| |
GM7HUD > SIX 22.12.05 01:29l 71 Lines 4285 Bytes #999 (0) @ WW
BID : 813167GM7HUD
Read: GUEST
Subj: CB to 6m conversion 3
Path: DB0FHN<DB0FOR<DB0MRW<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0RES<ON0AR<GB7YKS<
GB7LGS<GB7ESX
Sent: 051221/2224z 71792@GB7ESX.#31.GBR.EU $:813167GM7HUD [Witham, Esx]NNA V3.1
*CB TO SIX METRES* Part3 - The PLL Frequency Synthesizer continued
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*
In part2 , we had a brief look at the basic operation of the PLL frequency
synthesizer (If you want a more detailed description , do drop me a line)
and from that we can see how the channel frequency is generated . If now ,
for example , we change the binary input to the PLL chip to something else
, say 234 , it would result in the phase detector generating an "error" ,
which will drive the VCO to the new frequency so that the loop will once
again be in the locked state . As the VCO will now have moved 10kc , the
resulting sum output will also have incremented by te same amount ,
resulting in the transceiver effectively having changed frequency by 10kc .
This scheme is fine for changeing frequency round the design operating
frequency of the radio , but to change "band" to a frequency more than 1MHz
up , one has to watch out not to exceed the 3.5MHz maximum frequency input
to the divide-by-N counter . It should be clear that the synthesizer
frequency can be moved higher by changeing the 10.0525 Xtal to a new value.
There should be no need to rush out and buy one , just have a look in your
"junkbox" for a Xtal with a fundamental frequency round 10.400 . After you
have done the replacement the alignment is easy . Attach an RF voltmeter /
'scope / RF "sniffer" to test point3 (TP3) and , using the propper (!)
trimming tool , adjust the core of T3 for max deflection . Then , connect a
DC voltmeter to pin6 of the PLL02A chip (take care not to short any of the
pins) and carefully adjust the core of the VCO "block" until you get a
solid 5V on that pin . Flick the channel switch from CH1-CH40 while
checking the voltage on pin6 of the PLL ic and confirm that it stays at 5V.
The PLL is now locked . Connect the RF Voltmeter/'scope/sniffer to pin4 of
IC3 (C3001/TA7310P) and adjust T1 and T2 for maximum deflection on
Channel20 . Remember that the rig will not transmit if the PLL is not
locked . When pin6 of the PLL chip falls below 5V , it "switches off" Q7 in
the transmitter predriver stage , rendering the transmitter inoperative .
Set the rig in the transmit mode and realign the set as per the manual for
maximum output . Rig should now be transmitting somewhere around 28.600 and
the exact frequency can be checked with a frequency counter .(Please
remember to have the rig in the AM mode for measuring the frequency -
sounds obvious , but easy to forget in the heat of the moment)
You will now most probably be on a somewhat "wierd" frequency like 28.7842
or suchlike . (At no time touch the 10.692MHz oscillator trimming caps ,
CT4 and CT5 - I will explain their function later . They should not be
touched at all if your rig was operating 100 percent on 27Mc) . The easiest
way to go about the following procedure , is to have an HF reciever on hand
with SSB fascilities . In the previous example , say 28.7842 , tune your
reciever to either 28.780 or 28.790(prefered). Key up the rig again (in
AM mode) and adjust CT1 for a zero beat on your reciever (Which should be
set to USB) The rig should be keyed into a "Dummy load".
Now set your reciever to LSB and do likewise on your rig and while
modulating , adjust CT2 for a natural sounding voice on your reciever . If
necessary, experiment with the values of C20 and C21 to get the rig spot on
frequency . You should get no less than 4W on AM and 12W on SSB after the
mod , and if you want , you can realign the reciever strip now as well
using an adjustable signal generator and use the set as a 10M rig , or get
ready to continue the mod up to six metres . Good luck .
CU in Part4
73 de Andy GM7HUD
Read previous mail | Read next mail
| |
