Re: Simple UBITX Test Set

Gordon Gibby

Ashhar, do you think it would be worthwhile to make a small circuit board and sell that circuitry as an optional test unit?

On Oct 25, 2020, at 15:00, Ashhar Farhan <farhanbox@...> wrote:

I can document what we use at HF Signals. But , in effect it is like this ...
1. We have a dummy load (20, 1K 2/watt resistors) and an Rf sniffer with a 10K and a 50 ohms in series. the 50 ohms goes to the oscilloscope.
2. A 10 MHz crystal oscillator with a trimmer to net it exactly to 10 MHz. we calibrate it every month.
3. A zener diode biased with 1 ma of current as a noise source. it amplified by two stage of feedback amplifier and feb back to the transceiver through the dummy load through a 10pf. there are two 1N4148 back-to-back diodes to prevent high RF from entering the noise amplifiers. 
in rx mode, we can net the frequencies against the 10 Mhz oscillator.
the noise source allows us to set the BFO using the online BFO alignment tool.
on tx, we can monitor the RF to check for proper modulation.
We check it on 3.5 Mhz, 7 Mhz, 14 Mhz and 28 Mhz to exercise all the relays and LPFs.
- f

On Mon, Oct 26, 2020 at 12:22 AM Jerry Gaffke via <> wrote:
A standardized way to debug a uBitx would be a big plus for this forum.

Some kind of RF source is required that hits the operating frequencies of interest
plus the two IF frequencies would be good.
Perhaps using harmonics from a crystal oscillator is sufficient, though not always ideal.
Colorburst crystals are now archaic, but easy enough to get.
Plan B might be a $1 Si5351, plus of course an extra $10 for Nano and display and knobs.

VK4PP laid out a Raduino replacement with pads for an AD8307, though far as I know
nobody ever tried using the AD8307.
That looks like a really noisy spot for the AD8307.
With proper layout (perhaps a smaller processor) this could be made to work.

Jerry, KE7ER

On Sun, Oct 25, 2020 at 09:50 AM, Scott McDonald wrote:
You're right, I think, a simple cheap standardized UBitx test set would really be a great tool, not just for a Ubitx but for anyone starting out without any RF test equipment.
I started down that road for a club talk last year and built a single 2N3904 oscillator with a color burst crystal that gave signals in or very close to all the HF bands except 60 meters.  
By juggling a few values I was getting reliable output on all those freqs at -20 dBm plus or minus probably 5 dBm, and this was reproduceable with 5 different color burst crystals (tho from the same batch).  Not enough to drive a diode ring mixer but otherwise pretty useful and consistent, I think close enough that you could count on the output being what you expected without measuring it.  
As an option adding a buffer with another 2N3904 would make it more useful, or maybe just use one of Diz's inexpensive new SMT HF amp kits instead of reinventing the wheel.
Added an IF crystal from Kits and Parts (it was for a V3) with a cap and coil to fudge it a bit and got a useful IF signal at the same level. 
Added a couple 50 dB Pi attenuators to give a choice of 50 over 9, S9 and a few tenths of a microvolt for signal level.  Standard resistors used that way yielded decent attenuators plus or minus 3 or 4 dB.
Did it on a piece of perf board using 0.1 inch headers and jumpers to switch crystals and attenuators so there were no switches or other hardware needed.
It was dirt cheap and pretty handy, adding the diode detector would be simple, and the attenuators possible useful with that, same for adding an audio generator.  That's got to be a 10 dollar or less project with bulk parts purchases.
It would make a great first build project too.  
If I had any board skills I would make a board and put in Osh Park... :)  But if anyone with some skills wants to collaborate on it, I'd be happy to get together, as I expect people troubleshooting with a known test kit would make the great advice you, Evan, Allison and others give on here even quicker and more powerful.  
73 Scott ka9p

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