Sunday, November 02, 2014

Park Air Electronics VHF receiver external PLL control

When I bough the Park Air Electronics VHF receiver Model 2100, last year, the idea was if it was not worthwhile I would use some components and the box for another project.
The fact is the receiver worked and was programed for the Cork airport ATIS frequency.

Programing the frequency is a matter of changing some dip switches (red ones in the top of image), so, thinking of that, why not extending the dip switches externally and why not making a control box with digital readout for rapid frequency excursion!

So with some further delays I started codding the Arduino project to do that. After the software was finished was just a Mather of connecting 11 outputs of the Arduino to the dip switches on the receiver. The receiver uses two sets of dip switches, one for Mhz and another for Khz and are programed according to a binary table to generate the LO frequency which his less 21.4Mhz of the receiving one.

I finished codding last week and so this weekend was a mater of connecting all lines to the control box and testing, I was pretty sure it would work, worst case scenario there could be an issue with the run of cables from the PLL part to the outside of the box and that would be fixed by decoupling.

 (I didn't had to drill any holes the small cables leave the box in a small existing aperture)

And....guess what? it worked first time connected! Not a single issue!

I was a joy starting to scroll on the frequencies and starting to listen some air control traffic...

...until I ear the approach control directing a plane to the control tower frequency, so tuned to the control tower and didn't listen to the plane starting com's on that frequency, strange!

Fired up the TH-D7 on that frequency to see if there was no com's or the radio was deaf...

Yes your right, it was deaf at the tower frequency, so it could only be one of two: a bad connection on one of the control bits for the PLL or the PLL not locking.

Time to troubleshot, the PLL luckily has an internal indication of locking, and in fact was unlocked, so I tuned the main oscillator coil and started to receive....
Yes......No.....I lost lock 2Mhz up! bummer! The PLL lock range is too short! That's why this is a single frequency receiver and that's why in the instruction there's the procedure to re-tune the VFO coil until lock point! Previous I had thought: what nice thing, they even included the re-tuning procedure!

Now, I can keep it like this getting only a small subset of the full spectrum airband or keep it in a single frequency and use the Arduino control box for other project or I can disconnect the internal oscillator and try an external oscillator..... to be continued!

Have a nice week!

(p.s.; if you need the Arduino code for a similar parallel PLL (MC145152P) chip just let me know and I can email)

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