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DSB & AM on 222MHz with IQ receiver, can it be done? #building #vhf #si5351a #fst3253 #1.25M #fst3253 #ht #building #vhf #si5351a
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Justin Kelly
I'm looking for a way to build a simple single board HT with the Si5351a at it's heart, something that can receive CW,SSB and AM with a simple DSB and/or AM transmitter. the intended use for this would be for Hamfest and other events.
The general idea being to use a band with typically low saturation at these events, so 70cm, 2 meters and the commodity FRS rigs are out. this makes 1.25 meters in the US and 4 Meters outside the US appealing. some cost cutting measures might consist of using a POT as a stand in for a rotary encoder, and implementing a morse code readout of the current set frequency in place of or in addition to an option LCD display. Thoughts, comments? Justin N2TOH |
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ajparent1/kb1gmx
Ok the only more that the raw 5351 can do FM with a lot of signal processing to drive it.
Besides there are plenty of chips that can do that RX and TX already. The 5351 might serve as LO for that. For 1.25M the only other mode is DSB as that can be received on SSB RX AM is troublesome and likely you will be talking to yourself. So for a 1.25M HT you need DSB out and simple RX in using 5351 as LO. For any useful range 50mW (17dbm) out is a good start, a decent RX sensitivity is a must. Then again 2 china FM hts would be cheaper and certain to work on 70CM. I have a baofeng I paid 35$ (UV5R dual band now 4 years old) for and the battery alone is worth that. I have a pair of the old RS HTX-245 HTs that even at 1.5W they cover all of most ham events plus some on UHF. Either fit in a pocket easily. I've built small TRX and those HTs beat that being smaller by a huge margin. Commercial design and packaging is hard to equal. Allison ------------------------------- Please reply on list so we can share. No private email, it goes to a bit bucket due address harvesting |
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Justin Kelly
Allison
I am aware FM is no easy trick without a specialized IC, the goal here is a low cost single board badge style device to get people on 1.25Meters or 4 Meters at an event. for that kinda gimicky thing DSB TX with a SSB RX is fine. with some forethought footprints to add an SSB exciter later might be doable. I am aware of the copious offerings from China at very attractive prices, thing is everybody and their sibling has 2 Meters & 70cm rigs. as such the bands become saturated at places like the Dayton Hamvention! -Justin N2TOH |
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Arv Evans <arvid.evans@...>
Justin N2TOH For small conventions and such, it might work better to have a tiny hand-held HF transceiver. Could be DSB along the lines of a modulated Pixie rig. For near-field communications the antenna could just be a 4 to 6 inch loop or short loaded vertical. A couple of Pixies could be used to trial the concept. If CW works within 1000 feet or so, then the next step would be evolving to include DSB and a cell-phone battery pack(s). This came to mind because of the old pogo-stick cavalry radios. These had a sensitive AM receiver and were good for near-field work of up to a couple miles. Today we could make these to fit in the palm of your hand, instead of hanging off the side of a horse or mule. Arv _._ On Fri, Aug 28, 2020 at 2:59 PM <freefuel@...> wrote: Allison |
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Justin Kelly
Arv
could be done, about the only limiting factor with the Si5351a after it peters out around 290MHz or so is filtering, and antenna options. a creative type might manage a nice loop solution for man portable use, for 6 Meters only 3 to 4 feet of Cheap CATV coax should work nicely. Justin N2TOH |
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Justin Kelly
Now I have to wonder what DSB-SC would sound like if you actively inserted a proportional carrier to keep it at a fixed modulation level. say 99% all the time with audio drive and 0% without.
Justin N2TOH |
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Shane Justice <justshane@...>
Have you guys considered a mesh network topology for use at a hamfest? LORA transceivers are available in the ISM bands and are the size of a postage stamp. Pick 900MHz and there won't be that many hams on there. Use a codec like Speech for excellent quality speech encoding/decoding on a platform like a RPI W, tiny, etc. Use blue tooth mic/earbud. Might throw in VoIP server to connect people in conference mode- pick a group id and assign each person individual IDs so people can be in group or private conversations... Add some high tech to your high tech. 73, Shane Now I have to wonder what DSB-SC would sound like if you actively inserted a proportional carrier to keep it at a fixed modulation level. say 99% all the time with audio drive and 0% without. |
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Shane Justice <justshane@...>
My phone corrupted the codec name SPEEX. It is open source. Pick 7KHz encoding and it is virtually indistinguishable from landline quality audio. 73, Shane
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ajparent1/kb1gmx
Look up the term FREDBOX. basic idea is much like the pogo radio. It can
be built very small. Simple AM RX (many wer diode detector with audio amp) Some used a regen. Matched with low power AM tx (50 to 200mW). I did that for 6M AM using a mix of Jfets (rf amp and regen detector for RX) and a bunch of 2n3904s and 2n2222As for a bootstrap modulated TX of some 80mW (carrier). 2M or 222 even 432 is possible but for the higher frequencies the transistors used will have to be much better and suited for the frequencies. Suggest 2n5176 for very lower power work 2n5109 or similar for power over 50mw. Basically to work at 2M and up you need parts with FT of 1000mhz or higher many Jfets will work there for RX use. DSB is easy, Osc, audio amp (maybe 1 transistor and electet mic) balanced mod (DBM works ) and 2 or more stages of gain for TX power. RX you need superhet to receive DSB or if the other station is SSB a simple direct conversion. DSB-RC still need superhet or maybe Image reject DC frequency locked to carrier. Takes a bit of hard ware to do it acceptably. Allison ------------------------------- Please reply on list so we can share. No private email, it goes to a bit bucket due address harvesting |
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Hans Summers
Fredbox yes! I built one, must be more than 15 years ago now... And it got put on the shelf before I finished adjusting it and setting it up :-/ 73 Hans G0UPL On Sat, Aug 29, 2020, 21:33 ajparent1/KB1GMX <kb1gmx@...> wrote: Look up the term FREDBOX. basic idea is much like the pogo radio. It can |
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ajparent1/kb1gmx
With any tech on any frequency there is the issue of,
will you be talking to yourself. Keep it simple and at most erector set in complexity or you will have cool tech and only taking to your self. Do not ask How I know. The answer is that obvious. Allison ------------------------------- Please reply on list so we can share. No private email, it goes to a bit bucket due address harvesting |
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Justin Kelly
Yes keep it simple!
This is why I was looking at DSB-SC and AM. my intent was to keep it cheap&simple so it could be build in larger numbers for groups. for the higher bands there is this series of projects, https://github.com/travisgoodspeed/goodwatch/wiki for now I'll focus on SSB for 50MHz and up Justin N2TOH |
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Jens Groh
Hi, Justin, I think your idea to possibly use DSB-SC is a good one. But not only for transmitting: If you use a synchronous demodulator for reception, you get the same spectral power efficiency as with SSB, but with relaxed carrier frequency stability requirements similar to AM. The only disadvantage is the AM-like bandwidth, not good for a crowded band. Furthermore, DSB-SC is basically compatible with SSB stations in both directions. For SSB, frequency stability will probably become a issue as you go towards VHF or UHF. Probably feasible, but requires more efforts, like temperature control or GPS synchronization. For synchronous demodulation, consider the "Costas loop" demodulator. It is basically a PLL that locks to the sidebands pair instead of the carrier. (Google finds more explanation.) It should be possible to implement said PLL's frequency adjustment with the Si5351A in a similar way as GPS disciplining is usually done. Alternatively it can be done in the baseband with a kind of "quadrature BFO" around zero Hertz. Preferably all implemented by digital signal processing. Normally you could simply try to use the QRPLabs Receiver Module and the QRPLabs VFO to prototype your ideas, but regarding your idea to use a VHF band, I have some doubts that the FST3253 would be fast enough for the quadrature downconverter. Upconverter likewise. Hmm... Jens
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ajparent1/kb1gmx
On the most general level doing SSSB on 222mhz is trivial as
doing it at 14mhz. The LO is has always been the issue but recent generation chips makes that not at all onerous. Exotic mixers are not needed as the DBM is adequate in an uncrowded band and the amont of IF cain is not all that great and can be made up with audio gain. TX is still the same building blocks and high gain upper VHF and UHF devices are not an issue. Even 432 is not difficult as I've done that one. As to the QCX at more than low VHF no, its great at HF but to get to VHF you throw out far to much of the existing design. Allison ------------------------------- Please reply on list so we can share. No private email, it goes to a bit bucket due address harvesting |
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Justin Kelly
Not exactly the direction I was looking to turn, but this seems to have some merit, https://www.rtl-sdr.com/rpitx-v2-released-easily-record-and-replay-with-rtl-sdr-and-a-raspberry-pi/
-Justin N2TOH |
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