[ARDF] LZ1PPL ARDF receiver for 3.5 MHz

[Jay Hennigan]

On 8/15/19 07:09, Bruce wrote:
> Just keeping the thoughts going :)

> Those are indeed good points for a receiver that travels the globe
> following various ARDF competitions.
> 
> However, I suspect most receivers spend much of their time at minor local
> events, particularly a simpler, lower cost receiver such as this. I'm
> thinking maybe a bunch of receivers owned by a club for loan to beginners,
> for which this would be an ideal candidate.

I guess it depends on the frequency of use and the power consumption of 
the receiver. My Ukrainian 80M receiver lasts a very long time on a 
9-volt alkaline, dozens of hours and several hunts.

> In those use cases, a rechargeable setup is much easier to maintain, and
> could charge all receivers at once. Replacing all the 9V batteries for
> every event in that case would rapidly become expensive and inconvenient.
> Anyway, it's more just an example circuit to use whatever bits of it you
> wish.

Agreed. I doubt if the form factor of lithium batteries will match a 
standard 9V alkaline, so it's going to be either/or and not likely a 
universal solution.

> I wonder if you could use a USB cable to 3.5mm cable for 5V charging,
> making it something easy to supply anywhere?; you've likely figured out an
> international solution for charging your mobile phone anyway.

USB is pretty much everywhere so yes, a small boost converter would work.

> You may be right that I have misinterpreted the circuit. I assumed the
> sense went directly into the BF999, and then got mixed into the loop after
> that with some RF gain (and on/off switching)  provided by the little BF999
> PCB.

I suspect that there's an error with the schematic of the sense circuit. 
The drain of the BF999 is shown going to PAD4 which is ground, but it 
has the designation 3t (three turns on the loop, perhaps). Source and 
gate also are connected to ground through resistors. PAD5 which looks to 
be the switched DC supply for the sense is capacitively coupled to the 
gate, so there's no power supply to the FET. And where does the sense 
antenna itself connect? Perhaps the drain is supposed to go through the 
3-turn loop to PAD5 and the "Shtir" connection goes to the sense 
antenna. If so your earlier observation is right and the FET has no 
selectivity on its input. Of course it's a tiny antenna, but that means 
resonant at VHF/UHF which could overload the FET.

> Myself, I pretty much always use whoopee for sniffing, only listening to
> the modulation to ensure I'm listening to the right signal (and with ARDF
> that's not in question anyway). I also rarely (almost never?) use null DFs;
> peak hunting is just easier / quicker, and, with whoopee, nearly as
> accurate. Also easier for beginners to grasp - just like a 2m sniffer!

Probably personal preference, I've always used the null, but on the 
other hand have never had access to an 80m unit with whoopie.

>> I like the synthesizer vs VFO, but it looks like the frequency is
>> determined by a pot driving an analog input to the microprocessor. This
>>
> Agreed.
> 
>> Perhaps this pot is only for the case where you aren't using memories? (in
> which case not much point in a synth anyway?).
> I guess you could use a scheme where you set the frequency based on the
> memory and record the pot voltage at the time you press a memory button,
> and then the pot is used to only slightly tweak the frequency up and down?

As I understand it the pot feeds an ADC in the micro that then uses the 
resulting digital value to set the synthesizer frequency in 100 Hz 
steps. 100 Hz should be adequate resolution. You wouldn't need to store 
the pot voltage, just push the digital value to memory. I'm guessing 
that's what he does, probably a long push to store and a short one to 
retrieve. I don't see source code in the documentation to verify any of 
this.

> (supply variations then would be unlikely, as a %, to cause a 100Hz step)

To tune from 3.5 to 3.6 MHz would be 1000 steps of 100 Hertz, so a 0.1% 
shift in voltage would bump it a step. A flaky pot with a worn spot 
could also be trouble. I think the documentation says it's a 10-bit ADC 
for 1024 steps. I don't see any kind of analog fine-tuning, so digital 
100-Hz steps looks like what you get which would be adequate. One could 
shift to a finer step in software I suppose. I think it needs a micro 
with a few more I/O pins or a multiplexing scheme. I'd use a rotary 
encoder with a push-in function for tuning and eliminate the pot. Push 
and turn to tune, release to hold frequency, or push for coarse and 
release for fine, say 500 Hz and 50 Hz.

Of course it would make things more complicated, but with a more 
versatile micro you could then add an LCD display, generate the whoopie, 
etc.

Note that the analog-tuned variant has a varicap to tune the ferrite 
antenna across the band tracking the LO. This is lacking in the 
synthesized version. A micro with an analog output could restore this 
functionality to the synthesized unit. You could use the pot but that 
wouldn't work for memory channels.

It looks like you could use the same basic receiver design for both a 
no-frills but high performance analog-tuned VFO receiver and a 
bells-and-whistles synthesized microprocessor box with memory and display.

I'd still like to see some filtering between the squarewave clock 
generator and the mixer LO input on the synthesized unit. I wonder if 
he's going with high-side or low-side injection.

Ken, is LZ1PPL following this thread?

-- 
Jay Hennigan - jay at west.net
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV