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G8MNY  > TECH     11.07.18 10:03l 83 Lines 3341 Bytes #999 (0) @ WW
BID : 28640_GB7CIP
Subj: VHF Test Oscillator
Path: HB9ON<IW2OHX<UA6ADV<CX2SA<OZ5BBS<ON0AR<GB7CIP
Sent: 180711/0746Z @:GB7CIP.#32.GBR.EURO #:28640 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                     (Updated Jan 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is the circuit of a VHF marker oscillator that was used for a club on the
evening construction contest. (by G4VTD)

  ┌───────────┬─────┬────┬───┬─100─┬───┬─────┬───┬─o +9v
  │       __  (6T   │    │   │     │   │     │   │
  │   O/P __) (4mmÝ │    │  10k  100k 100k  10k  │
  │           │ L   │    │   │     │   │     │   │u1
 39k   ┌──────┤     │    │   │     │   │     │  ===
  │  │/ 2SC   │     │ 10n│   ├─┤├──┤   ├──┤├─┤   │
  ├──┤  549   │     │   ===  │ 10n  \ /  10n │   │      2SC549 &
  │  │\e  3p3 │    ===   │   │       X       │   │       2SC540
  │ T1 ├──┤├──┤_    │10n │    \│    / \    │/    │         .-.
1n│    │      │/|   │    │  T2 ├───┘   └───┤ T3  │        /...\
 ===  100    ===    │    │   e/│           │\e   │        └───┘
  │    │     /│1-10p│    │   │    ASTABLE    │   │        e b c
  └────┴──────┴─────┴────┴───┴───────────────┴───┴─< 0v
           tube type

HOW IT WORKS
T1 is a ground base amp to form the VHF Colpitts Osc, with L tunned by the 2
current gain/tap caps 3p3 & 1-10pF, & putting part of the Collector output back
into the emitter. The 2x 10nF across the supply completes the high Q tunned
circuit RF path. Base current is from 39k & together with the emitter 100R
forms a stable osc bias.
Tone modulation is from the 1kHz 2 transistor (T2 & T3 2SC540) astable osc,
where T2 square wave collector current slightly varies the Osc supply via the
shared 100R. The 2x 10n & 2x 100Ks set the on-off ratio & the frequency.

TIN CAN
A flat piece of tin plate 3 x 4cm or PCB is used to attach all the components.

       ┌───────────────────────┐
       │_                    C │
 Output│_).     C─┬100┬───┬──┤+│Power in
   Coax│  │  L  C 10k │   │ 10k│
       │ _)())))  │ 100k 100k│ │
       │   /\     │   │ X │  │ │
Trimmer│(0) T1 30k├  C     C ┤ │
       │   C 100   T2      T3  │
       └───────────────────────┘

COMPONENT MOUNTING
All were standard small components as the design is ugly constructed & the lead
wires used to support all the components. All wire lengths kept at a minimum
in the "best VHF practice" in the oscillator stage.

TESTING
The oscillator runs between 120-160MHz with this coil, & the trimmer can be set
with a Rx, or a frequency counter from the nearby small coax pick up loop.
The square wave AF oscillator is about 1kHz & provides a 1% power rail change
to give AM/FM mod. Deviation is about 20kHz p-p of FSK. Output level around
-20dBm for single turn pick up, & haronics are all >-30dBc.

FINISHING OFF
If all goes well & you want to box the unit, try adding a tin top cover...

         ┌────────────────────┐
         │''''''''''''''''''''│
         │                    │ DC
    ┌────┘˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙└─u───┐
  RF└──┐ :                    :    :│
Coax┌──┘ :                    :    :│
    │:   :                    :    :│
    │:   :                    :    :│
    │:   :  _ trim            :    :│
    │:   : (_)hole            :    :│
    │:   :                    :    :│
    └────┐˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙┌─────┘
         │                    │
         │....................│
         └────────────────────┘


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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