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G8MNY > TECH 25.02.08 00:00l 192 Lines 9291 Bytes #999 (0) @ EU BID : 48038_GB7CIP Read: GUEST Subj: 198kHz Off Air Standard Path: IZ3LSV<IW2OHX<OE6XPE<DB0RES<ON0AR<GB7CIP Sent: 080224/1016Z @:GB7CIP.#32.GBR.EU #:48038 [Caterham] $:48038_GB7CIP From: G8MNY@GB7CIP.#32.GBR.EU To : TECH@EU By G8MNY (New Feb 08) (8 Bit ASCII graphics use code page 437 or 850) I currently use an analogue TV lock reference system (see my buls on "Off Air lock for Ref osc", "Simple Crystal Oven" & "Crystal Drift Compensation") for calibrating RF gear, but as this TV source will end in the not too distant future in London, I took the opportunity to buy someone's old homebrew veroboard construction version of the 198kHz off air reference project. (from a Practical Wireless article Dec 1995 by G8JVE, to go with the Robin counter). WHY WAS IT CHEAP? Well it sort of worked, but there was far too much PSK on the reference output following the low speed PSK DATA on the 198kHz broadcast. As well as that it was quite insensitive despite an extra RF buffer stage over the original article. It also had a rearranged divide chain (same ICs) to give more useful reference frequencies. ÚÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿ÚÄÄÄÄÄ¿ÚÄÄÄÄÄÄ¿ÚÄÄÄ¿ÚÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄÄ¿ ³External³ ³ RF ³³ ³³ 2kHz ³³ IF³³ IF ³³ IF ³ Rx ÚÄÄÄÄÄÄÄÄÄÄ¿ Lock ³ Active ÃÄ´BufferÃ´MixerÃ´ LC IFÃ´AMPÃ´Tuned Ã´LimiterÃÄÄÄÄ´ ÃÄLED ³Ferrite ³ ³ ³³ ³³Filter³³x10³³Filter³³ x100 ³2kHz³ Phase ³ ³Rod Ant ³ ÀÄÄÄÄÄÄÙÀÄÄÂÄÄÙÀÄÄÄÄÄÄÙÀÄÄÄÙÀÄÄÄÄÄÂÙÀÄÄÄÄÄÄÄÙ ³ComparatorÃÄ¿ ÀÄÄÄÄÄÄÄÄÙ ³200kHz S meter ³ ³ ³ ÀÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ ³ ÚÄÄÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄ¿ÚÄÄ¿ÚÄÄ¿ÚÁÄ¿ÚÄÄ¿ÚÄÄÄ¿ÚÁÄ¿ÚÄÄ¿ Local 2kHzÀÄÄÄÄÄÄÄÄÄÄÙ ³ ³Variable³³ ³³ ³³ ³³ ³³ ³³ ³³ ³³ ³ ÚÄÄÄÄÄÁÄÄ¿ ³ 10MHz Ã´BufferÃ´ö2Ã´ö5Ã´ö5Ã´ö2Ã´ö10Ã´ö5Ã´ö2³ ³Multiple³ ³Xtal Osc³³ ³³ ³³ ³³ ³³ ³³ ³³ ³³ ³ ³ Slow ³ ÀÄÄÄÂÄÄÄÄÙÀÄÄÄÂÄÄÙÀÂÄÙÀÄÂÙÀÄÄÙÀÂÄÙÀÄÄÂÙÀÄÄÙÀÂÄÙ ³ Loop ³ ³ 10M 5M 1M 100k 10k 1k ³ Filter ³ ³AFC Frequency reference/marker outputs ÀÄÄÄÄÄÂÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ FAULTING On testing I found the mixer was hard clipping very early, & carefully following the circuit I found it was incorrectly biased, due to uncut track! Fixing that showed me there was room for improvement here, a there was 2kHz IF beats on several of the Mixer bias points, I changed those u1 caps to u47 & reduced the IF in inputs symptoms. The RF buffer was rebuilt to a simpler less lossy circuit. (see below) The problem with the PLL was initially crudely solved with a 100uF & series 2k2 across the AFC line, then it gave a steady 10MHz note. IMPROVEMENTS 1/ PLL Filter Looking into the PLL loop jitter problem, the was a very high Z circuit using a 40 Meg ohm as the R for a 40 second time constant. This obviously was not working correctly on this layout, possibly leaky old PCB or varicap diodes? Anyway I redesigned that bit using electrolytics, something the original designer avoided. But by using 2 identical electrolytics in series across the 5V power rail, I avoided the most of the leakage problems & I gained instant half rail AFC voltage on initial power up, for fast lock up time. +5VÄÄÂÄÄÄÄ +³100u 4x 1N4148 === ÚÄ´<ÃÄÄÂÄ´<ÃÄÄ¿ Scope Varicap ³ ³ ³ ³ o IC CD4046 AFC <ÄÄÂÄÄÄÄÄ)ÄÄÄÄÄÅÄ´>ÃÄÄÅÄ´>ÃÄÄÅÄÄÂÄÄ39kÄÁÄÄ< Phase Comparator ÃÄ2k2Ä´ ³ ³ ³ ³ Pin 13 ³ ³ ÀÄÄ1MÄÄÁÄ220kÄÙ ³ 1u === +³ === u1 ³ ===100u ³ ÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄ I enhanced the variable CR system with 2 more diodes & resistor, so out of lock you have 39k to 200uF (8S), @ 1V from lock add a 220k (50S) & then @ less than 0.5V add a 1M (200S). The 39k & 1u (39mS) is needed to remove the 5V 2kHz pulses & reduce the 10Hz phase shift to below 0.5V. This however is a loop time constant that would oscillate, so the 2k2 is added to damp this oscillation, but not to big to let through to the AFC line, any significant amount of the PSK data. At lock, the scope point using a ö10 or ö100 probe & locked to a fraction of 50Hz mains (e.g. 12Hz> you see. _ +5V ³³³³³³³³³ 2kHz phase ³³³³³³³³³ correction ³³³³³³³³³ needle pulses \/ PSK Ripple ..ÁÁÁÁÁÁÁÁÁÄ-...__________.. Under 0.5V Mean 2.3V/ ³³³³³³³³³ /\ Swing. ³³³³³³³³³ 0V_ ³³³³³³³³³ SLOW P.S.K. DATA The PSK data pulls on the stored AFC voltage several times a seconds. 2/ As there was an ugly hole in the box & a S meter/centre zero meter & switch fitted, I developed a circuit for these, a manual Centre Zero mode with a pot to fill the hole. Varicap AFC pot ÚÄ27kÄÄ´ oÄÄÄ< S meter circuit +5V on/off ³ \³/ Loop Green ______/ Ào\___o\__ÃÄÄÄÄÄ47kÄÄ6k8ÄÂÄ<Filter LED ³ oÄÄÄÄÄ22kÄÄÄ>CD4046 ³ Pot ³ AFC ³ ÃÄÄÄÄÄ¿ CZ mode Pin 2 ³ ³/ ³/ /~~~\ +³ ÃÄÄÄÄÄÄÄÄ3k9ÄÄÄÄÄÄÄ´ T1 ÚÄ´ T2 ³Meter³ === ³ ³\e ³ ³\e \___/ ³100u ³ _³_ ³ _³_ _³_ _³_ ³ ³ ÀÄÄÄÄÄÄÄÄ3k9ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ The switched log pot I used had a double make e& I needed a changeover. Leaving the pot in circuit & using T1 to short incoming AFC line I achieved the same function, & T2 shorts out the Green in lock LED to indicate it is in manual off lock mode. As the pot was log the varicap angle-frequency action was not quite linear so 27k is put max to centre & 6k8 on min end, gave a really even feel to the offset (ñ10Hz @ 10MHz) with a 2.3V centre. The Centre Zero meter action was take from pulses directly from the unused IC pin 2 via a 22k to set the meter sensitivity. If you scope that pin see. ÚÄÄÄ¿ ÚÄ¿ÚÄÄÄ¿ ³ ³ ³ ³³ ³ The average changes a little depending on the two 2kHz phases. Ù ÀÄÙ ÀÙ 3/ I added an AGC, as I found the mixer & other stages could overload & possibly add phase mod from the AM signal before the signal was tightly filtered. I did not want to remove RF buffer gain, as this is useful when in building with weak signals. So I used a NPN to short out the external FET preamp supply & this gave a very good AGC no distortion AGC action & gave the S meter with usable log scale. ÚÄÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄ+12V ÚÄÄÄ>S meter d ³ ³ 2k2 100K || g ÃÄÄÄÄÂÄ15kÄ¿ 22k 1M ³ 4n7 preset IF ||ÚÄÄÂÄÄÂÄ>³ ³ ³ ³ 4n7 ³ ÃÄ´ÃÄ>Mixer ³ u1 Opamp ||( ³ ³\ ÃÄÂÄÄ)ÄÄ´ÃÄÁÄ>>ÄÅÄÄÄÄ´ÃÄÄ´ ³/ ÚÄÄÄÂÄ100kÄÁÂÄ´ÃÄ*ÄÄ< pin1 ||( === ³/\ s³ ³ 1u \³ ÀÄ´ RF Buff ³ ³+ _³_ No ||( ³ === ³ === AGCÃÄÄÂÄÄ¿ ³\e ³ === /_\ Series ||( ³ /³ 4k7 ³1u e/³ === ³ ³ ³ ³10u ³ R ÀÄÄÁÄÄÁÄÄÄÄÁÄÄÁÄÄÄÄÄÄÄ>>ÄÁÄÄÄÄÁÄÄ)ÄÄÄÄÁÄÄÄÄ ³ ÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄ External Active Aerial BNC u1 ³ AGC ³ ÀÄÄ100kÄÄÄÄÄÄÄÄÙ 4/ Frequencies As this design was not quite to the published wiring here is the arrangement. The narrow 1:5 200kHz pulses fed to the mixer has been ramped a bit with an additional 4n7 & 1k, instead of a 10k, this yields 3dB more mixer gain (ideally a 100mV square wave is best). 4n7 ÚÄÄ>Mixer 0vÄ´ÃÄÄ´ ===10n 74LS132 74HC390 ³ 74HC390 +5VÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄ ³14 16³ 1k 16³ 2kHz 10MHz ÚÄÄÁÄ¿ 10M ÚÄÄÁÄÄ¿ Ão200 ÚÄÄÁÄÄ¿ o Buffer>Â´1 11ÃÄÄÄÄÄÄÄÄ´1 15ÃÄ´ kHz Ú´3 15ÃÂÄÁÄÄ>to Phase À´2 8Ã¿ ³ 9ÃÄÙ 50À´4 9ÃÙ Comparator Ú´3 12Ã´ ÚÄÄ´3 ³ kHz³ ³ Ã´4 13ÃÙ ÃÄÄ´4 13ÃÄÄÄÄÄÂÄÄÄÄ´1 13ÃÄÄÄÄÄ¿ À´5 6Ã¿ ³ ³ ³ ³ ³ ³ ³ ³ 9Ã´ ³ Ú´14 12Ã¿ ³ Ú´14 12Ã¿ ³ ³ 10Ã´ ³ Ã´2 7Ã´ ³ Ã´2 7Ã´ ³ ÀÄÄÂÄÙ³ ³ ³ÀÄÄÂÄÄÙ³ ³ ³ÀÄÄÂÄÄÙ³ ³ 7³ ³ ³ ³ 8³ ³ ³ ³ 8³ ³ ³ ÄÄÄÄÄÄÁÄÄ)ÄÄÄÄ)ÄÁÄÄÄÁÄÄÄ)ÄÄÄÄ)ÄÄÄÁÄÄÄÁÄÄÄ)ÄÄÄÄ)ÄÄ o o o o o o 10M 5M 1M 100K 10k 1k /³\ ÀÄÄ´ÃÄ470ÄÄÂÄÄÄÂÄÄ(o Output 33n _³_ _³_ ³ 1V p-p \_/ /_\ ³ ÄÁÄÄÄÁÄÄÙ 5/ Output Protection By adding a the 1N4148 diode clipped attenuator the TTL chips are protected from external static damage & small amounts of accidental RF (10W?). RESULT I now have a very accurate marker for HF, & I can calibrate Frequency counters or lock then up to this source, as well as lock up my 100Hz-1GHz PLL sig gen. For VHF & UHF, Xtal oscillators age & can't be relied on to maintain high accuracy over several years. This accurate source (better than 1 in 10^7) enables checking of standards. Why Don't U send an interesting bul? 73 De John, G8MNY @ GB7CIP
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18.10.2024 11:21:26l

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