressed carrier signal centered around 10.695 . This signal is now fed
into the crystal filter where the USB signal is heavily attenuated and the
LSB part of the signal is only slightly attenuated (filter insertion loss).
We are now left with a LSB SSB signal centred on 10.695 .
What if we want an USB signal ? The answer is to lower the carrier
frequency to below the filter passband : 10.692 is about right . The DSB
signal will now be generated centered around 10.692 , and after passing
through the Xtal filter , will only have the USB components remaining .
Note though that the USB signal will be centered around 10.692 and the LSB
and AM signals will be centered around 10.695 . Voila ! we generated both
USB and LSB with the same filter !
Now you can see that if the frequency from the synthesizer stays constant
between "mode" switching , The LSB and AM signals will appear on the band
"on freqency" , while the USB will be 3kc "off" . The designers got round
this "problem" by shifting the synthesizer output frequency by 3kc when the
USB mode is selected , thereby making all of this transparrent to the final
user , and all the "channels" RIT exactly at "12'o'clock irrespective of
the mode you have selected .
More to follow on this in Part6
CU in Part6
vy 73 de Shawn ZR1EV JF96ic
73 de Andy GM7HUD
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