Decoding VOA Radiogram content usually involves a patch cord from the radio’s headphone jack to a computer’s microphone jack. This, however, can introduce noise from the computer, or the computer’s power supply, to the radio. A solution is an isolating patch cord, which (after some searching) can be purchased. Or you can build one yourself, using these plans kindly provided by VOA Radiogram listener Al Holt in Florida …

After my first few weeks decoding Radiogram programs I noticed a distinct increase in the background noise level when I connected my computer to the radio, something on the order of 6-9 dB on its S-meter. I thought this might be a detriment to the quality of decoding Radiogram content so I started looking for a solution.

There’s enough electrical noise generated within the case of the computer that when the two units are connected with a common ground (earth) point that noise gets induced into the signal path. Sometimes this can be in the form of 60 Hertz hum, but in this case it was RF noise passing through the receiver to its audio output.  The solution is to isolate the grounds between computer and receiver and this is done with a transformer.

An email message from Kim Andrew Elliott suggested some commercial solutions:

http://www.packetradio.com/catalog/index.php?main_page=product_info&cPath=46&products_id=160&zenid=ket94plgp9q73fim8nt1ik5qf0

http://www.sonicelectronix.com/item_26692_Axxess-AGL3.5.html

http://www.gromaudio.com/store/accessories/noise_filter_-_ground_loop_isolator_with_3_5mm.html

8th item down…

http://www.mixitproducts.com/product_list.php

I happened to have a few old miniature transistor radio transformers so I decided to try making one.  Here’s a view of the completed unit and basic schematic:

I wanted to build it in such a way so it would integrate nicely with my equipment and not have to resort to using a lot of adapters. My radio does not have a separate “line out” so I used the headphone out connector which is 3.5mm stereo otherwise known as 1/8” mini-plug. Essentially it’s a tip-ring-sleeve or “TRS” connector. Since we’re dealing with shortwave and a monaural signal, only one transformer is needed and either left or right channel out of the receiver will do; so you use just one channel, left or right…tip or ring. I’m using the cord & plug side as the input and this plugs into the radio’s “headphone out” jack. For the output side of the box I used a “stereo” 3.5mm jack. I connect a cable from my soundcard’s line-in at the rear of the computer to this jack. This arrangement can be modified as necessary depending on the one’s individual’s needs.

You’ll want to try and use a transformer with a 1:1 turns ratio with an impedance between 600 and 1000 Ohms. I used an old interstage transistor radio transformer I had in my junk box. Unfortunately, I don’t think there’s too many of these still in circulation. Radio Shack used to sell one, PN. 273-1374, they’ve been discontinued but may be in stock in some stores.

The Bourns LM-NP-1001-B1L 600:600 Ohm transformer available from Mouser Electronics looks like a good option, sized at 12.6x17x17.7mm (HxWxL):  http://www.mouser.com/ProductDetail/Bourns/LM-NP-1001-B1L/?qs=ulUe94RKhgtJXLP5iCDxaA%3D%3D

Also, the Tamura TTC-5017 data/voice coupling transformer, available from Digikey, could be used. It’s also 600 Ohm 1:1 and sized at 12x14.5x15mm (HxWxL).

http://www.digikey.com/product-detail/en/TTC-5017/MT7207-ND/674286

I mention Radio Shack mainly because they’re a brick and mortar store here in the U.S. and parts are quickly available although somewhat expensive. For the rest of the project I used RS’ Case PN. 270-1801. I clipped one of the plugs off the cord, PN. 420-080, and wired it directly to the input side of the transformer.   Using a cord gives you a molded-on connector as well as some extra cable for internal wiring. The output jack, PN. 274-249, is wired with both tip and ring connected to the “high” side of the transformer to feed both channels of the soundcard. The “perfboard” I had on-hand but is a trimmed-down section of PN. 276-147.

Radiogram data transmission is pretty noise tolerant but using the audio isolator will help a great deal when  shortwave RF noise level is high by eliminating any additional locally produced noise between computer and receiver.

Happy Decoding!