8589357276

Błock diagram of G3WXO's phaae-locked 2m vfo

Filter

Oscillator

SN7400N

Osc.

P.S.D.

400

-600

I.F.

400-600 kHz

ssb from thc 2m cxcilcr and the diflerence frcquency of 70-208MHz is cxtraclcd. This puls F.16AS just insidc thc Rcpublic of Ircland allocation on ^ł*Four**.

It is apprcciatcd that thc diflerence frcqucncy is rathcr closc to the output frcqucncy (74MHz) of thc oscillator Chain, but a scrics of tuncd circuits at thc transmit frcąucncy cnsurcs rcjcction of both thc 74 and 144MHz components. Thcrc arc four such circuits in all; at thc mixcr anodę, drivcr grids and drivcr anodcs, and of coursc in the finał QQVO6/40A anodcs.

a    a    a

From G3WXO (Phil Harman, Wolvcrton, Bucks):

Havjng scen thc commcnt “What Oscillator?'* in April FMD I thought readers would bc interested in my phasc* locked 2m vfo, a błock diagram of which is shown.

The required 2m frequcncy b generated directly in a simplc voltagc controlled LC oscillator. The output is mixed with 140MHz dcrivcd from a crystal oscillator. The rcsulting i.f. of 4 to 6MHz is divided by 10 in an integrated Circuit, SN7490N, to producc a finał i.f. of 400 to 600kHz. A fet Vackar vfo tuning 400 to 600kHz together with thc i.f. is applied to a SN7400N which is used as a phasc sensitivc detcctor. The dc output from this psd is fed to a varicap diodę in thc 2m oscillator, thus controlling its freąucncy. Freąuency modulation is easily achicvcd by applying audio to thc varactor in thc 2m oscillator; a fcw millivolts produccs fuli deviation.

The output of the vfo is very clcan as thcrc arc no multi-plication proccsscs involvcd, the 140MHz componcnt bcing bufTered from the output by thc mixer.

Construction of a slabie vfo tuning 400 to 600kHz is vcry simplc and rcquircs very little in thc way of voltagc or temperaturę compensation, this being thc main advantagc of this particular 2m vfo ,Thc drift at 2m is cqual to thc drift at 400 and 600kHz times 10. However, it is much casicr to build a stable oscillator at this freąucncy than it is at 8 or 24MHz.

When thc vfo is phase-locked thc coil in thc 2m oscillator can be “twanged" with no eflect on the output freąuency.

Here and there

Notę for thc diary: thc Scottish VHF Convention will be hcld at thc Carlton Hotel, Edinburgh, on Sunday 3 October. Tickets will be avatlabłc in Scptembcr.

RADtO COMMUNICATION June 1971

4-146 MHz

v.c.o.	^ ^ to Power Amolifier.
	1 ^- i	-w- ’	r~	-1
	Mixer—<—| x2 —Osc.			i
		140
		MHz
SN7490N			—►—140 MHz
-MO h"	-4-		output	for
_1—i	4-6 MHz I.F.		conwerter
kHz

70 MHz

Holdings Photo Audio Centrc. of Blackburn, ask us 10 statc that Trio JR310 receivcrs sold by thcm can bc supplied eon-vcrtcd to covcr ihc rangę 28—30*1 MHz lo function as i.f. strips for the 2m band. Price of convcrsion: £3.

• • •

‘*Telcgraphy gives better rcsults Ihan tclephony becausc a.m. only convcys intelligcncc on one«sixth of the available power (one sideband) whereas the cw system uses all of the avail-able power. For the same reason ssb will give results com-pa rabie with cw”—BRS31448 (LREM C. J. Taylor, on HMS Arethusa).

25 YEARS BACK

, afttr many watka of fłno achlwemon) on 5m wllhfn tha country, tha contłnant ha* at la«1 baan contactad. G3M0 <Uvarp00l) workad MIRA (Navarro. N. Italy) on May II, aignala balng SI boih ways with tha llallan alatlon uilng tCW"—W. A. Scarr, G2WS. In ⁽*Tha Month on Flva". ao-paarlng lor tha ftrat tlma lr> RSGB BulM/n. Junt 1941.

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When Luton Scouts Council held a racruiting campalgn, Dunatable Downa Radio Club gave taiaviaion coverag© for four houra of Scouta* outdoor compatitiona. From G6AFK/T in tha cantral park want picturaa 1ł milaa ovar a non-viaual path to G6AGE/T. Str urnach loanad a 6Sft Veraatowar to lift tha 70cm aignal ovar (ocal obstructiona. At taaat half a doi en local Stroke T man alao »avy tha picturaa at thair homa aitas. Commentary was by G8CBU, v*ho Is a Luton Scout leader. Thia waa on 2m. Photo ahowa G8AEV/T (ha ia alao G3VZV) oparating one of tha o/b cameras.

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