LC7137 based CB to 10M FM conversion ==================================== RIGS USING LC7137 ~~~~~~~~~~~~~~~~~ Rotel 220, 230, 240; York 861, 863; Fidelity 2000, 2001(with low power switch at rear) Amstrad 900,901; Binatone 5-star; Cobra 21XFM; Comtron CB40F; Cybernet Beta 1000, 2000; Maxcom 4E; Midland 3001; SMC Oscar-1; Transcom GBX4000; Plus others! Most of the popular transcievers are based on the LC7137 PLL chip, specifically designed for the UK FM CB allocation. The LC7137 is a 20 pin PLL chip which generates 40 channels from a single 10.240MHz Xtal. This xtal frequency is divided by 2048 to give 5kHz steps. The VCO is injected at pin 19 and divided by a number which is determined by the channel switch and the internal ROM and is then compared with the 5kHz reference. The channel switch generates BCD (Binary Coded Decimal) codes ie. the TENS digits and the UNITS digits are represented by four bits. Two of the tens digits, however, are always zero, so a total of SIX BITS only are required. In BCD, the decimal digits 0,1,2,3,4 --9 are represented by binary codes 0000=0 0001=1 0010=2 0011=3 0100=4 0101=5 0110=6 0111=7 1000=8 1001=9 The combinations from 10 to 15 are not used and if attempted the ROM chip detects these codes are produces a mis-program code fixing the PLL to channel 9. Here are the full 40 channel codes and division ratios Dial BCD TX DIV RX DIV CB FREQ 10M FREQ NO. CODE OF VCO OF VCO MHz MHz 1 000001 2760 3381 27.60125 29.31 2 000010 2761 3383 27.61125 29.32 3 000011 2762 3385 27.62125 29.33 4 000100 2763 3387 27.63125 29.34 5 000101 2764 3389 27.64125 29.35 6 000110 2765 3391 27.65125 29.36 7 000111 2766 3393 27.66125 29.37 8 001000 2767 3395 27.67125 29.38 9 001001 2768 3397 27.68125 29.39 10 010000 2769 3399 27.69125 29.40 11 010001 2770 3401 27.70125 29.41 12 010010 2771 3403 27.71125 29.42 13 010011 2772 3405 27.72125 29.43 14 010100 2773 3407 27.73125 29.44 15 010101 2774 3409 27.74125 29.45 16 010110 2775 3411 27.75125 29.46 17 010111 2776 3413 27.76125 29.47 18 011000 2777 3415 27.77125 29.48 19 011001 2778 3417 27.78125 29.49 20 100000 2779 3419 27.79125 29.50 21 100001 2780 3421 27.80125 29.51 22 100010 2781 3423 27.81125 29.52 23 100011 2782 3425 27.82125 29.53 24 100100 2783 3427 27.83125 29.54 25 100101 2784 3429 27.84125 29.55 26 100110 2785 3431 27.85125 29.56 27 100111 2786 3433 27.86125 29.57 28 101000 2787 3435 27.87125 29.58 29 101001 2788 3437 27.88125 29.59 30 110000 2789 3439 27.89125 29.60 31 110001 2790 3441 27.90125 29.61 32 110010 2791 3443 27.91125 29.62 33 110011 2792 3445 27.92125 29.63 34 110100 2793 3447 27.93125 29.64 35 110101 2794 3449 27.94125 29.65 36 110110 2795 3451 27.95125 29.66 37 110111 2796 3453 27.96125 29.67 38 111000 2797 3455 27.97125 29.68 39 111001 2798 3457 27.98125 29.69 40 * 000000 * 2799 3459 27.99125 29.70 Here is a block diagram of a tranceiver synthesiser circuit. Frequencies shown are for channel 30 selected on the channel selector. 旼컴컴컴컴컴컴컴컴컴컴컴� � LC7137 PLL chip � 넬엿 10.24046285MHz � � 납납 reference xtal oscillator � � 2048 쳐컴컫컴눴냉컴컴 � � � 납납 � � TX RX � � 냅冒 \ � / � �2789 �3439 � \ � / 읕컴컴컴컴컫컴컴컴컴컴컴� � \�/ � � Antenna � � � � � � � 旼컴컴컴커 旼컴컴컴� � 旼컴컴컴컴컨컴컴컴컴컴컴� � � � � � � V.C.O. � � � (tx) � � � � � 13.945630 (tx) 쳐컴컴컴컴캑 쳐컴캑 P.A. 쳐컴� 27.89125 � � � � x 2 � � � � 17.195777 (rx) � � � � � � 읕컴컴컴컴컫컴컴컴컴컴컴� 읕컴컴컴켸 읕컴컴컴� � � � � � � 旼컴컴컴컴컴� \ � / 旼컴컴컴컴컨컴컴컴컴컴컴� � RX � \�/ � � � � second � � � RX first mixer 쳐컴컨컴컴캑 mixer 쳐컴�> � � � � � 0.455010 � � � � � second I.F. � 읕컴컴컴컴컫컴컴컴컴컴컴� 읕컴컴컴컴컴� � � � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The block diagram above shows a very simple conversion process. On transmit, the VCO frequency (13.945630MHz) is doubled then amplified by the P.A for output on 27.89125MHz. On receive, the incoming signal (27.89125MHz) is mixed with the VCO to produce the first IF of 10.695MHz. This is then mixed with the 10.240MHz reference xtal oscillator to produce the second IF of 455kHz. The reference oscillator is actually offset to 10.24046285MHz in order to generate the TX frequencies almost exactly with a minimum error of 15Hz. On receive, however, there is an error but because of the width of the filters is of very little consequence. Because of the ROM within the LC7137 chip, it is impossible to change the binary coding from the rotary channel switch, so we are stuck with the 40 channel selections. If we tried to alter the reference oscillator from 10.240 to say 10.860 to get to 29MHz, the tx frequencies would have a channel spacing of 10.86kHz which would throw the frequencies further off when going from channel 1 to channel 40. But the receiver would be offset by too much, and it would give a second IF of 165kHz instead of 455kHz. DOWN MIXING ~~~~~~~~~~~ The solution adopted must be attributed to G3XSE, which is to raise the VCO frequency, but use down mixing to fool the PLL that it is still on the correct freqs. So, for channel 30 the VCO would be on 14.800000MHz for TX and 18.903795MHz for RX instead of 13.945630 TX and 17.195777 RX. If we calculate the difference, we get 0.85437 for TX and 1.708018 for RX If we take 10.24046285 and divide it by 6 we get 1.706743808 which is very close to our figure for RX and dividing 10.24046285 by 12 we get 0.85337 which again is close enough, especially if we pull the 10MHz xtal slightly, to say 10.2411530 which is virtually perfect for our needs! Divider and Mixer circuit ~~~~~~~~~~~~~~~~~~~~~~~~~ L1 is 23 turns on 3/16 former +12V rail +12V rail 컴컴컫컴컴컴컴컴컴컴컴컴컴컴컴컴컫컴컴컴쩡컫컴컴컴컴컴컴컴컴컴컴컴컴컫컴컴� 쳐컴컴� out 旼컴컴컴퓁n� � � 旼컴� � 1 � � 1 旼캑 쳐캑 憫� � 컨� � 憫�2 旼컨컴옭캐컴 N � � 7805 � 컨�1.2k� � � 1000pF 컫� � � �0 쿝ELAY� / \ 4 � � REG � 컫� 읓� � 컴컨컴 � 읓楙 읕컫컴� � 0 � 읕컴쩡컴� � � 읕컴컴컴컴컴컴커////// 읕캑 쳐컴컴� 0 쳐캑쳐컵컴컴컴�0.1�F � 쳐컴� 旼컨커 컴좔� �.1 컴좔� � 憫�1 � ( 컨� \ / 녔F //// � 10K � �2憫� (L1 컫� \ / � gnd � 읓�0� � 읕컫� 컴냐� � � � 遠쬔 �15pF � 1N4002 � 쳐컴컴컫컴컫컴커 � 읕컴컴엿 � � � 憫�5 � 컨� 읕컴컴컴� 납 \ / � � � �6 � //// ___ � 냅컴� to 0 Volts � � 읓�0 � 渼___첼 � � � on tx � 旼컴쩡컴컴컴� � � � � �120� � � 읕� � 5� �8 �14 쿝LA1 � 넬컴좔컴컴좔컴컴좔컨컴엿 � 旼좔컨컴컨컴� o rx � 33pF 납 8 2 3 5 납 � � IC1 � o컴컴o�)컴늘컨� IC2 6쳐)탠� 0.01�F � 74LS92 �12 � tx � 33pF � 납 � � >컴컴컴>컴늘컫컴캑 쳐컴켸 旼컴�)캑쳐쩡�1 MC1496P 납 � � 10.240 � � 6 10 � 旼컴� 憫� 5憫엿 12쵬 � � from pin 11 � 읕컫컴컴컫컴� � � � 6� 납 4 14 � � � of LC7137 憫�\ � � � 820� � � 0읓牡컴컫컴컴컴쩡컴컴컴컴� � � � �/\ � � � 읓� � 읕컴컴� 퀃rimmer旼컴켸 � � / � � � 쳐컴컴컴컴컴� � 컨� � �/� � � � 憫� � � 컫�50pF � /쬔1.2K � � � � � � � � � / 쿾reset� � � 1K � � 컨� � o o tx � � � � � 읓� 1000 pF컫� � rx / � � � � � � � � / RLA2 � 컴컫컴컴컴컴컨컴컴컴좔컴컴좔컴컴�)컴컴컴좔컴컴컴컴컴좔컴컴컨컴컴좔컴컴컴컴�)� � � � 컨� to VCO <컴켸 � //// gnd to pin 19 <컴컴� of LC7137 SORRY THE DIAGRAM IS SO SQUASHED UP, BUT HAD TO MAKE IT FIT IN! N.B. Use screened cable from pin 11 to circuit and from pin 19 to circuit. Relay is a DPDT ultra miniature Maplin NO.YX95D. Unfortunately it is not possible to draw an accurate sized PCB track layout, but I will attempt to draw one BIG one and it will give you the general layout of the board. Again, I claim no originality for these circuits and give credit to G4UTB for his layout. I will use the letter 'o' to represent a pad, and where it is impossible to join two pads vertically, I will us Capital o 'O' to show that they are joined together. Two small pads like 'oo' must be taken as joined together. Relay 74LS92 *** = coil MC1496P of relay o컴컴쩡컴컴컴컫컴컴컴컴컴컫컴컴컴컴컴컴컴컴컫컫컴컴컴컴컴컫컫컴컴컴컴컴쩡횽 � � � o컴컴컴컴컴컴� � � O O O � � o � o훟 O O � O � 1o컴컴oo o컴컴컴컴컴컴o � 쳐훟 o컴� o 읕컴컴컴컴컴컴커 � o � o훟 � � o� 쳐o O O � 읕o � o훟 � o캑 o� � o 旼컴컴컴O O � o O o컴컴o컴훟oo � � o� � o커 O�***o컴o훟 o� 旼o o컴컴컴훟 O � O � O o� � o 읕훟� �o 旼컴� o 旼o � O � O컴훟� � o컫컴o� 쿽 o컴컴� o컴o� � o O � � 1� � O � � o혃 o혃 O � oo컴컴컴컴햛 � o캑 o훟� � 旼켸 � o흌 o흌컴켸 o컴컴컴컴컴컴컴컴o컴컴� � O훟 � oo 픬o o햛 o흌 o컴컴컴컴컴컴컴컴컴컴훟 � � o � 읕컴컴컴컴컴컴컴컴컴켸 旼컴컴컴컴컴컴컴컴컴컴컴컴켸 o O � O O o컴컴컴훟켸 O O O O 읕컴컴컴좔컴컴컴컴컴좔좔컴컴컴컴컴컴컴컨컴컴좔좔컴컴컴컨컴컴컴컴o Hopefully you should be able to determine what components go where! Remember that this diagram is the TRACK view, and would be turned over, right to left. I will attempt to give a component overlay but it will not be easy! PTT 0V on TX 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 �0V .001�F mc1496p ^ 74LS92 � � gnd � � 1旼커旼커 � 1憫왯컴컴커 旼컴컴� � �>� 1k� � 33pF 5 1� � 잎 � � 2� 납 � � � � � � � � 캑쳐�6� 2� � � � 0읓冒RLA � � � � �1k2 Gnd� � � 0� 0� � � 33pF 읕 � � � � � 憫� � � �8 怨 � � � 컴늘컴 k k � � � � � � }�* � �2 � � � ___ D D � � � � � 읓� � � �0 � � � 캑___쳐 2 1 � � � � 悶 � � .01�F � � � � 읕컴컴켸 560� 읕컴컴� 읕牡컴�1 컴늘컴 � � 憫� /컴컴� 15pF � � � � X cap 컴늘컴 캑쳐� 컴늘컴 � � � 10k� � /.1�F 캑쳐uuuuuu 50pF 憫� both � 쿲nd 1k2� 읓� 6p8 1 trimmer � 쳐 .1�F � �<컴 � � � / 憫� 2 읓� 컴늘컴 � � � .001�F X � � 0 � �+12V / 읓� � � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴켸 7805 regulator * = input from pin 11 PLL chip (10.240) D1 and D2 are the relay diodes and 'k' marks the cathode ends. � = VCO input. � = output to pin 19 PLL uuuuuuu = L1 23 turns on 3/16 former with ferrite slug. Output from a double balanced mixer consists of the sum and the two difference frequencies and output of the original frequencies should be suppressed. The relay is connected to a point in the radio that gets grounded on transmit, (MIC SOCKET). On receive the relay is not activated and the 74LS92 divide by 6 output (1.707MHz), is selected. This is passed to the MC1496 carrier input whilst the VCO signal (18.904MHz) is applied to the signal input. The difference frequency on 17.197MHz is selected by the tuned circuit connected to pin 6 and passed to the PLL programmable divider. On transmit, the relay switches over and selects the divide by 12 output (0.853MHz) from pin 12 of the 74LS92. This is mixed with 14.800MHz from the VCO and the difference frequency (13.946MHz) is selected by the same tuned circuit which has had an extra trimmer capacitor switched across it by the second pole of the relay. INSTALLATION ~~~~~~~~~~~~ Examine the rig you are converting, and find the capacitor that connects from pin 19 of the PLL to the VCO. Remove this capacitor as the two empty lands will we used to connect the board between the VCO and PLL chip. Short screened leads will be used to connect the mixer board between these two lands, but do not connect to pin 19 YET. Connect a screened lead from pin 11 of PLL (10.240) to the input on the mixer board. Most rigs tend to have a trimmer capacitor to alter the 10.240 xtal frequency. If there is not one then connect one of about 20pF depending on the other capacitors going to ground around the xtal, making a total capacitance of about 100pF. Having located the VCO coil look for some circuitry that reduces the VCO frequency on transmit, usually a transistor that switches a capacitor across the VCO. It may be fixed value or a trimmer. If it is fixed replace it with a trimmer of about 30pF. Alignment of the tuned circuits requires a certain minimum test equipment. A calibrated GDO or absorption wave meter and a general coverage receiver with an ACCURATE frequency readout! A frequency counter with a high impedance probe would also prove useful. Remember that before adjusting any coils use a hair drier or other heat source to melt the locking wax. BE CAREFUL NOT TO MELT THE COIL FORMERS. First the VCO must be tuned to 18.9MHz on recieve. The PLL will be unlocked and the signal from the VCO may seem unstable and warbly. Now switch to transmit and adjust the trimmer capacitor for a VCO freq of 14.8MHz. Still with no connection to pin 19 of PLL chip, tune the coil on the new mixer board to 17.2MHz on recieve and 13.9MHz on tx using a GDO. That is, providing you remembered to connect the relay RLA to the PTT line, to switch in the trimmer on the mixer board. Apply a voltage to the mixer board and then adjust the coil and capacitor for final freq. N.B. the lead length connecting to the 7137 will affect the tuned circuit to some extent so some adjustment might be needed to the fixed capacitor on recieve, as well as re-trimming on tx. Next connect the 10.240 from pin 11 of the 7137 to the driver IC. The 1.2k preset which is across the input may need adjusting to ensure that the 10.240 xtal continues to oscillate and that the divider is not swamped and operates cleanly. If the preset reaches a maximum resistance and the 10.240 still fails to oscillate try increasing the preset value to 3k3 or 4k7. With a general coverage rx or a frequency counter make sure that the relay is selecting 1.706MHz on rx and 0.853 on tx, using channel 30 on the CB. Connect up the output from the VCO to the mixer board and check that the VCO fre q of 18.9 on rx is still OK. Now connect the mixer board output to pin 19 of the PLL chip and peak the mixer coil for maximum signal on 17.197 using a general coverage receiver or an absorption wave meter. The PLL circuit should now lock up as the mixer coil is peaked and both carriers on 18.905 and 17.197 will become T9 on the GC rx and should be rock steady with no warble. Turn the channel selector and the freq of the VCO should move up and down as each channel is selected. If necessary, readjust the VCO to get full lock across all 40 channels. It may prove very tricky to get the VCO to lock up. Note there is no lock detect on rx so you have to be in tx mode to adjust and lock up the PLL. Once the VCO is locked up on tx and hopefully rx the recieve coils can be peaked for a signal on 29Mhz using a signal generator or a local ham. WATCH THOSE COILS - remember to melt the wax first and use a proper trimmer and not a metal screwdriver. Next, attach a dummy load and tune the up-mixer whilst transmitting for maximum signal from the tuned circuit, to 13.946 using the trimmer on the mixer board and a GC rx or absorption meter. STILL ON CHANNEL 30 of course. Then, providing the PLL is locked, peak the tx stages for maximum of 3-5watts. Rotate the channel selector and ensure the PLL is staying locked on all channels and adjust the mixer board trimmer for tx andr the mixer board coil for rx . Finally, the transmit and receive frequencies will be low, so now is the time to adjust the trimmer across the 10.240 xtal on the CB board until a tx frequency of 29,600 000 is obtained on channel 30 (use a dummy load) so as not to cause interference to any other operators. If the 10.240 stops oscillating before 29.6 is reached try altering the fixed capacitors, especially the one which is in parallel with the trimmer. I hope you are able to get your CB converted with this information, and maybe I will hear you on 10FM some time. This article would not be complete without the acknowledgement of the original people:- G3XSE For the original design published in SWM. G4UTB For the track design. AND to all at G3BRS Bury radio club for compiling most of the information. I thank you one and all and for myself only claim credit for the text files which I uploaded to my local BBS GB7SAM. 73 Mark G0NMY @ GB7SAM (Downloaded from GB7SAM)