30m QRSS Grabber Receiver
The basic design is a mix of ideas gathered from pa2ohh, the ARRL
Handbook and publications made about the subharmonic mixer of Prof.
Polyakov (ra3aae).
Since I am a really lazy dawg, thinking came first...
What we want to observe is 10140000 to 10140100Hz. Let's see what is
available.... 10.140MHz crystals, close, but too close for comfort.
Pull those crystals, and oscillators will become less stable. 10.125MHz
crystals... too far off! 10.135MHz, closer, possible, but still pretty
far off; 5kHz, oh my goodness!
But we also got really cheap and well available crystals and
oscillators (!) at 5.0688MHz. Multiply by two and we are at 10.1376MHz,
that's just 2.4kHz off the lowest frequency we want to receive.
2400-2500Hz also is in the comfort zone of any random soundcard at even
low sampling rate.
To use this particular frequency, all we need to do is, multiply by
two, or in other words, double it.
Jan (pa9qv/oz9qv) pointed me towards Prof. Polyakov's mixer, also known
as the Russian mixer. Great readings from late la8ak, check this out: http://noding.com/la8ak/c21.htm
Thus, there we are, ultra-cheap 5.0688MHz CMOS-oscillators combined
with a Russian mixer receives the 30m-qrss-band at about 2.4kHz. Let's
design something!
Final design
A Polyakov mixer is supposed to work best with a sine signal. The
CMOS-oscillator's signal however looks more like a squarewave,
involving a lot of harmonics. The easiest and cheapest way, as it
seemed to me, to transform that into a sine, is a crystal in series
resonance.
The unwanted lower
sideband has been taken care of by including a 10.140MHz-crystal
behind the preamp.
Behind the filter, a RF-amp is added to compensate for losses. Due to
this, the filter response is now shifted up again, I will look into this, but
for now, I will let the receiver run and collect data/info.
As promised, I looked into it, shifting a crystal up is done by
capacitive load. Reducing this load will bring the crystal closer to
its series resonance frequency. To be a little bit on the adjustable
side this time, I added a trimmer found in my junk box. I thought it
would have a max. capacity of 18pF. This trimmer allows for adjustment
of the crystal filter pass band frequency.
Consideration for operations w/o the filter: we are looking at a
bandwidth of
100Hz and the received LSB will be (about) 10135100Hz to
10135200Hz. I figure, there is just the occasional Feldhell QSO,
but other than that, the LSB
range of the DSB receiver seems to be an empty spot.
A jumper is added in order to enable support of an active antenna via
coax cable.
The ideas for an active aerial are floating presently. It will be a
shielded resonant loop with a preamp, that's for sure.
The loop will be made from "YMVK-as 2x2.5mm²" which is underground
twin-lead mains
cable, doubly shielded. In an earlier experiment I successfully made a
40m-75m RX-loop
from that stuff.
Photo of the 30m direct conversion receiver
Audio mod
OK, the receiver is now receiving successfully since months, I will therefore
not change it. Should I build it from scratch now, I would replace the transistor
audio stage with one that employs a low noise operational amplifier. In a similar
design to the above receiver, I used a TL082 dual opamp, needed two channels...
I figure a TL081 will do fine for a single channel.
Note, the schematics does not show the supply and ground connections to the op-amp.
Last modified 10.01.2010
