A 2.3GHz to 432MHz
receiver converter
1.
What is it good for?
It
is a very small receiver converter that can be plugged to the backside
of the battery powered portable transceiver FT817 from Yaesu. It runs on
the batterys of the transceiver (no changes to the transceiver necessary).
So the FT817 together with the converter forms a very small, high perfromance
receiver for 2.3GHz amateur radio signals. The system is ideal for portable
and/or outdoor usage. Normal setting is that 2320MHz is converted down
to 432MHz, but with only a minimal change in the software of the microprocessor
in the converter, it can also be used for satellite (OSCAR) signals.
2.
How does it look like?
The
saying goes that a picture tells more a thausand words, so please have
a look on my picture gallery.
Closeup
of the converter on the backside of the FT817
Another
closeup from a different angle
Just
the converter, showing the BNC and mini-DIN connectors to the FT817
3.
How does it work?
Well,
the priciple of converters is always the same: The incoming signal is amplified
first in a LNA (low noise amplifier). Then it is filtered to avoid unwanted
mixing products later on (the so called image-frequency is suppressed).
After another amplifier, the signal is fed into a mixer where it is mixed
with a local oscillator signal. One of the mixing products is the input
frequency minus the local oscillator frequency. That product is fed to
the FT817 where it can be received just like any other signal that has
never been on 2.3GHz.
Most converters
us a crystal oscillator signal that is mulitplied a couple of times to
generate the local oscillator (LO) signal. The advantage of this principle
is an extremly clean LO signal. The disadvantage is the large boardspace
that is required for the filtering after each multiplication stage
and the fact that the LO signal is fixed on one frequency. In my approch
I have used a 13MHz crystal oscillator signal as a reference, a 1880MHz
VCO and an integrated PLL IC to generate the LO signal. A small microprocessor
tells the PLL which frequency it has to generate (this happens only once
at power-on, then the micro falls asleep and does not consume any significant
current). So it is just a matter of changing the software and retuning
the adjustable input filters to change the frequency, that can be downconverterd
to 432MHz for the FT817.
Here
is a block schematic for a better understanding of the explainations above.
Ok,
now lets talk about the building blocks that are mentioned in the block
diagram. First of the all: the LO signal generation. A 1880 MHz VCO (Voltage
Controlled Oscillator, sensitivity around 45MHz/V) is controlled by a National
Semiconductor PLL IC LMX2320. The PLL is programmed by an Atmel AT90S1200
AVR microprocessor. The AT90S1200 has the RCEN (RC-oscillator ENable) bit
set, so it does not need any external components, no crystal, no resonator,
real zero component count. The AT90S1200 can be ordered with that bit set
(it is called AT90S1200A then), or you can set that bit yourself. It is
stored in a non-volatile memory, so it will stay set, even without supply
voltage. A circuit for setting
the RCEN bit is extensively explained here.
Have
a look on the schematic to see how the PLL, the VCO, and the processor
are connected.
The
LNA and the buffer amplifier are build using BFP420 bipolar RF transistors.
At the chosen bias point, they have their best impedance for minimal noise
right at 50 Ohms (at frequencys around 2.4GHz), so no input matching is
necessary. For filtering the signal, I have used readily available helix
filters from Neosid. The mixer is an integrated downconversion IC from
Maxim, the MAX2680.
Here
is the schematic of the amplifiers and the mixer.
4.
How is made? Is there an etched board?
The
converter is handmade as a prototype, just one part for myself. And no,
there is no etched board. And you do not need an etched board for 2.3GHz
designs if you do not use stripline technics. Just make everything as tiny
as possible and 2.3GHz circuits are no black magic. Making everything so
small may not look so nice, but it works very well. For all of you that
have a hard time to imagine how it may look like, I have a few more photos
here:
Top
side of the board (VCO, PLL, amplifiers, mixer)
Bottom
side of the board (reference oscillator, microprocessor, helix filter)
The
microprocessor is in a DIP20 package. A SMD package would have been also
available, but that costs a few cents more and the DIP part did also fit.
So I used the PDIP chip and mounted it dead-bug stile, i.e. glued down
with the legs in the air. So it looks like
that.
The
reference oscillator and the helix
filter are mounted just as defined by the manufacturers.
The
mixer and the componentes around it are done as compact SMD design.
From
the LMX2320 PLL IC I have broken off the
pins that I did not need. This way it was not very tricky to solder to
the remaining pins of that fine-pitch IC.
The
amplifiers may be a bit hard to recognize, therefore I have marked
them in the picture. The helix filter is mounted from the backside between
both amplifiers.
5.
So, what about the software for the microprocessor?
The
software if of course also available, even with the source code for those
who want to experiment with it
