[ARDF] LZ1PPL ARDF receiver for 3.5 MHz

[Jay Hennigan]

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 can't imagine an ARDF receiver that attenuates only the audio
>     being much use at all (as mentioned in the text) !  You might get
>     away with it for FoxOrs maybe, but for ARDF it'd just be useless.
>     Hard to believe.

The schematic shows a 10K pot RV1 connected to G2 of both the RF input 
FET and the IF amp so this one indeed has a real RF gain control that 
should have plenty of dynamic range. There's some pretty junky stuff out 
there, I wouldn't be surprised if there are some units that just 
attenuate the audio.

>   * 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 
worldwide, etc. In addition you need to worry about battery degradation, 
travel restrictions for lithium batteries, etc. A decent 9v alkaline 
battery costs $2 or less and you can get them worldwide.

Agreed that you don't need hi-fi bass response, in fact that could be a 
bad thing in terms of static crashes, etc. Commonly available earbuds 
are designed for music and that's all about the bass these days. You 
really don't need audio response below about 300 Hz. Even FM 
communications receivers roll off there to prevent CTCSS from being 
annoying.

>   * The sentence "The capacitors C49, C53, C54 and the resistor R46 are
>     used to tune the ferrite antenna." appears twice.

Yep. Minor edit needed.

>   * 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.

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 
mode would be of much value as the audio level conveys the same 
information. The ear will detect small changes in pitch more easily than 
small changes in volume so I suppose there might be some benefit to it.

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?

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 
means that small supply voltage variations will affect frequency so 
you're really kind of back to VFO-ish behavior. Random 100-Hz jump 
changes in pitch would be more distracting to me than typical VCO drift. 
I suppose the memory function will prevent this if used. As long as 
there's a microprocessor and synthesizer I'd like to see a digital input 
for tuning, perhaps a rotary encoder, and more than two memories. For a 
sprint and some fox-or events multiple frequencies are used.

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

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