Friday, March 04, 2011

ARNSW Home brew 80m DC challenge receiver

The current "challenge" project for the Home Brew Group is to build a direct conversion receiver for the 80m WIA sunday broadcast on 3595KHz SSB.

While this is pretty trivial for many in the group it's appropriate for my level of construction and happily I have a working receiver which I'm listening to as I type.

Front panel

The box is a CDROM drive box with the guts ripped out and a simple aluminium front panel added. This is an idea I first saw from Stephen VK2BLQ. At Wyong I picked up a few drives just for this purpose.

The circuit is the classic NE602 / LM386 design from early on in EMRFD. Construction is "ugly" but it seems quite stable.

Inside

Ergonomics leaves something to be desired, I think I'll move the antenna socket to the back, and the tuning needs to be less sensitive. (I have a lovely reduction drive but I'll save it for something more substantial).

In operation

Here it is achieving its objective, listening to the Sunday broadcast.



Stability is surprisingly good.

As requested by John, here is the circuit from Experimental Methods in RF Design. The only change for 80m was to use 40 turns of #33 wire on the two toroids.

DC receiver

Since the video I have made the tuning less sensitive by adding series capacitance but it probably still needs a fine tuning knob. Also, I'd love to have another go with smaller components and try to get this down to a more compact size suitable for travel.

Here's how it sounds listening to the latest broadcast on a speaker this time.



I've completed installing the radio in its case and fitted a fine tuning knob and power switch. Here's how it came out.

Front closed

Minimalist back.

Back closed

Old CD ROM drives make an excellent home brew box.

Front open

The perfect size for many smaller projects, that is ones too big to fit in an Altoids tin.

Side open

The battery clip is a little dodgy.



All boxed up, with a new fine tuning knob, listening to the broadcast.



I'm done with this project now.

7 comments:

John said...

Well done Peter, nice and compact with good audio. Any details of circuit or is this part of the challange ?

Regards John

Peter Marks said...

John,

I've added the circuit as you requested. I assume this is "fair use" and I most definitely recommend anyone interested grab a copy of the book. This example is almost a throw away early on before the authors get serious - but as you see it works remarkably well.

73s

Peter

Anonymous said...

Sounds very good for SSB

Dave
vk3ase

robototechnik said...

Hi, Im Alex from Germany, I have built the same receiver using your circuit, its pretty cool and seems to work, all the components Ive used are smd. Can you tell me please what do all the 3 variable capacitors control? The 180 pF, 80 and 50. Which of them should I use for tuning and how can I make tuning fine?
Thank you very much for your answer!
Regards.
Alex

Peter Marks said...

Hi Alex,

the largest variable is used for course tuning and the smaller ones for setting frequency or as a fine tuning knob.

In my version I played around with these values to get the tuning "feel" I wanted.

Peter

robototechnik said...

Thank you very much for quick answer=)
For capacitor 180 pF Im using 5-285 pF. When Im tunring it I can only hear the changes in loudness of sound. Its very strange. If this one should tune the frequency, I would say that mu tuning is too short. The other two capacitors can tune well. Yesterday I have used the 80 pF for tuning and 50 pF for fine. Is it wrong? Can you tell me please which of variable capacitors should be on on the circuit board and which on the box, to tune. Tell me please also what can I do to make my 265 pF capacitor tuning? How can I set the right frequency with the 80 and 50 pF capacitors? At which pin can I measure it? Am I allowed to use the 50 pF capacitors as a fine tuning? Or should I connect one more f.e. 30 pF capacitor in serie to the 265 pf for fine tuning?
Excuse me please for such an amount of questions! And thank you very much for your help!
Alex

Peter Marks said...

Alex,

that doesn't sound right - having the capacitor change the volume of the sound. If you have a CRO or frequency counter you should be able to see the local oscillator at pin 6.

Do you hear any stations? Maybe the local oscillator isn't running for some reason?

I'm sorry but I'm travelling at the moment and can't make comparisons with my version for you.

Peter