[ARDF] LZ1PPL ARDF receiver for 3.5 MHz

[Bruce]

Just keeping the thoughts going :)

On Wed, 14 Aug 2019 4:16 pm Jay Hennigan <jay at west.net> wrote:

> On 8/13/19 22:06, Bruce wrote:
> > On 14/08/2019 5:16 am, Kenneth E. Harker wrote:
> >>         Plamen LZ1PPL is looking for feedback on his latest 3.5 MHz
> >> ARDF receiver project.  The article itself is reasonably technical,
> >> for those interested in receiver electrical design.
> >>
> >> http://www.lz1ppl.com/projects/ardf-fox-huntreciver-3-5mhz/
> >> <http://www.lz1ppl.com/projects/ardf-fox-huntreciver-3-5mhz/>
> >
> > Some random feedback ideas:
>

> >   * I wonder if you could reduce the number of plugs (and hence water
> >     entry points into the receiver) by charging using the headphone
> >     connector. The only reason you couldn't do this with the existing
> >     circuit is the 100uF isn't bipolar. You could make a cheap /pretend/
> >     bipolar by using two 100uF in series with opposite polarity, and a
> >     high-R to gnd in between. In fact, you probably don't need such a
> >     large cap really for headphones. Perhaps 2x33uF (effectively 16uF)
> >     would do for bass response.
>
> Personally I prefer a 9-volt primary battery that is replaced before the
> competition as opposed to a rechargeable battery. A charger is one more
> thing to break or lose, it has to be compatible with voltages and plugs
>

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.
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.

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.
The charger becomes a bit more complicated as you'd need a boost converter
to charge the two series batteries (or a scheme to electronically 'rewire'
the batteries in parallel when charging), but such converters/chargers are
pretty common. Or even on prebuilt PCBs from eBay.

>
> >   * Could the BF999 overload from off-frequency (or out of band) large
> >     signals, and cause spurious responses, as it effectively has no
> >     tuned circuits protecting it ?
>
> The ferrite loop and associated capacitors, etc. is a tuned circuit. The
> documentation tells you that twice. :-) I think the sense is coupled via
> the 3-turn winding but I'm a bit confused about the BF999 and associated
> parts. That's obviously the sense circuitry, but I don't see the sense
> antenna itself on the schematic. PAD4 is ground and PAD5 appears to be
> where the sense switch turns on the BF999. Perhaps the sense antenna is
> connected to the other side of the 3-turn winding on the ferrite??? If
> so the ferrite resonance will offer some selectivity. I don't see any
> kind of trimmer to set the sensitivity of the sense. Perhaps the antenna
> length itself is used for this.
>
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.

>
> The tone generator seems to be generating a whoopie by simply rectifying
> the received audio to drive a VCO. An IF chip with a logarithmic RSSI
> output would be more useful in my opinion. I'm not sure whether the tone
>

I've used both schemes in the past (nowadays I'd probably get the micro to
generate the whoopee tone, so you can create whatever response you want).
Using the RSSI to feed the whoopee VCO directly results in a logarithmic
(volts proportional to dB), which in practice is a but 'unexciting'. In the
old Ultra sniffer we used another 'resistance' FET and the phase comparison
(spare) section in the 4046 to exponentialise the response, making the
overall a linear response.
It sounds complicated,  but it's only a few extra parts, and better matches
the linear response of the rectified audio used here, but with a much
better dynamic range.

> I've not had a receiver with whoopie on 80M and think it might be
confusing to me unless the pitch is inverted so the highest pitch occurs
at the null. Is whoopie of value on 80 to others?

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!

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

>
> Overall it looks like a good design but like everything it could use a
> few tweaks.
>

A certainly a good open reference design starting point.

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