[Laser] led transceiver

[C. Turner]

Hello Martin,

The trick of resonating across the diode's capacitance works quite well 
if relatively narrowband results are needed - or if the Q is adequately 
low.  This is exploited in Max Carter's FM laser link:

http://www.maxmcarter.com/lasrstuf/lasermodulator.html

It is also used in one of the receivers used by the Germans for their 
Laser TV linking (although they apparently use Photomultipliers for more 
serious, long-distance work) and I've seen it used yet again on an Azden 
IR wireless mic system that used a pair of FM carriers in the 2-2.5 MHz 
range.

While it may be that pure reactances can be made to be "noiseless", the 
"D" of the photodiode's capacitance and the "Q" of the resonating coil 
are not and in truly broadband cases (such as high-speed data or video) 
obtaining the needed bandwidth can be problematic:  Traditional 
resistive "Q" spoiling will decrease the signal from the photodiode 
output and/or increase noise and more-broadband types of matching 
networks are a bit tricky to design and can be rather lossy.

When this is done, one has the advantage of having a limited tuning 
range by adjusting the back-bias on the diode:  The BPW34 will vary from 
around 70pF at zero volts to around 10-15 pF at its maximum voltage and 
that makes for a rather wide-range varicap.

***

In the Azden system mentioned above they use a simple "T" type matching 
network on a JFET source-follower - followed by an emitter-follower.  
The capacitance of the photodiode formed the input capacitance and a 
variable capacitor on the other side was used to "tune" the circuit.  
This particular circuit has zero voltage gain and could be considered to 
be more of a line-driver, matching the output of the photodiode to the 
75 ohm coax used to provide power and conduct signal to the receiver.

This source-follower technique sidesteps one aspect that sets the 
frequency limitation of the K3PGP receiver- and that is Miller 
capacitance, which causes additional, aggressive, high-frequency 
rolloff, but that wasn't of serious concern considering the purpose for 
which the circuit was designed.  A more-appropriate circuit for 
high-frequency purposes would be the use of cascode and bootstrapping to 
minimize these effects:  A cascode is used on my Version 2 and 3 
circuits to good effect, but bootstrapping was not used owing to the 
fact that it would have introduced noise sources - and low noise was 
paramount in the design.

***

More recently I've been doing tests with an APD version of my "Version 
3" receiver.  As with other as-built circuits, I have an output from the 
first unity-gain follower stage, before the pre-emphasis (and thus, not 
subject to the intentional lowpass-filtering of the emphasis stage) and 
with a high APD bias voltage (near the maximum for the APD) it was quite 
sensitive at several MHz, easily able to receive noise-free 
frequency-modulated signals from weak, indirect bounces across the room, 
eventually falling flat at about 10 MHz.  I'd not really made any 
attempt to optimize its operation at such frequencies and were I to do 
so there would be several design changes (perhaps broadly resonating the 
APD, reworking the cascode, using higher-speed amplifiers, bootstrapping 
- any of which would have seriously degraded its performance at audio 
frequencies) but it was an interesting experiment nonetheless.

Keep experimenting!

73,

Clint
KA7OEI


> Hello - another long time lurker puts his head above the parapet!
>
> When I was playing with radio over light systems several years ago, I
> used a high Q 455kHz IF coil and a reverse biassed BPW34 exciting the
> tuned primary. Then I took a lo-Z link straight to a receiver (Kenwood
> R5000 IIRC) or buffered the tuned circuit with an FET source follower
> and then to the RX.
> The transmitters were LEDs or laser pointers modulated with a PIC CW
> sender and a Weaver SSB Tx using a ceramic resonator as oscillator.
>
> I reasoned that I could tune out the photodiode capacitance and the coil
> DC resistance would take care of ambient and 100Hz light. It seemed to
> work OK but I never had the patience to extend the range beyond a few
> Km. I gave a 'daylight' demonstration at the Hereford radio club in
> about 1999 but I haven't used it since.
>
> I'd like to know from the experts what fundamental penalties this system
> suffers in comparison to a photovoltaic K3PGP-type receiver with a
> similar BW? Is it more than just the increased pd leakage shot noise?
>
> Regards
>