R1155



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Tuning Tone Oscillator
for the BITX20A


osc circuit The audio oscillator left (click images to enlarge) provides a sinewave tone of around 900 Hz to the input of the mic amp when the BITX20A is set to tune and the PTT switch is pressed. This provides constant modulation so that the SWR can be minimised (the level will be adjusted so that the PA output is only a couple of watts). The SWR bridge will be built into the transceiver and a simple five section LED bargraph used to indicate relative SWR (this same indicator will be used to show relative signal strength on receive).

The circuit is a simple twin T sine wave oscillator utilising a BC109 NPN transistor but almost any small signal NPN transistor will work in this circuit and provide a reasonably clean output as the 'scope snapshot shows (the blurred trace is a camera effect). output waveform The output is taken from the collector and then divided by 20 by the potential divider formed by R4, R5 & VR1. Output to the mic amp is variable from zero to around 70 millivolts peak to peak.

Construction: I constructed the oscillator on a piece of 0.1 inch matrix SRBP board measuring 40 x 60 mm, layout is not at all critical but it helps to start off laying out components in the manner of the circuit diagram, see the image below. output waveform
(This picture was taken before the preset, VR1, was added to the board.) The three bolts are M3 and, with PVC pillars, support the oscillator board on the auxillary board which will also carry relays.

Note: The circuit was actually tested at 13.8v so it is perfectly happy running on a fully charged SLED (sealed lead acid battery).



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