OpenBCM V1.07b12 (Linux)

Packet Radio Mailbox

IZ3LSV

[San Dona' di P. JN65H]

 Login: GUEST





  
G8MNY  > TECH     23.02.10 09:21l 78 Lines 3326 Bytes #999 (0) @ EU
BID : 49628_GB7CIP
Read: GUEST
Subj: Comparing Off Air Freq Standards
Path: IZ3LSV<IK2XDE<DB0RES<ON0AR<GB7CIP
Sent: 100222/2307Z @:GB7CIP.#32.GBR.EU #:49628 [Caterham] $:49628_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, Terminal Font)

    I now have 3 standards to compare. BBC 198kHz LW, Analogue UHF TV syncs &
Amateur beacon GB3VHF based on GPS time.

I can use BBC 198kHz LW PLL ref Rx or UHF Main Tx Analoge 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 TV                                          VHF 2M Beacon
 \│/                                      144.43  \│/ 
  │      ┌─────┐          TV or ┌─────────┐  │     │ ┌─────┐   /│
  └──────┤ VCR ├─────── /─────>─┤   PLL   ├>─┘     └─┤2M AM├─[] │LS
  |      └─────┘      | │  Ref  │  SIGNAL │          │  Rx │   \│
  |    ┌────────────┐ | │ input │GENERATOR├>─┐       └─────┘ 
  └────┤FREEVIEW STB├─┘ │       └─────────┘  │ ┌─────┐
BBC LW └────────────┘   │                1MHz│ │DUAL │
 \│/ ┌───────┐          │1MHz                └─┤BEAM │
  │  │198kHz ├────>─────┴──────────────────────┤SCOPE│
  └──┤Ref PLL│                                 └─────┘
     └───────┘

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 (Digital TV) 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                                    10.000000MHz
 \│/                       10.000000MHz    \│/
  │  ┌─────┐Video ┌─────────┐  │            │  ┌────┐   /│
  └──┤ VCR ├────>─┤   PLL   ├>─┘            └──┤ HF ├─[] │LS
     └─────┘input │ Signal  │                  │ RX │   \│
                  │Generator│                  └────┘
                  └─────────┘  │
 \│/ ┌──────┐                  │
  └──┤198kHz├──────────────────┘
     │Rx Ref│            10.000000MHz Ref
     └──────┘

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..
 _                         _   _ 
/ \_/~┐_┌─┐__──--─-─┐_┌~\_/ \_/ \
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. ˝5Hz (can't do that in
graphics!) as the hop path length changes, but also sudden phase inversions
when another path becomes stronger.


See other TECH buls "Off Air Lock for Ref Osc.", "198kHz Off Air Standard"
"Calibrating Frequency", "Crystal Drift Compensation" & "Simple Crystal Oven" 




Why don't U send an interesting bul?

73 de John, G8MNY @ GB7CIP


Read previous mail | Read next mail


 23.10.2019 04:33:03lGo back Go up