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G8MNY > TECH 25.02.08 09:30l 70 Lines 2971 Bytes #999 (0) @ EU
BID : 48102_GB7CIP
Read: GUEST
Subj: Comparing Off Air Freq Standards
Path: IZ3LSV<IW2OHX<OE6XPE<DB0RES<ON0AR<GB7CIP
Sent: 080225/0042Z @:GB7CIP.#32.GBR.EU #:48102 [Caterham] $:48102_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@EU
By G8MNY (New Feb 08)
I now have 3 standards to compare. BBC 198kHz LW, Analogue UHF TV syncs &
Amateur beacon GB3VHF based on GPS time.
I can use 198kHz LW PLL ref Rx or UHF TV syncs (64uS pulses) from a VCR to lock
up my PLL osc, & it can output a carrier on top of the GB3VHF VHF beacon
frequency 144.430MHz & the slow beats can be heard in a seperate Rx (noise goes
up & down on AM mode), I can also lock the scope to 1MHz ref derived from the
TV or LW to see which is creeping ahead.
UHF VHF
\³/ 144.43 \³/
³ ÚÄÄÄÄÄ¿ TV or ÚÄÄÄÄÄÄÄÄÄ¿ ³ ³ ÚÄÄÄÄÄ¿ /³
ÀÄÄÄÄÄ´ VCR ÃÄÄÄÄÄÄ /ÄÄÄÄÄ>Ä´ PLL Ã>ÄÙ ÀÄ´2M AMÃÄ[] ³LS
| ÀÄÄÄÄÄÙ | ³ Ref ³ SIGNAL ³ ³ Rx ³ \³
| ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿ | ³ input ³GENERATORÃ>Ä¿ ÀÄÄÄÄÄÙ
ÀÄÄ´FREEVIEW STBÃÄÙ ³ ÀÄÄÄÄÄÄÄÄÄÙ ³ ÚÄÄÄÄÄ¿
LW ÀÄÄÄÄÄÄÄÄÄÄÄÄÙ ³ 1MHz³ ³DUAL ³
\³/ ÚÄÄÄÄÄÄÄ¿ ³1MHz ÀÄ´BEAM ³
³ ³198kHz ÃÄÄ>ÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´SCOPE³
ÀÄÄ´Ref PLL³ ÀÄÄÄÄÄÙ
ÀÄÄÄÄÄÄÄÙ
With this set up is quite rewarding to hear on the 2m Rx both the 198kHz & BBC
TV input to the PLL signal generator give an almost zero beat (much less than
1Hz) on 144.430000MHz, despite the RF paths involved.
Using the Freeview STB as a source was hopeless as expected, more than +3Hz in
1MHz out, as its TV timebase is just an internal crystal & nothing to do with
Rx data rates/clocks at all. But out of the 5 analogue TV channels BBC & CH4
were the closest to LW & GB3VHF. Ch 5 was the next (London Tx has a direct non
digital sat feed!) & ITV was well out as bad as the freeview box!
USING HF BEACONS
Doing the same sort of thing but with the 10MHz beacon, is far less accurate.
UHF 10.000
\³/ 10.00 \³/
³ ÚÄÄÄÄÄ¿Video ÚÄÄÄÄÄÄÄÄÄ¿ ³ ³ ÚÄÄÄÄ¿ /³
ÀÄÄ´ VCR ÃÄÄÄÄ>Ä´ PLL Ã>ÄÙ ÀÄÄ´ HF ÃÄ[] ³LS
ÀÄÄÄÄÄÙinput ³ Signal ³ ³ RX ³ \³
³Generator³ ÀÄÄÄÄÙ
ÀÄÄÄÄÄÄÄÄÄÙ ³
\³/ ÚÄÄÄÄÄÄ¿ ³
ÀÄÄ´198kHzÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³Rx Ref³ 10MHz Ref
ÀÄÄÄÄÄÄÙ
The 10MHz beacon HF path is quit indirect & multipath, so not only do fad outs
reduce the signal, but other paths can invert the phase..
_ _ _
/ \_/~¿_ÚÄ¿__ÄÄ--Ä-Ä¿_Ú~\_/ \_/ \
Strong w e a k Inverted
_ _ _ _ _ _ _ _
/ \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/
Local Ref
This is easily shown up with an AF beat into a reference locked scope, the AF
not only can be seen shifting to & throw in phase e.g. ñ5Hz (can't do that in
graphics!) as the hop path length changes, but also sudden phase inversions
when another path becomes stronger.
Why don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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