YET ANOTHER TRY TO MAKE A
SUPER LOW NOISE PREAMP FOR 10 GHz
by Marko Cebokli S57UUU
EME conference Paris 98
ABSTRACT: After some disapointing results with SMA / microstrip preamps,
I have decided to build an preamplifier with waveguide input circuit. This
paper presents the design of this preamp, together with test results of the
first prototype. I haven't yet achieved the hoped-for results, but decided
to publish it anyway - for the pesimists as a warning that this may be a dead
end, and for optimists as a stimulation for their own experiments.
1. EXISTING DESIGNS
When I was building the preamp for 10 GHz EME, with SMA input and microstrip
matching circuit, I found it impossible to get closer than about 0.45 dB to
the theoretical minimum noise figure of the transistor. I got about 1 dB system
NF with NE 32684, which has 0.45dB optimum noise figure at 10 GHz. I tried two
different designs, measured with two different low ENR noise sources, so I'm
quite confident in the results. A survey of the last few years of DUBUS
magazine confirmed my findings. All published designs ,, achieve
noise figures of about 1dB, with similar transistors. The best reported value
 is 0.68 dB with NE 329 (a 0.37 dB device), but that is probably pure
(single stage) NF and the design was not published.
Since the best SAT-TV LNB's, where the microstrip is coupled directly to the
waveguide, have about 0.7 dB system NF, it is safe to conclude that 0.3 dB
above device NF is the 'sonic barrier' for microstrip, on 10 GHz. On 24 GHz,
things will be even worse.
0.3 dB may not seem much, and indeed, when transistors have had 2 or 3 dB NF,
it was negligible. But with a 0.4 dB device, adding 0.3 dB of loss means
almost doubling your noise!
Theoretically, considering the second stage noise contribution, receivers with
0.5 ... 0.6 dB system noise should be possible with these transistors in a low
loss circuit, so I decided to try and make one.
2. DO WE REALLY NEED SUCH LOW NF's?
The opinion can often be heard, that on 10 GHz EME, because you can hear Moon
noise, you are Moon noise limited, and further reduction of receiver noise
makes no sense. But, in most cases, this is not true.
Let's consider a typical 10 GHz station: 3m dish, 80K (1.06dB) receiver, 40K
antenna noise. A 3m dish will gather about 50K of Moon noise, so Moon / cold
sky will be 170K/120K or 1.5 dB of Moon noise, a typical value for such a
That means that out of 170K of total noise, only 50K, or 29% is Moon noise -
quite some room for improvement! A 40K (0.56dB NF) receiver would mean 130 K
total noise (including the Moon), a 1.1dB improvement in total sensitivity.
Moon noise in this case would be 130K/80K or 2.1 dB.
So where is the limit, when it really doesn't pay to improve the receiver
any more? My suggestion is 3dB of Moon noise - that is, when half of your
total noise comes from the Moon. In that case, you are still 3dB less
sensitive than the (nonexistent) ideal noiseless receiver, but the
improvements you can make become smaller and smaller and cost more and more.
If we someday get a satellite with a 10 GHz downlink (phase 3D), this will
be another use for super low noise preamps. The absence of Moon noise will
make them even more attractive, to have the smallest possible antenna.
3. CAUSES OF LOSS IN A MICROSTRIP PREAMP, AND SOME CONVENTIONAL CURES
In a SMA / microstrip preamp, there are several places where losses occur:
- SMA connector. This can be most easily eliminated, as is the case
in SAT-TV LNB's, where a probe goes directly from microstrip into
- Coax / microstrip transition. Without a special 'microstrip
launcher' type SMA connector, it is very hard to get a decent match.
The solder bump also causes radiation losses.
- Microstrip line itself. 0.15dB / cm is typical, and it is hard to
make an amplifier with less than one cm of input line. Thick
substrates usually have less ohmic losses than thin substrates,
but more radiation losses than thin ones.
- Input capacitor. Not so much absorption loss, but the cap makes
a radiating bump on the line. Because most preamp inputs are
connected to a coax / waveguide transition, this capacitor can
usually be left away. Only noise measurement is problematic then,
since noise sources have resistive attenuators at their outputs,
that will short the DC. A good DC block for measuring purposes
can be made of two SMA / waveguide transitions, screwed together
back to back. I have measured the loss of such a combination on a
HP 8510, and it was 0.15 dB. (0.075 dB each transition)
- Bias circuits. Their loss is mostly radiative. G3WDG told me that
he spent much time optimising these in his preamp.
In the above list, many losses are radiative, and some improvement could
be made if we could prevent these things from radiating. Here we can learn
another thing from SAT-TV LNB's. If you open one of these, you will probably
see that the preamp is mounted inside a separate very narrow metal 'tunnel'.
The trick is that if you put something inside a waveguide that is subcritical
at a given frequency, it won't be able to radiate at that frequency, because
the energy simply cannot propagate away. This also eliminates the problem of
instability due to radiative interstage coupling, that people usually try to
cure by inserting absorptive material. But it is BAD practice to put
absorbers into a low noise gadget.
4. WHAT ALTERNATIVES DO WE HAVE?
I looked through lots of literature (IEEE periodicals, microwave conference
proceedings etc) to see what people have done in this respect. Several people
found microstrip inadequate for low noise amplifiers, and proposed a variety
of alternatives, like coplanar lines or suspended substrate striplines.
Two of them , proposed mounting the transistor directly into waveguide.
Marti  has mounted the transistor on a vertical (E-plane) substrate in a
waveguide and made the matching structure using asymetrical finlines.
Fig 1. (7kB GIF)
Author achieved 2.2 dB NF on the 11.7 - 12.4 GHz band, with NE 710, a 1.6 dB
device. But it is not clear from the paper, if this includes a bandpass filter
in front of the amp. The weak points of this design are the gate bypass
capacitors and the problem of assuring ground continuity across the substrate.
Angelov et al.  propose a variety of low noise structures. For a 22GHz LNA
they have coupled the transistor to the waveguide without using any dielectric
Fig 2. (7kB GIF)
They have achieved 2.3dB NF with MGF1403. 21GHz is above
manufacturer's data for this transistor, by extrapolation we can estimate its
minimum NF on 21 GHz to be about 2.4dB. This is probably an error in
extrapolation, but it could also mean that the test fixture that the
manufacturer uses to measure noise is lossier than this waveguide structure.
5. MY DESIGN
I have decided to make the input matching according to , and to use normal
microstrip circuit on the output, where losses are not so critical.
The preamp consists of three main mechanical parts: the base
Fig 3a. (44kB GIF)
, the wavegude transformer
Fig 3b. (42kB GIF)
and the transistor clamp
Fig 3c. (6kB GIF)
. These parts are machined brass with silver plating.
The gate bias is supplied through a coaxial low pass filter type choke. The
choke consists of two turned pieces of brass as shown in
Fig 3d. (5kB GIF)
linked by soldering to a piece of 0.5mm copper wire, and inserted in a hole
in the base part. A 25 micron (1 mil) mylar film is wrapped around for
insulation. A piece of 0.1mm copper foil, cut as shown in
Fig 5. (6kB GIF)
top end of the bias choke to the gate terminal of the transistor and serves
also as the RF coupling loop. Its shape is optimised for a good impedance
match. This match is quite sensitive to the shape and position of this
piece of copper foil!
The transistor is mechanically fixed by the source leads that are held in
place by the transistor clamp. The drain terminal is soldered directly to a
small microstrip circuit board. This board provides drain matching and bias.
A SMA female flanged connector serves as the RF output.
The whole assembly is shown in
Fig 4. (13kB GIF)
And here are two photos:
Photo1. (27kB JPEG)
Photo2. (34kB JPEG)
The input is a stepped height waveguide impedance transformer  that lowers
the impedance to a value more suitable for transistor matching.
My idea was to make a good impedance match with this transformer and the
shape and position of the gate coupling loop, and then shift the impedance
with tuning screws in the waveguide towards the optimum noise point.
With modern (1997) HEMT's, the optimum impedance match and optimum noise
points aren't very far apart, so this should be possible without introducing
significant additional losses. (i.e. without putting the screws too deep
into the waveguide)
6. MEASURED RESULTS
As of the time of writing, I haven't yet achieved better than 0.87 dB NF
with a NE 326. That is about 0.13 dB better than my microstrip version with
the same transistor, but still short of the desired results.
I have measured the return loss at the input with the transistor removed, and
it was well below 0.1 dB (such values are hard to measure even with a HP 8510)
So, it seems, the input circuit losses of less than 0.05 dB are not the
The next possibility was that I'm shooting in the wrong direction
(impedance-wise) with the tuning screws. But there is one well defined
impedance point, namely that of matched impedance, which can be easily
identified on a network analyser. Even at that point, the measured NF was
significantly higher than the calculated value (circles of constant NF etc.)
For now, I'm trying to devise some more measurements to find the missing
tenths of dB's. However, I'm also beginning to suspect the transistors.
5 to 10 years ago it seemed that the big volume users (Sat TV LNB makers)
were buying all of the first-class devices that the device manufacturers could
produce. The DEVICE NF's of the best transistors you could buy were equal to
the SYSTEM NF's of the best LNB's you could buy at the same time! Also,
the small volume resellers usually don't have the strict input quality
control that the big volume users do. They are often hunting the bargain,
which seldom takes the form of the first class merchandise. In Italy, even
transistor label falsification was popular some time!
7. SOME NOTES ON NOISE FIGURE MEASUREMENT
Today, noise performance of LNA's is almost exclusively measured on
microprocessor controlled PANFI's like HP7890B. These instruments tend to have a
bewildering multitude of measurement modes, calibration options, special
functions and so on. So, especially if you don't use the same instrument every
day, it tends to be very confusing. That's why I have developed a 'standard
procedure' for measuring noise figures on a HP7890.
It goes like this:
- Let the NF meter warm up at least for one hour, with noise source connected.
- Enter your room temperature under special function 6.0 (Tcold)
- if the frequency is above HP7890B's range (1.6 GHz), I use for downconversion
the transverter that I normally use for QSO's, complete with the same
cable between the LNA and transverter that I use in real life.
- Despite external mixing, I use mode 0, input frequency 144 MHz, and type in
the ENR value of the noise source at the input frequency (10 GHz) as special
function 5.3 ('use spot ENR')
- I DO NOT use calibration, to IMPROVE accuracy!!!
- If there are losses between noise source and DUT, I dont use the special
function 34.2 for that, but just type in the ENR of the source minus the
losses. For example, if the source has 5.55 dB ENR and I have a 0.075 dB
coax/wg transition, I type 'use spot ENR', 5.47 .
This may seem odd, but here is the explanation:
- Why no calibration? There is a big difference between calibration on a
network analyzer like HP8510 and calibration on a noise figure meter. On a
network analyzer, calibration removes lots of systematic errors (remember the
name '12 term error model'), and can improve accuracy for orders of magnitude.
In contrast to that, calibration on a noise figure meter only enables you to
simultaneously measure gain, and to subtract the second stage noise
contribution. If you made a bad calibration, or because of thermal drift, the
calibration on a NF meter can actually DEGRADE your accuracy!
Also, because one is usually interested in his total SYSTEM noise figure, it
makes sense NOT to subtract the noise contribution of later stages. If these
later stages are your real life transverter, by not subtracting its noise
contribution, you will actually measure your system noise figure, and tune
the preamp to the optimum compromise between noise and gain that minimises
your total system noise!
Of course, if you want to measure the 'pure' noise figure of your preamp, like
on a NF contest, or want to characterize a device for noise / gain, you MUST
use calibration. In that case, use lots of averaging during calibration, and
measure your DUT as soon as possible after calibration. After measurement
re-check the calibration! It can drift away a few tenths of a db in ten
- Why not use the appropriate measuring mode for this type of external mixing?
Well, just for simplicity, and to reduce the chance for errors (like
forgetting to tell the machine that you are using SSB mixing etc). The purpose
of these special modes is to tell the machine the input and IF frequencies.
It needs to know them only to control an external variable LO, that we don't
have, and to know which ENR to use from the table. Since we are measuring
only on a single frequency, it is simpler to use spot ENR.
It still does make sense to go for the lowest possible NF. Since conventional
designs fail to deliver what is theoretically possible, I scanned the literature
for alternative designs, and made a waveguide input LNA. The improvement was
measurable, but still not what I was shooting for. So, I plan to continue work
on this design. The measurement of such low NF's means squeezing the last out
of today's PANFI's, so I have described a measurement procedure that tries to
simplify things and reduce the possibility of errors.
 C & P Suckling G3WDG/G4KGC: Modern 10GHz Transverter System (Part II)
DUBUS 4/93 pp 21-46
 Silvano Ricci I0LVA: HEMT Preamplifier for 10GHz. DUBUS 3/94 pp 52-57
 Michael Kuhne DB6NT: LNA for 10GHz. DUBUS 3/95 pp 5-10
 Microwaves USA: the '95 Microwave Update, noise figure contest results
 Albert Marti: Ridge Guide Preamplifier for DBS Outdoor Unit;
European Microwave Conference 1983 proceedings pp 221-225
 I.Angelov, A.Spasov, A.Stoev, L.Urshev: Investigation of Some Guiding
Structures for Low Noise FET Amplifiers; European Microwave Conference
1985 proceedings pp 535-540
 J.Uher, J.Bornemann, U.Rosenberg: Waveguide Components for Antenna Feed
Systems: Theory and CAD. Artech House 1993.
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