{"id":185,"date":"2018-08-31T16:47:09","date_gmt":"2018-08-31T21:47:09","guid":{"rendered":"http:\/\/ad7i.net\/main\/?page_id=185"},"modified":"2020-01-09T20:30:27","modified_gmt":"2020-01-10T01:30:27","slug":"qst-go-digital","status":"publish","type":"page","link":"https:\/\/ad7i.net\/main\/publications\/qst-go-digital\/","title":{"rendered":"QST: Go Digital!"},"content":{"rendered":"

Thanks for your interest in my \u201cGo Digital\u2026\u201d article in the October 2018 issue of QST magazine. The article text that I submitted to QST was way too long, about eight pages and QST\u2019s limit for publication is five pages.\u00a0 I want to thank the article\u2019s QST editor, Kai Siwiak, KE4PT, for doing a great job of trimming and massaging my submitted text down to size and getting the article in proper shape for publication in QST.<\/span><\/p>\n

As mentioned above, there was extra info that we couldn\u2019t include in the article that might be useful to those who want to construct the Go Digital interface.\u00a0 I\u2019ve placed that info in the area below.\u00a0 Some is based on info that I would like to share with others, as well as responding to questions from readers of the article.\u00a0 With that as background, here we go, in no particular order.<\/span><\/p>\n

The info below was last updated 09-Jan-2020<\/em><\/span><\/p>\n

What USB Dongle Did You Use? \u2013 Or What Dongle Do You Like?<\/strong><\/span><\/h2>\n

I tried several dongles for this project.\u00a0 My only limitation was that I stuck with dongles that cost less than $5.\u00a0 I got all of them, except one, on ebay.<\/span><\/p>\n

The low cost dongle that I liked best is shown in the photo below.\u00a0 I\u2019ve seen these on ebay on Buy-It-Now for anywhere from 80 cents to $4.\u00a0 Not all dongles that look like this on the outside are the exactly same on the inside, but they are fairly similar to each other.\u00a0\u00a0 This particular dongle includes an LED that binks when connected to the computer.\u00a0 It’s a nice feature but the on\/off current draw from the LED caused audio noise in my system.\u00a0 If you experience rhythmic periodic noise in the audio from the dongle that’s in cadence with the LED flashing, then remove the LED and see if that solves the noise problem.\u00a0 It does on the dongles I’m using.
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Testing the USB Sound Dongle<\/strong><\/span><\/h2>\n

When you get a dongle (or several dongles) in preparation for building this project, you should first test the audio input and output capabilities of the dongle as a standalone device, to ensure that it behaves as expected.\u00a0\u00a0 Do some Googling and you will find MS Windowstm<\/sup> programs for sound cards that will function as an audio oscilloscope and signal generator.\u00a0 Two Windows programs that I found useful are Christian Zeitnitz’s Soundcard Scope ( https:\/\/www.zeitnitz.eu\/scope_en<\/a> ) and RingBell’s Tone Generator (\u00a0http:\/\/www.ringbell.co.uk\/software\/audio.htm<\/a> ).\u00a0 With these programs, or similar programs, you can see how much audio AC voltage output you can get from the dongle audio output before clipping occurs with a low resistance load.\u00a0\u00a0 Use the oscilloscope function to see how much audio AC voltage you can apply to the microphone input before it clips.\u00a0 An interesting thing about the white dongle that I used is that the Windows 10 device driver (that driver seems to be included as part of W10, I didn\u2019t install any special drivers for this dongle) shows an option for AGC.\u00a0 With AGC option selected in the device driver the microphone input would clip with about 30 mV rms at the input, but with AGC option un-selected the dongle became fairly deaf, requiring about 1 V rms before clipping would occur.<\/span><\/p>\n

Below is a test fixture that I used to test USB sound dongles I was considering using for my interface.\u00a0 To test the audio output of the dongle I would connect TP1 and TP2 to an oscilloscope to observe output level, clipping and distortion, when using a Windows-10 signal generator program.\u00a0 To test the audio input I would connect an audio signal generator to TP3 and use a Windows-10 audio oscilloscope program to determine the input level at which clipping would occur.\u00a0 The two pin headers near TP3 allowed me to select different attenuator values, depending on the capabilities of the audio generator I used.<\/span><\/p>\n

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Attenuators<\/strong><\/span><\/h2>\n

Do you really need to have 12 steps of attenuation at 5 dB per step?\u00a0 Well, no, probably not.\u00a0 But with that range of possible attenuation you are almost always going to be able to sufficiently knock down whatever signal comes from your radio receiver, even if it\u2019s driving 5 watts into an 8 ohm load.\u00a0 Although it takes a bit of time wiring the step attenuators (maybe 15 minutes each) they are cheap to construct.\u00a0 I found 12 position rotary switches on Ebay for about $1 each.\u00a0 Those $1 rotary switches are not high quality, so I wouldn\u2019t expect them to survive 100,000 operations, but in most cases once you adjust the attenuators for a particular radio, further attenuator adjustments will be infrequent.\u00a0 One alternative to the step attenuators that\u2019s worth trying is two cascaded potentiometers; one for coarse control and the other for fine control.\u00a0 Given that both the radio\u2019s speaker driver and the dongle’s audio output driver are intended to drive relatively low impedance loads, we can use the shunt resistor on the wiper trick to get linear pots to function somewhat like log taper pots, which will increase the range of controllable attenuation.\u00a0 For the coarse control I\u2019d suggest a 100 ohm pot with a 22 ohm wiper shunt resistor, and then the fine control pot (the second pot) consisting of 1000 ohms (high side connect to the wiper of the 100 ohm pot) with a 220 ohm wiper shunt. \u00a0I haven\u2019t yet tried this, but if you try it, please let me know how it works for you.\u00a0 One issue with this approach is that it\u2019s hard to find inexpensive 100 ohm pots.\u00a0<\/span><\/p>\n

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And here’s a link to a terrific article about potentiometers:\u00a0<\/span> http:\/\/www.geofex.com\/article_folders\/potsecrets\/potscret.htm<\/a><\/p>\n

Another lower cost alternative to obtain variable attenuation would be to use a 10K pot connected to an inexpensive LM358 op-amp.<\/span><\/p>\n

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One might even want to consider an IC based digital potentiometer with push-button up\/down controls (for one example, see https:\/\/datasheets.maximintegrated.com\/en\/ds\/DS1802.pdf<\/a> ).\u00a0 There are many ways achieve the needed loss while allowing user control.<\/span><\/p>\n

One other consideration for the attenuators:\u00a0 I found that I didn\u2019t need a \u201cfine\u201d control for the receive path.\u00a0 Getting the level to within 5 dB seemed to be good enough on receive.\u00a0 For the transmit path, I like having a fine control knob so that I could easily and quickly adjust my transmit power up or down to hit 20 watts of output power on the nose.\u00a0 I realize that both WSJT-X and fldigi<\/em> provide an on-screen slider to adjust TX audio (and thus RF transmitter power), but I find it easier to grab and rotate a physical knob rather than using a mouse to move an on-screen slider.<\/span><\/p>\n

CW Transmissions<\/strong><\/span><\/h2>\n

In the Go Digital QST article I didn\u2019t say much about CW keying.\u00a0 But I would like to point out some things about using the interface on CW that are radio specific.\u00a0 I designed the interface for use with my 30+ year old TenTec Omni C and TenTec Triton IV HF transceivers.\u00a0 With those radios the Key input is ignored whenever the radio is *not* set to CW mode.\u00a0 So it was no problem to leave the interface key output connected to the radio\u2019s key input.\u00a0 The interface would only activate the transmitter via the key input when the radio was in CW mode.\u00a0 Life was good.\u00a0 I\u2019ve subsequently tested the interface with a modern radio, a Yaesu FT-891.\u00a0 With that radio, regardless of the \u201cmode\u201d the radio is in, if the key input is active the radio will transmit CW.\u00a0 That\u2019s not a bad thing; actually it\u2019s probably a good thing.\u00a0 But, because the Go Digital interface\u2019s key output is active whenever the interface engages the PTT for digital modes, you may want to either add a switch to your interface in series with they key output (to disable it when you don\u2019t want to send CW) or just disconnect the CW plug on the back of your radio when you don\u2019t want the interface cause your transmitter to send CW.<\/span><\/p>\n

Non-Polarized Capacitors<\/strong><\/span><\/h2>\n

Do you really need non-polarized capacitors for C3 and C6?\u00a0 Maybe not, but when I don\u2019t absolutely know for sure what I might be connecting the interface audio ports to, I like NP caps on the audio input and output.\u00a0 I found some inexpensive ones on ebay, but you can also get them from Mouser or Digikey.\u00a0 Do you really need the 470K resistors across C3 and C6?\u00a0 Yes, you really should include those.\u00a0 But the resistors don\u2019t have to be exactly 470K.\u00a0 Anything from 47K to 4.7M is likely OK.\u00a0 One of the basic rules for any electrical design is \u201cno floating metal\u201d, and that includes the net between two capacitors connected in series.\u00a0 We don\u2019t know for sure, in every case, that the interface will connect to a radio with a DC path to ground for both its input and output, so we need to provide one, and that\u2019s the purpose of the 470K resistors.\u00a0 In the case of most speaker outputs from modern radios, those are almost always capacitively coupled.<\/span><\/p>\n

Monitoring Your Transmit Signal<\/strong><\/span><\/h2>\n

Regardless of what you are transmitting (CW, SSB, Digital Signals) it\u2019s always helpful to have a way to monitor your signal.\u00a0 An easy was to do that is with an inexpensive Software Defined Radio module connected to your station computer, an old laptop computer or even a Raspberry Pi.\u00a0 To monitor my transmit signals I use an SDRplay RSP1A connected to an old laptop.\u00a0 That allows me to monitor the modulation of my transmitted signals and spot any problems that might crop up.\u00a0 You can also ask a nearby station to monitor your signal with any program that shows a waterfall.<\/span><\/p>\n

RFI Problems?<\/span><\/strong><\/h2>\n

One of the best indications that you have an RFI problem is when the RF power does not smoothly increase or decrease as you smoothly adjust R15, the fine level control on the TX attenuator.\u00a0 If you think you have RFI problems be sure to double check that your transmitter and your interface are connected to a common ground point by means of short ground wires, something relatively fat like #14 or #12 solid conductor (don’t rely on the ground wire in the audio cable that connects the radio to the interface for RF grounding, and don’t use braided wire for RF grounding).\u00a0 Keep in mind that for RF ground wires of the same diameter, solid wire is almost always better than stranded wire.\u00a0 I spent hours trying to resolve an RFI problem with one of my interfaces, only to find that I had inadvertently left the ground wire on the radio disconnected;\u00a0 when I reconnected the ground wire to the radio the RFI was gone. \u00a0 If simply having proper ground wires in place are not enough to solve your RFI issues, then Ferrite mix 31 is usually a good place to start for mitigating RFI problems in the HF range.\u00a0 FT240-31 torroids will usually let you pass a few turns of cable through them.\u00a0 Only use the clamp-on style of ferrites when using a torroid is not an option (usually due to big connectors at both ends of the cable that won’t fit through the torroid).<\/span><\/p>\n

—ooOoo—<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Thanks for your interest in my \u201cGo Digital\u2026\u201d article in the October 2018 issue of QST magazine. The article text that I submitted to QST was way too long, about eight pages and QST\u2019s limit for publication is five pages.\u00a0 … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":146,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-185","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/pages\/185","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/comments?post=185"}],"version-history":[{"count":42,"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/pages\/185\/revisions"}],"predecessor-version":[{"id":282,"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/pages\/185\/revisions\/282"}],"up":[{"embeddable":true,"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/pages\/146"}],"wp:attachment":[{"href":"https:\/\/ad7i.net\/main\/wp-json\/wp\/v2\/media?parent=185"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}