All members are urged to attend even if it’s just to enable us to put a name to a face or a voice!

Any other interested parties or prospective new members are welcome to come along and meet us for a chat.

The first ever event organised by the comparatively new radio magazine Monitoring Monthly, together with the Dorset Police Amateur Radio Society and their caravan, will be held on Portland, up the Grove round the back of the Y.O.I. on Monday 29^th May. They will be operating on or around 7.07MHz depending upon QRM. For further details and updates see their website: http:\\www.monitoringmonthly.co.uk

Chairman’s Chat

Welcome once again to all our members and readers. Please one and all try and make it to the AGM. It is your club and this is the main chance to let us know what we are doing right or wrong or what direction you`d like us to point our aerials to. See you there.

As reported elsewhere by Graham we have the Monitoring Monthly team transmitting from the top of incline road on the 29th May. This is a chance to meet their team and say what you think of the new Mag.

Well, as has been mentioned in previous newsletters, slow but steady progress is being made with the BITX20 project VFO. As described last month, the prototype has been built on stripboard, designed using the excellent ‘VCAD’ software.

On powering up the prototype, the tuning inductor and capacitor values were tweaked so that the circuit operated in the necessary range (around 4MHz). However, one or two small problems were immediately noted. Firstly, using an LED as a simple Varicap diode simply did not give the necessary tuning range for this project. It was therefore decided to obtain some ‘proper’ high capacitance Varicap diodes. Now, such things are still to be found in any ‘AM’ radio not utilising a variable capacitor for tuning, although, such are things in electronics today, they are most likely to be ‘surface mount’ types (i.e. miniscule little specks, difficult to handle and use!).

However, there used to be a readily available part in a standard plastic transistor-type package, made by Philips, called a BB212 – but unfortunately these are now obsolete. As with a lot of obsolete parts though, they can still be obtained, albeit at a price (around £8 each!). Luckily, good old Ebay provided a source of these for a much more reasonable price of just under £6 for two, so a pair were duly obtained.

I now had the necessary tuning range (4.0 to 4.4MHz), but another problem was apparent. Whilst the ‘huff and puff’ circuitry was operating as anticipated (see the January 2006 issue of the PARCing Space for a description), and producing an error-correcting signal for the VFO, the VFO simply was not locking as it should. Indeed, greater stability of the VFO was obtained by disabling the correction circuitry altogether!

After much puzzling, I surmised that the high capacitance Varicap diode was over correcting, and instead of gently pushing the VFO frequency back in line, it was pushing it too far in the opposite direction, caused by the magnitude of the DC correction voltage being too high.

This was addressed by increasing the series resistance that feeds the integrating capacitors, and also the resistor that couples the resulting voltage to the Varicap diode. In all, a major increase in value of about 20 times was required, but with this, the VFO now locks nicely over its tuning range. Perhaps some ‘fine tuning’ of the component values would improve the performance even more, but that can wait for now.

Below is a photo of my prototype.

I hope you can see that it follows the stripboard layout shown last month. Visible is the oscillator module (top left), the digital delay line next to it, the diode-transistor exclusive-OR logic gate, and the twin integrating capacitors with the Varicap diode below them. On the right hand side of the board is the oscillator and digital divider IC, the VFO tuned circuit, with the power supply circuitry above that. The potentiometer visible above the circuit board is a 10-turn type, used as the main tuning control.

What’s next? Well, being a little scared of getting stuck into more RF circuitry too soon! I will probably put together a frequency counter, based on a microcontroller and LCD display, for use with the VFO. But more of that next time……..

……..what marketing suggested

what management approved……..

what was manufactured……..

what the customer really wanted……..