Message 26567 in TECH.WW
From: G8MNY
Date: Sat, 25 Jul 26 05:45:00 Z
Newsgroups: TECH.WW
Subject: VHF Test Oscillator
Message-ID: <221_GB7CIP>
Path: N2MH4|N3HYM|W9GM|KC9UHI|VK2RZ|GB7CIP
R:260725/0547Z 34319@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260725/0547Z 10971@N3HYM.#FRDK.MD.USA.NOAM BPQ6.0.25
R:260725/0546Z 5321@W9GM.#SWWI.WI.USA.NOAM BPQ6.0.24
R:260725/0546Z 30099@KC9UHI.#ECWI.WI.USA.NOAM LinBPQ6.0.25
R:260725/0546Z 19398@VK2RZ.#SYD.NSW.AUS.OC LinBPQ6.0.25
R:260725/0545Z @:GB7CIP.#32.GBR.EURO #:221 [Caterham Surrey GBR] $:221_GB7CIP
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)
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.
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...
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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