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Old 2nd Dec 2014, 3:58 pm   #1
Wendymott
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Default Airband receiver

Hi guys. Just thought you might like to see the Final version of an "Airband" receiver I built, with the help of an austrailian software engineer. I posted details of the PIC micro controller and VFO a little while ago, when it was under development.
After several months, and different versions the "final" version has been completed.
The radio consists of 5 separate modules, this made development much easier, as pcb material is not cheap.
RF front end. This is based around 2 x dual gate fets (BF998). The coils are made on pcb substrates and tuned with varicap diodes BB121. The tuning voltage is "trimmed" to match the VCO voltage swing, to enable reasonable tracking.
The IF Amp, finally finished with 2 x Plessey SL612 rf amps with a ITT 10.7Mhz crystal filter in front and a smaller 10.7 Mhz inter stage filter to narrow out of band noise. A SL623 as AM detector and AGC control, and a LM328 as a Scan start and stop control derived from the AGC line.
Audio output. This is a conventional LM386 IC. There is an internal loudspeaker, but unfortunately creates audio feedback to the VCO if set too high, but as a headphone socket is also fitted, I am not too concerned at present.
VCO. This is a separate module with the inductor element printed onto the pcb, tuned again by 2 x BB121. I found this the most stable of any configuration, with minimal "noise" except if the Audio level is too high as noted above.
MICROCONTROLLER. This is based on a PIC18F46K22, driving a LCD Display
2 line x 16 character 1602 industry standard. The PLL chip and Loop filters are combined on the controller pcb. The PLL chip is based on the Analog Devices range of ADF411x ic's and the loop filter is a TL072 opamp.

There have been a number of software versions, each improving on the last. The final version allows scanning of either memory banks or frequency steps, pre settable scan and wait times from 0.2 seconds to 10 seconds.

Interconnects between RF modules is by SMB and SMC plugs and sockets, I have fortunately a number removed from old test equipment, but normal coax should be ok.

I have posted photo's and the schematics. If anyone wishes to contact me about PCB's or the pre programmed PIC.. please PM Me. The software source code will NOT be available.
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Old 2nd Dec 2014, 4:42 pm   #2
G6Tanuki
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Default Re: Airband receiver

I like the 'modular' construction! Doing it that way does make things a lot easier than working with one massive PCB.
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Old 2nd Dec 2014, 6:16 pm   #3
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Default Re: Airband receiver

Nice work! How I wish I had availed myself of some Plessey products in the days when I worked for them. One thought is that I guess that Rx could be moved on to the 2m amateur band without too much work, but then would need either FM or SSB detection added?

I'm right under one of the main flight paths out of Heathrow to the US (though they are at considerable height by the time they are overhead) but don't often listen to them (just watch them and wish I was on board).

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Old 2nd Dec 2014, 6:27 pm   #4
Wendymott
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Default Re: Airband receiver

Yeah Tanuki, I decided early on the modular construction was the only way, as the VFO was separate anyway.
Ok Bazz,I used to request samples from Cheyney Manor, and they were very helpful. As far as the different modes are concerned, there are IC's that will demod all the different modes.. one kind member sent me a link to download the old data book with much useful info that I used to have in the dark distant days.
2 metres is no problem, the way the PIC controller works, its just a case of re setting the VCO, and tweeking the front end slightly.
I am going to make a 40 metre SSB transceiver next using all the techniques used on this project.
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Old 3rd Dec 2014, 4:20 pm   #5
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Default Re: Airband receiver

Just out of interest - do you find front-end selectivity an issue with only 2 RF tuned-circuits? And is your LO on the high or low side of the desired reception frequency?

Reason I ask is that in the past I've had significant issues with airband receivers suffering FM Band-II broadcast image signals when fed from a broadband antenna like a discone. If you're using low-side LO then subtracting double-the-IF from much of the civilian airband drops you into high-power broadcast-transmitter territory.

I made a similar mistake some years back. Thinking I was clever I redesigned a "Practical Wireless" crystal-controlled 2-metre MOSFET converter to use 120MHz injection rather than the original 116MHz so 144-146MHz was tuned at 24-26MHz on the main receiver.

This redesign put the image-response nicely into the realm of FM broadcast. Fortunately, Toko at the time offered some rather nice 2-metre 4-stage helical filters and a pair of them [one between antenna and the RF-amp BF981, the other between RF-amp and mixer] neatly solved the problem.
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Old 3rd Dec 2014, 9:26 pm   #6
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Default Re: Airband receiver

Hi Tanuki.
The L/O is working at + 10.7 MHz thus 128.7 - 148.7, pre programmed into the pic, to show the correct 118 - 138 Mhz on the display. I have a local FM transmitter quite near and I can detect slight "splash" but nothing significant. The RF front end as seen on the schematic is as you say 2 pole, I thought about adding an extra pole, but decided the complexity of getting the tracking to work correctly may be counter productive. David of "Volcanic island fame" suggested that commercial units have 4 pole filter with each pole individually controlled from a D/a with look up from eeprom, my solution is to use PCB coils mounted vertically. These seem to have a better "Q" than wound coils, in fact I had to detune slightly as a little instability was found in the front end. But as it goes it seems to work well. I have a 4 ele beam pointing south from Hudds, LBA being East of me, and I use a wideband discone as well. Early days yet, but I have no complaints so far If it needs modifying, the modular construction comes into its own.
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Name:	FET VCO 5.jpg
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Old 3rd Dec 2014, 9:30 pm   #7
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Default Re: Airband receiver

Sorry Tanuki. I thought I was putting the PCB pictures on, then I realised I cannot post those. Grrr
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Old 3rd Dec 2014, 9:45 pm   #8
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Default Re: Airband receiver

Hi Wendy, just a thought about the coverage.
The Nav section of the airband from 118MHz down to 108MHz also has ATIS information on it in terms of a specific airport's runway states, wind speed direction etc.
This could also be useful if it is not too late in the design to add this feature.
These are usually VOR beacons, often paired with DME, but only the ones on the actual airports have ATIS, and not the ILS beacons which are in the same band.

Ed
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Old 3rd Dec 2014, 9:52 pm   #9
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Default Re: Airband receiver

Hi Ed. Due to the tuning range of the VCO, it may go down to 108 Mhz, but as I am not bothered about the VOR etc, its not part of the remit. However if someone was to request a "look at it" I could consider it. The micro controller would certainly do it, even on a separate memory bank. Another issue would be does the front end track down to 108. Again its not in my remit.
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Old 3rd Dec 2014, 10:47 pm   #10
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Default Re: Airband receiver

You should be able to find ATIS also in the COM part of the band. Not all VFR flying is done in aircraft with NAV receivers, and they all need to check conditions.

Aircraft radios need to be compliant with ED-23C to be used in certified aircraft and that was revised some years ago to require serious filtering to prevent intermod and desensitisation in the wanted band if flying near FM broadcast stations. Four stages of varactor-tuned preselector is normal, hiding behind a good fixed filter whose job is to shave several dB off of even the higher frequency broadcast channels.

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Old 4th Dec 2014, 10:38 am   #11
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Default Re: Airband receiver

What a nicely turned-out project, Wendy. The front panel is a pleasure to look at and something to be proud of.
I've found that receiving AM properly is a bit like black magic, as getting weak AM signals NOT to sound all wishy-washy is a matter of getting everything just right - gain distribution, bandwidth and audio filtering seem far more important than they are on NBFM.
What BW are the IF filters? Did you consider a second conversion to 455 KHz where you could have used inexpensive CFU-type ceramics?
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Old 4th Dec 2014, 10:56 am   #12
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Default Re: Airband receiver

Hi Andy. Thanks for your nice comments. My first version of IF strip was using a double conversion, but I was not happy with the performance, either in gain or bandwidth, so I reverted to single conversion. The filter is a ITT +/- 6 KHz from ebay, very reasonable at only £5 each. I bought 2.
I do know that the CAA and worldwide, is changing the channel spacing to 6.66Khz in 2015, but as this is an "amateur" receiver it is not necessary to change things, and anyway the PIC software will take care of the channel spacing if required, it is so flexible.

As an aside, I have just paid £25 for a 455 KHz filter from ebay with a +/- bw of 1.1 K at 6db, but that is for my next project.
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Old 4th Dec 2014, 10:58 am   #13
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Default Re: Airband receiver

Hi David. Thanks for your comments, and noted.
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Old 4th Dec 2014, 7:14 pm   #14
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Default Re: Airband receiver

It's 8.33KHz channel spacing (actually 25/3 kHz but called 8.33) and it's here now as a requirement for all new aircraft radios.

There are also some funny tricks with offset ground transmitters. As a plane flies along a route, the same ground service comes from spaced transmitter sites along the route and each transmitter is offset a different amount within the 15kHz filter width of a '25kHz' aircraft radio. (the exact centre and 4 offset values are used) The pilot hears continuous service unaware of the trickery going on. Needless to say there is a terrible strong audio heterodyne tone made in his demodulator from the beating carriers, but the offsets make that high enough in frequency for an audio filter in the radio to kill the unwanted tones.

In 8.33kHz channels, they can fit a centre and two offsets (one highside and one lowside)

David

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Old 5th Dec 2014, 4:56 pm   #15
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Default Re: Airband receiver

Hi David. I stand corrected, thanks. As you say, things are getting complicated, I am glad I am not doing it for a job.
So if I am getting this right, say LBA (Leeds Bradford) is on 133.125, there are slave transmitters on route to allow the pilot more distance range, rather than being on top or near the airfield before reliable contact is made, or is it for "sector" radar controllers.
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Old 5th Dec 2014, 5:45 pm   #16
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Default Re: Airband receiver

The offset carrier trick is used where one traffic controller has to manage a long corridor or where you get low altitude aircraft flying along long or curved valleys. Usually airports are in flattish areas and get as much coverage as they need from just a single transmitter site

I do do this for a living nowadays... http://www.trig-avionics.com/ty92.html

David
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Old 5th Dec 2014, 6:39 pm   #17
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Default Re: Airband receiver

It's an elegant and pragmatic solution compared to the old "closely-aligned-carrier" approach used with multiple-transmitter crystals-in-ovens-controlled AM PMR base-stations in the past. In the absence of any kind of inter-transmitter AFC they *never* stayed closely-enough-aligned, so most users experienced a horrible rise-and-fall of signal strength over much of the coverage area.

[Pye made some valiant attempts at distributed AFC so all the transmitters were on the same frequency to within 0.1Hz or so - but it never really worked well]
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Old 5th Dec 2014, 10:34 pm   #18
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Default Re: Airband receiver

Yes, I can confirm it was a long and fiddly job tweaking a multiturn pot on the front of the site frequency standard while waiting for the Rubidium referenced frequency counter to update it's sample every ten seconds or so. You still used to get a "chuff" every few seconds on the transmission, and then there was the phasing and delay to set up on the audio. The offset is detectable on the London Information transmission on 125.475 where I live. I had to skew the xtal oscillator about plus 3KHz on a home built airband receiver I made a while back. I proved the offset on a couple of other receivers as I originally assumed that it was poor IF filter matching in my design causing the problem.
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Old 10th Jul 2015, 10:24 am   #19
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Default Re: Airband receiver

Hi there.

Is it possible to get the PCB layouts and a programmed IC still and at what cost?

Chris Brand.
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Old 22nd Jul 2015, 2:21 pm   #20
Wendymott
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Default Re: Airband receiver

Hi Chris. Sorry for the delay in answering you. I have just read your comments. If you PM me I can send you the artworks and the HEX code so you can programme the micro yourself. If you cannot programme the PIC yourself I could do one for you, but I would need to research the costings.
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