BLOCK DIAGRAM AND DESCRIPTION
The Mix Unit we can build with a modified LNB needs few
other blocks to make a working transverter. The most important is a
precise and low phase-noise Local Oscillator (LO), then we need a PA and
an antenna relay. I also added the automatic RX/TX switch over the IF
cable (requires a modification in the IF transceiver), an output power
indicator that also serves as a DC voltage check, a remote control
connector (for pole mounting in fixed station) and a PLL to lock the LO to
an external 10MHz frequency standard. Here is the block diagram of the
Let's now see how each section of the transverter has
As told before, the MIX Unit has been made from a modified AE2 Cambridge LNB.
Each block is indicated on the photo of the PCB (shield is removed), and
for some of them I give some comments.
20dB attenuator: it is in place of one of the
IF buffers, and works in both RX and TX. It is needed to reduce the power
from the IF transceiver to a safe level for the RX/TX diode switch. The attenuation it
introduces in RX is recovered by the gain of the IF RX buffer (~20dB).
sustain the 1W max power from the IF (0.5WPEP operating), the 55ohm
resistor is made with 4x220ohm 1/2W SMD resistors in parallel.
RX/TX Switch: the original switch was a
matrix arrangement, that has been simplified to a SPDT arrangement.
Original design significantly cuts signal below 700MHz, so, to
work well at 432MHz, two stripline chokes have been replaced with SMD RF
6dB attenuator: it is in place of one of
the original IF buffers (mark C1H) to further reduce the power from the IF
transceiver to a suitable level for the TX mixer (~2dBm). Low power SMD
resistors are OK.
TX Mixer: it works reversed, but no
modification is needed because diode mixers are bi-directional. The little
piece of fiberglass laminate is glued over the stripline balun for a
better balance, in a position where the maximum LO rejection is obtained.
This trick is not needed for the RX Mixer
Image BPFs: one for RX and one for TX.
Both have been re-centered to 10.5GHz using Polyimide adhesive tape as
TX Buffer: it is a gain block that has
been reversed as described here. The GaAsFET is
not the original one and Drain resistor has been reduced to 27ohm to get a
little more power. Two tiny copper foil stubs have been used to optimize
gate and drain impedance matching.
TX Driver: another reversed gain block.
The GaAsFET is not the original one and Drain resistor has been reduced to
10ohm. Luckily no retuning has been needed to reach the target output
RX LNA: these two stages have not been touched
until an upgrade was done. I have just added a 1/4lambda short to GND at the RX IN connector
for ESD protection.
IF RX buffer: also this stage has not
been touched. Its gain (~20dB) recovers the losses of the 20dB
LO splitter: the only modification has
been to put the SMA connector to inject the LO frequency. The components
of the original DR Oscillator have been removed.
GaAsFET bias: apart of the Gate/Drain bias wires
swap for the two reversed gain blocks in the TX path, this block has been
modified to properly activate the RX and TX RF stages together with the
PIN diode RX/TX switch and the Antenna Relay. This modification is quite
simple, just requiring to selectively apply the Vbb to the PNP bias
transistors using the inverters of the 74HC14 (see the two 8.2K resistors
and the two brown wires).
Unfortunately I did not have any LO source ready, like a cavity multiplier,
so I had to build a good 10GHz LO source from the scratch, and this took
the larger part of the time I've allocated to the whole project. My
approach has been a "counter-stream" design: from the final
frequency to the oscillator because I had no previous experience on
10GHz multipliers and I tend to solve first the biggest problems...
Once determined that the optimal LO power for the Mix
Unit was something between 2 and 4dBm, my idea was to
reach a 2.5GHz frequency by affordable multiplications, further multiplied
by 4 with a portion of LNB implementing a saturated gain stage
followed by a stripline filter. This is very similar to what proposed by
G4HJW, so I made some experiments, discovering that
this kind of multiplier is quite deaf and critical, but
can work, provided that two gain stages are used and that the stripline
filter accepts to be retuned down to 10GHz. With a +4dBm output level at 10GHz,
spurious x3 and x5 multiplications were well below -40dB, so next
efforts were concentrated on earlier multiplications, oscillator stability and phase
For this transverter a good fit is a 5th overtone 92MHz
multiplied by 108 goes to 9.936GHz: the exact value for
a 432MHz IF. So the multiplication scheme I chose is:
This is basic schematic of the LO Multiplier:
First two multipliers are a classical design, and have
been realized "Manhattan style" on a dual-sided FR4 laminate.
The 276MHz coils are on the backside, and all other components plus the
the 828 MHz filter are on the front side. Last two multiplications have
been implemented reusing some sections of a modified LNB. An interesting
point is that the 2484MHz filtering is greatly simplified by using
miniature monolithic filters for WiFi, that I took from an old dismantled
WiFi access point. Two 2484MHz stages were needed to reach enough
drive level for the GaAsFET x4 multiplier (~5dBm). This last multiplier is a 2-stage gain, where I
added a trimmer to adjust the bias of the two FETs to let them work as much as possible in a
non-linear region. Besides, this trimmer can adjust the LO level to an optimal value: if too low there will be less TX power and less RX gain, but if too much the LO leakage and the image level will rise
dramatically. Of course, the image filter frequency has been
lowered by putting some dielectric material on it.
In later 10GHz LO multipliers I made (for Andrea's transverter and for the CW
beacon), I got a much higher efficiency for this "LNB multiplier"
creating an intermediate "weak" x2 multiplication.
Working at relatively low frequency, this section in not
particularly critical, and could be as simple as a one-transistor crystal
oscillator. But for best performance a more complex design is mandatory.
The key parameters, as widely described in literature, are the low
phase-noise, the frequency stability and the frequency accuracy. To get
the first one I simply took the project of a 5th overtone
oscillator+buffer by I2SG, who has optimized it for phase-noise
performance. Then I have added the varicap frequency adjust and a
prescaler buffer to phase-lock it to an external 10MHz reference, like a
good quality OCXO or my GPS-based
Clock Generator. This last would ensure a very high absolute frequency
accuracy (around ±100Hz at 10GHz), but to keep a reasonable degree of
accuracy when no external source is available, I put a small TCXO that
keeps my transverter within ±10KHz with a very compact design. The TCXO
insertion happens automatically when no external source signal is detected.
I've seen in practice the advantages of
having a fairly good accuracy without relying on auxiliary equipments
during quick lab tests or for line-of sight contacts, but with the
possibility to stay within 10ppb when challenged by a long-range
contact or for beacon monitoring/hunting.
The Oscillator and the PLL have been realized on
experimental pre-drilled small boards, mounted inside a tinned shielding
box. An internal shield keeps the oscillator separated from the PLL.
The big 92MHz crystal is on the backside of the oscillator board.
Similarly, the TCXO sub-module, that has been added later, has found his
place at the backside of the PLL board. Room is quite tight, but I like
this compact arrangement that still allows a good access to all internal
components and adjustments. The TCXO is now replaced by an OCXO
following an upgrade work.
and Antenna Relay
The PA unit is built from a part of another LNB. Since only one gain stage
is needed, the simplest LNB is the best. The modification could have
been easy, but the 20dBm GaAsFET I had was a flange-mount type and I
also wanted to add an RF detector. So I carefully cut a rectangular
slot in the PCB exactly the size of the GaAsFET flange (a little, sharp
cutter does the job), and now the GaAsFET is tightened to the aluminum
plate with two small screws. The RF coupler is done with a small strip cut
from a copper foil, glued to the PCB close to the output stripline. At one
end of the added strip there is the 50ohm termination, while at the other
end there is a microwave schottky diode taken from the unused mixer of the
LNB. During RX, the GaAsFET is forced to cut-off, again by playing with
the Vbb of the PNP bias transistor.
The GaAsFET matching has been optimized seeking the maximum (stable) power
by cut and trial with small stubs cut from a copper foil. This technique
is normally used when tuning microwave amplifiers, but needs patience and
extreme ESD precautions (here GaAsFETs start to be highly priced...) . At
the end I got the 20dBm with 9dB gain: very close to the transistor
specifications. Here is the picture of the PA unit PCB (without
A new 2-stages PA has replaced this little boy, and
currently output power of the transverter is a respectable 1WPEP. A
picture of the new PA here.
The Antenna relay is an HP8765A I bought on the surplus market (probably a
NOS). It is specified at max 4GHz, but on the the same
model HP has a "B" version specified up to 20GHz with slightly derated insertion loss
and "just" 90dB insulation. So at the SNA bench I verified how the HP8765A works at
10GHz, measuring <0.5dB loss and an insulation higher than the maximum I can measure with my bench (60dB). That clearly
says how large are the margins that HP takes over their specifications and
that I got a good antenna relay for this transverter. Luckily, with
Andrea, we bought enough relays to build another transverter for him and keep a
couple spare... This relay is of the latching type, with the two coils
specified at 15V. This is not a real issue because at 12V it still
works fine (thanks again, HP), but a static drive wastes a lot of current
and causes severe overheating. Therefore the relay coils of my Transverter
by a dedicated circuit of the AUX Unit.
This last block includes a few circuits that are not part
of RF processing:
- Antenna relay driver
- Generation of -5V supply
- Remote control and monitoring
- Supply Voltage check
The antenna relay driver gives a short current
pulse to each relay coil by discharging the energy stored in a 220uF capacitor,
that was previously charged at 17V (nominal) through a 1K current limiting
resistor . The
-5V generator is not strictly necessary, but I've put it to have the negative supply
ready when I (hopefully) will add a more powerful
PA. In the meanwhile, having the -5V rail put in series with the Transverter
supply, I can reach the specified 15V relay coil drive also when the supply
voltage drops from the nominal 12V down to 10V. This gives a good margin for
portable operation with a Pb-gel 12V battery. And for a pushbutton check
of the battery voltage, a simple indicator circuit drives the same
microammeter used for RF TX monitoring.
Not strictly necessary is also
the DC amplifier block, that I have included to remotely monitor the RF TX
power when the Transverter is pole-mounted in a fixed station. For a complete
operation monitoring, also the LEDs drive voltages are made
available at the Supply/Remote connector.
The AUX unit is also a kind of hub for supply
rails and for the connections between the various other units. I built it
on a pre-drilled experimental board, and is mounted inside the Transverter
without any shielding box.
Being the only 10GHz Transverter in my ham station, I
wanted it to be a "one size fits all". For portable outdoor use
it has to be practical and small, but rugged at the same time, able to be
powered by a Pb-gel battery, and to be used with or without an external
precision frequency standard. For fixed home station (that means
outdoor, close to the antenna) it has to be small to fit easily in an
hermetic box, and able to be operated/monitored remotely. Moreover,
being an experimental built, it has to be easily opened for lab
verifications/tuning, with each section easy to be disconnected and
removed for individual checks/service.
This form-factor is different from the more common
"desktop style", and in fact you cannot stack anything on it,
but I found it more suitable to be mounted close to the antenna. A
foldable collar on the back allows to hang the transverter to the vertical
mast that holds the antenna. Alternatively, with a U-shaped bracket, it
can be steadily attached to the back of a dish, and the two front handles
that protect the controls and the connectors can serve to steer the
The enclosure I found perfect for the purpose is a
1590D diecast aluminum box by Hammond, that I have used
"reversed". All modules, switches, connectors LEDs and the
microammeter are mounted on the box cover that actually is the front
panel, while the box has become in turn the cover. So, when the enclosure
is opened, everything comes out in one piece with the panel: you have 5
sides open, therefore all modules and internal connectors are easily
accessible while the whole Transverter is still operational.
|These are 5 views on the inside of the
Transverter. You can identify each block of the diagram.
And the next two photos have been taken with the MIX
Unit removed, to show the inner view of controls and connectors.
No project is perfect, and homebrewing gives a lot of
room for upgrades. Three main parts of this transverter have been
addressed for improvements:
- TX power
- RX conversion gain
- Replace the TCXO with an high stability OCXO
The original 100mW PA is a
single stage with 9dB gain, so to reach a significant increase of
power it has been replaced by a dual stage PA. This time, the LNB
reuse was abandoned, and a specific teflon PCB from an italian
hamfriend was adopted. Luckily, a very small dual stage PCB was
available, so it has been possible to replace the old 100mW PA
keeping the same mechanical arrangement. The new PA delivers 1W
(30dBm) with the same driving power from the Mix Unit (11dBm). Its
full project is described in another
of this website, so here we just show how it fits inside the
Because of the architecture chosen in the TX/RX PIN
switch section of the Mix Unit, the 20dB gain of the IF buffer just
compensates the attenuation of the 20dB patch from/to the IF transceiver.
As a result, overall conversion gain in RX is just 12dB, that could be low
if the transverter is mounted on the rooftop with a long IF cable. To get
a little improvement, also on the NF side, the GaAsFETs of the 2-stages RX
front-end have been substituted by an higher gain pair. Original
transistors of the AE2 Cambridge LNB were FHX06LG (10.5dB gain, 1.1dB NF),
now replaced by two NE32584 (13dB gain, 0.4dB NF). Measured conversion
gain shows a 4dB improvement given by the new transistors: not
a big jump, but not bad without making radical changes in the
architecture, and with a theoretical improvement in the NF.
|Last upgrade was the replacement
of the TCXO with an high stability Ovenized Oscillator. In the
starting concept of this transverter, mostly driven by a compact
design, the only reference frequency source should be
external, like a GPS Disciplined Oscillator. Then, as a back-up, a
small TCXO was placed inside the 92MHz PLL oscillator to give a decent
accuracy without relying on an external source. Having recently
bought on e-Bay some nice small-sized OCXO by Piezo Technik,
I found a little room inside the transverter to put one of them and
finally get rid of the GPS DO unit when operating outdoor.
Furthermore, after warm-up, current drain of the OCXO is much less
than the GPS DO, leaving longer operation time with
batteries. Of course, the OCXO is routinely calibrated in my
lab with the GPS...
All test results of next section reflect the upgrades
A thorough test has not been possible, because not all
necessary instruments are in our hands. For example, RX Noise Figure
should be measured now that new Low-noise transistors have replaced the
original ones. The few tables below give the
main characteristics of the transverter, plus the breakdown of the contributions
to overall signal gains or levels.
|Ham band covered
|Usable RF BW
|10MHz EXT REF lev
||based on orig. LNB spec
|RX Gain (RF to IF)
|TX driver power
|TX power (PA out)
|TX power (ANT port)
|TX LO leakage (ANT port)
|TX Image rejection
|IF TX drive level
||1W CW max safe level
|Supply Current (RX)
|| After OCXO warm-up
|Supply Current (TX)
|| 1W Pout, after
GAIN BUDGET RX
|PIN diode switch
|IF power attenuator
Total RF to IF gain
GAIN BUDGET TX
IF power attenuator
|PIN diode switch
|2nd IF attenuator
Total TX gain
|IF drive level
Ant. TX power
Another transverter, based on the same concepts and
similar architecture, has been built by Andrea (IW9HJV). It's shown working with the IF transceiver and the GPS frequency
reference. With these two transverters we have started our experimentation
on the 3cm bands, establishing interesting contacts. This activity is
described in our Ham Radio pages.