Noise Source
Below is shown the circuit of a typical noise source
on which my version is based.
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At this stage I plan to
use a BGM1013,115 in place of the
BGA2817 and a 6 volt battery with
appropriate changes eg a different zener diode.
I should explain the purpose
of building this noise source as it's not intended to compete
with expensive commercial items, but merely as a project with
which to familiarise myself with the interesting topic of noise
in radio receivers and perhaps to see how it compares with a
tracking generator for helping with RF filter design. That said,
read on to hear how I built it then attempted various design
improvements.
Click
here or scroll to end of this page to see the final circuit diagram.
To eliminate local RF pickup
it will be put in a diecast box and the construction
will be on a piece of tin bent under the BNC output socket
securing nut with the chips mounted on grounding wires and the
battery supply decoupling capacitor(s), The zener diode and various
resistors will be soldered directly to the tin. The power supply
will be three or four AA cells mounted in a holder within the
diecast box with a tiny LED and on/off switch. The choice of
the BGM device was partly because it can accommodate 6 volts
whereas the BGA devices are limited to a 4.5 volt battery supply
as they can only handle 5 volts, however I'll try and extend
battery life by balancing the supply voltage with total current
drain. These types of chips sink current depending on the amount
of signal at their input. As the BGM chip has a higher gain than
the BGA, maybe one chip will be enough?
Up to now, I've not considered
a suitable zener diode. In the circuit above you can see the
front part of the circuit has the title "Avalanche"
because a zener diode can have a couple of basic designs and
the avalanche type is said to have a better noise characteristic
for our noise source. Many years ago, back in the 60s and 70s
our factory made noise sources for use in cryptographic equipments
and their design was based on zener diode breakdown noise, so
relatively early zener diodes would have performed quite adequately.
The noise source I'm planning will use a 6 volt battery so I'm
looking for a zener diode having a rating of something less than
6 volts. A check on my junk box reveals 11 zener diodes rated
below 5 volts including some ancient obscure types. I also have
a good selection of 5.1 volt and over a hundred others. The plan
is to try a selection and if none below 5 volt are good enough,
raise the battery voltage to 12 volts and try others. If any
specific types are significantly better I can order a lower voltage
rating. This will also apply to TVS diodes, as I only have a
few rated at below 9.1 volts. The diagram above shows 100nF capacitors
in parallel with "uF" implying you need fairly low
impedance coupling for lower frequencies. For RF work 100nF to
1uF should be OK. The output matching network shows a PI configuration
with a 36 ohm series resistor and a couple of 360 ohm shunt resistors.
Another solution is to use 3 AA cells plus a further 5 AA cells
for the zener. |
I ordered the following parts including an SR2.8
device with an interesting spec
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PART |
QUANTITY |
NOTES |
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BGM1013 |
5 |
RF amp, 35.5dB gain SOT363 |
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Diecast box |
1 |
112 x 62 x 31mm (later changed to 121.2 x66 x 35.3mm) |
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BNC connector |
1 |
bulkhead screw fitting |
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Battery holder |
2 |
4 x AA cells (later changed to 6 x AA cells) |
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SR2.8.TCT |
2 |
TVS array, SOT143 low cap with 3V punch through (not used) |
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The amplifier chip I've chosen
needs a voltage at its output pin which is supplied via a choke.
In the application notes this is shown as 100nH which at 100MHz
represents an impedance of about 60 ohms. If the noise source
is to work well down to say 100KHz this choke needs to be 100uH
and at say 465KHz around 20uH. This means that I'll need to experiment
with the output crcuit, perhaps using a load resistor in place
of, or in addition to, the choke of say 60 ohms. The value of
the resistor will result in a voltage drop at the output pin,
but as the output current isn't specified some experimentation
will be necessary. Also needing checking is the manufacturers
"K factor" which appears to be concerned with instability,
so where it drops to less than unity at 100MHz might result in
a problem getting the amplification down to 100KHz?
The parts arrived but I'd forgotten
to check the dimensions of the amplifier chips which looked fine
in the suppliers catalogue but almost invisible when I looked
in the packet (the body is 2mm x 1.25mm and the 3 legs on each
side spaced 0.65mm apart). Back to the drawing board so I can
work out a layout because I'll need to use mostly chip resistors
and chip capacitors to balance the construction. Also, remembering
the design of those TRF receivers using those fancy high gain
tetrodes with a metal screen between input and output, I've decided
to use a similar technique with the noise source to prevent it
oscillating. Below.. a rough idea of the layout. The shaded areas
are grounding wires soldered to the baseplate. Practical difficulties
(mainly eyesight) meant I had to deviate from this plan. |
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It looked easier than
it actually was. First I found my magnifying desk lamp wasn't
powerful enough to even see the dot on the chip marking pin1.
I have a very high power lens which did the job, then I must
have breathed too heavily and the chip vanished. Fortunately
I'd noticed it was magnetic and I eventually found it on the
floor together with umpteen ends of wire etc. I first soldered
a pair of 100nF chip capacitors to the tin sheet, one either
side of the central shield, then used one of these on which to
mount A1 by soldering Pin 1, the power supply leg, Then I soldered
ground wires to pins 2, 4 and 5 and soldered another 100nF near
the output (Pin 3) and a 100 ohm chip resistor between this and
the output, together with a thin wire whose end passed through
the centre shield hole. Next I fitted a BZX79-C3V9 (3.9 volt
zener diode) and a 1.2Kohm load resistor, connecting their junction
to a 100nF capacitor to A1 Pin 6. Before adding A2 I then tested
the A1 circuit by terminating this into a 1Kohm resistor via
another 100nF capacitor. After setting the voltage to 5 volts
and limiting the current, then gradually increasing the trip
I found the current measured around 39mA.
Next I connected my spectrum
analyser to see the results. I could see some large RF spikes
even before connecting to the noise source. The main spike was
around 600MHz and this was accompanied by a strong second harmonic
at 1.2GHz. Clearly the high amplifier gain was resulting in feedback
and, by intially adding a metal screen over A1, then trying different
cables to the SA I settled on a BNC cable with a very short end
which I soldered to the noise source output. By experimenting
I found the noise introduced by the zener diode was very low
(no more than a 1 or 2dBm increase over the minus 90dBm baseline)
and oscillation occurred very easily. I swapped the 3.9 volt
zener for a forward biased OA90 diode and found this was useless
because I got an untameable 1.2GHz output signal at around 2dBm
with zero noise. Next I tried a chip zener diode rated at 5.1
volts (see below for details). I increased the supply voltage
to around 6 volts and found this zener did produce some noise,
but with a very lumpy shape and still prone to oscillation. I
added an extra metal screen which helped settle the instability.
As the output was still running
into a 1.2Kohm load, I changed this into a pad comprising three
100 ohm resistors. This reduced the tendency to oscillate. I
could now see RF broadcast pickup so I added a 330uF capacitor
at the power input and passed the supply leads around a ferrite
ring. The results were much improved and stable enough to see
that significant (wanted) noise was being produced when the power
was applied. Here are a set of pictures showing spans of 30,
100 and 1500MHz without power, then with the noise source switched
on. |
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Above, the span is zero
to 30MHz with the noise output 24dB above base level. |
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Above, the span is zero
to 100MHz with the noise output 28dB above base level. |
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Above, the span is zero
to 1.5GHz with the noise output dropping rapidly to base level.
Because RBW/VBW has been increased tenfold for the greater span,
the start of the trace is not as well defined (about 10dB higher)
as the previous pictures, but you can clearly see the noise isn't
nearly as great as in the lower frequency ranges. Note: One of
the problems during testing was hand capacity and at this point
the circuitry was mounted on a small tin sheet and later, even
with this in place inside the diecast box, the circuit was prone
to instability, however, once the lid was screwed in place it
was fine.
The noise source isn't complete
yet as A2 hasn't been fitted but it's clear that building it
in this fashion is not easy and it's quite possible that the
addition of the second amplifier may make the project impossible
to complete unless the overall gain is reduced to prevent instability.
Stray RF pickup is also a problem and will remain so until the
circuit is fully screened, however the choice of zener looks
OK. This is a tiny chip device type MMSZ5V1T1G.
The next step is to vary the zener load resistor to see if the
noise output can be improved. It's also important that the range
below 1MHz is reasonably flat because that range is most useful
for IF amplifier alignment. Below..
not much to see. |
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The supplier's picture of the
BGM1013, 115
2mm x 1.25mm x 1mm
Right, sitting on top of a 6BA
screw.
Below, a view of testing the
first stage A1, then adding the second stage A2 is shown in the
following pictures. |
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Steps in fitting the second
amplifier chip A2, are very similar to fitting the first chip,
now under the metal cover to prevent RF feedback. Note the output
wire from A1.
Top left.. securing pin 1 of
the chip to the 100nF power supply decoupling capacitor
Top right.. adding the 100 ohm
output load resistor between Pin 3 and a second 100nF power decoupling
capacitor
Left.. adding the 100nF output
coupling capacitor and ground wires to Pins 2, 3 (input) and
4 (output) of the chip.
Later, the 50 ohm RF output
load (two 100 ohm resistors in parallel to provide mechanical
stability) and power supply connections were added.
You'll realise that two soldering
iron bits (and temperatures) are required.. one for use on the
tin sheet and the second for soldering parts. |
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Again, the noise source
was tried outside its screening box and I found it was prone
to oscillating at between 600 and 1000MHz. Below are scans at
a span of 1MHz with the noise source power turned off and then
turned on. The centre is at 500KHz, and the area around this
is about -90dBm (S6, OFF) then -40dBm (better than S9+30dB, ON)
giving a noise level of 50dB above baseline which is about double
that for the first stage. If you look carefully at the first
scan below you can see spikes indicating the presence of Radio
4 at 198KHz and Smooth Radio on 828KHz. A few of the other spikes
are from local interference sources. The falling off to the left
of the second curve is where the impedance of the 100nF coupling
capacitors is within the same order as the output impedance of
50 ohms (ie. 100nF = 16 ohms at 100KHz and 160 ohms at 10KHz).
The value of the capacitors could be increased from 100nF but
this may have introduced annoying stray inductance at higher
frequencies. The capacitors
I'm using are from a batch I bought a few years ago for a
few pence each and are physically ideal for this method of construction.
I guess the use of suitable 470nF capacitors throughout would
improve the LF end of the noise spectrum but this remains to
be proven. |
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The final breadboard test
was to plug the noise source into a general coverage receiver
(my Kenwood R2000) and I found the noise level was a pretty constant
S9 +30dB (as predicted in the above scan) from 100KHz to 30MHz.
The supply was 6.4 volts and the current consumed about 79mA
which makes the zener current a little over 1mA. The next step
is to fit the chassis into the diecast box, but this proved difficult
because of the size of the AA cells. AAA cells would have made
the task a lot easier.
I found a suitable larger box
(Hammond 1590N1, approx 121mm x 65mm x 40mm) which will just
accommodate the noise source plus twin 4 x AA battery holders
to take 6 NiMH rechargeables and I'll fit a 2.1mm DC socket with
a suitable series resistor for feeding a charging voltage. The
surplus green box will be re-assigned for a new project.
While I'm waiting for the replacement
box so I can finish testing the noise source...I wondered what
I'd see on an oscilloscope. The power measurement by the spectrum
analyser at any specific frequency (or range of frequencies)
is about -40dBm. But that checks the level across its internal
50 ohm input circuit, and at the moment I'm using two 100 ohm
resistors in parallel at the noise source output which is 50
ohms. Surely half the output power is dissipated at the two 100
ohm resistors and half in the spectrum analyser, so the total
power being developed is twice that shown in the scan above?
I recall the handbook for my TF2008 signal generator explains
at length that the output voltage shown on the dial is out by
a factor of two when connected to a 50 ohm input, but accurate
if recorded on an oscilloscope using a high impedance probe.
Minus 40dBm represents about
2.2mV RMS or 6mV pp across the pair of 100 ohm resistors (=50
ohms) so this voltage should be visible on an oscilloscope. My
GDS1102U has a maximum sensitivity of 2mV RMS per vertical division,
and should therefore produce a 3.3cm pp display and if I use
a 50 ohm Tee to connect the spectrum analyser the voltage at
the Tee should be 1.5mV RMS or 4.5mV pp. In fact this all matters
very little. Half the voltage is -3dB so the spectrum analyser,
when it tells me it can see -40dBm across 50ohms is really saying
the ouput is -37dBm because its input is shunted by my noise
source output of 50 ohms. To read the true output I could change
the pair of 100 ohm parallel resistors into say a single 500
ohm resistor. The resulting impedance would then be 500 and 50
ohms in parallel or 45 ohms and about half a dB instead of a
3dB error. The reading on the SA will now be closer to the noise
source output when its terminated into 50 ohm, but if I now disconnect
the SA and instead connect an oscilloscope probe, the display
will now show a larger trace than before because the output will
be across 500 ohms instead of 50 ohms. Assuming the same power
is being produced this will mean (V1 x V1)/R1= (V2 X V2)/R2.
Assuming the power output is -37dBm across 50 ohms, V1 = 3.1mV
and -37dBm across 500 ohms, V2= 9.8mV. So the trace will increase
from 3.1mV when the output was terminated in 50 ohms to 9.8mV
with the 500 ohm termination. The problem with this is that when
plugged into a receiver having an indeterminate input impedance
the noise level will be equally indeterminate so leaving the
terminator at 50 ohms will at least guarantee the noise input
is between -37 and -40dBm at 50 ohms. In the end I decided to
use a 100 ohm load within the box as this compromise would better
tame the circuit and prevent it oscillating. |
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I worked out the best
layout for the batteries in the new diecast box and marked out
drillings for the tin chassis, a small on/off switch and an LED.
I have a collection of LEDs so looked through the box and tried
a few. A yellow one in a holder drew 2.5mA at 2.4 volts. I tried
another and it failed to light so rather than unsolder its connections
I temporarily switched over the red and black plugs at the power
supply (a really bad idea). This also failed to light te LED
so I tried a third which was fine, drawing only 1mA at around
2.2 volts. This had a formed shoulder so was suitable without
a special bulkhead fitting. Next I drilled three holes in the
end of the diecast box and ftted the new LED, the miniature switch
and the BNC connector. Behind this connector I fitted a tin sheet
with a shoulder to which I'll solder the noise source tin chassis
suitably trimmed to minimise the space required.
A tip here in mounting parts
to the diecast box... to ensure a tight fit for the LED I drilled
a hole slightly smaller than the LED diameter then used a scissor
blade to carefully enlarge the hole for a tight interference
fit. Diecast material is an excellent material with which to
work except for soldering. Tin sheet is ideal for soldering and
an excellent material for quickly rigging up a circuit. Once
I was happy with the circuit I snipped off excess tin sheet,
cut a second piece of tin, in which I drilled a hole for the
BNC connector then bent it to fit the end of the box. I then
soldered to chassis to this. I found I had to ground the opposite
end of the chassis so drilled a hole through the chassis and
box and secured the two with a 6BA screw. That completely tamed
remaining tendency to oscillate.
Before going further and powering
the noise source I moved the amplifier supply connections (two
each to the pair of ampifiers) so that they are fed via a series
180 ohm resistor so that the amplifier 6 volt max supply voltage
wouldn't be exceeded by the new 6-cell battery. Turning on I
found a problem. The new assembly drew far too much current (upwards
of 400mA) and a monitor receiver showed complete lack of noise.
After experimenting a little I discovered the zener diode was
sitting at half a volt and each amplifier independently drew
well over 100mA. Now, during tests I'd noticed that my PSU had
developed a fault... as the rotary control is turned to increase
the voltage it would sometimes add a few volts to the output
instead of allowing this to increment by 100mV per step. Had
I inadvertently destroyed the amplifiers? I gave up for the day,
intending to use two spare amplifier chips but then thought back
to the zener diode. This surely couldn't fail as it's fed via
1.3K ohms. Then the penny dropped... I'd switched over the leads
to the PSU when checking LEDs so red was negative and black positive
and I'd forgotten to put them back. I switched leads around,
set the output to 5 volts and 100mA, turned on the monitor receiver
and switched on.... the receiver roared into life showing a steady
S9+30dB right across the range from 200KHz to 30MHz, and dipping
only slightly down to 100KHz. The current being drawn was very
low so I set the output to 7.2 volts (the intended new battery
voltage) and it drew 22mA. Dropping the voltage below the zener
voltage turned off the noise output which is what I'd expect.
The received noise hardly changed with supply voltage above 5.5
volts so I can now confidently finish assembly work. |
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Above are pictures of
the completed noise source, or at least the provisional model,
as I'm not sure about the final output level. With a reduced
power consumption of 22mA the output has dropped from 30dB to
around 20dB above the minus 90dBm baseline across the shortwave
band to 30MHz. No doubt this will help to reduce any tendency
to oscillate as was the case when it was drawing close to 80mA.
My previous diecast box project used alkaline cells which need
replacing every so often, so I've now used rechargeable cells,
adding a charging circuit based on a convenient 12 volt power
supply. This feeds the battery via a 47 ohm resistor to provide
100mA charging current but I decided to add an orange charging
LED. This proved slightly tricky to arrange circuitwise so I
fitted the LED in series with the 47 ohm resistor. This reduced
my initial 100mA charging current, but there's 2.8 volts across
the resistor so that charging current works out at 2.8/47 = 59mA
for 1.2 volt cells which is OK and to fully charge the 1300mAh
battery will take around 12 to 24 hours. I used a handy single
pole 2-way on/off switch so that the noise source is either turned
on or, if the 12 volt power supply is plugged in, and the switch
is in the off position, the batteries are charging. The scans
shown below show that the noise source is suitable for aligning
radios up to 100MHz with an ideal flat response up to 30MHz. |
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This picture shows the
noise source being turned on. On a monitor receiver you hear
a loud rushing sound with the S-Meter registering S9 +20dB. This
level remains at this level from about 200KHz to 30MHz. |
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Later I'll show the results
of further experiments.
The only problem in final construction
I had was one of the new battery holders had an open circuit
between its negative output pin and the spring contact. It looked
OK but had a faulty crimp so I had to solder a link from the
spring to the output tag. I shouldn't have selected these parts
by lowest price!
Below is the final circuit.
A1/A2 load resistors are 100 ohm surface-mount chips, 100nF are
chip capacitors. I used a small 330uF electrolytic capacitor
as a convenient anchor point and this is certainly not a critical
value. The charging resistor isn't critical either and can be
chosen to match the power supply and desired charging current.
Switch S1 happened to be available and conveniently allows an
OFF position for charging. Because the A1 and A2 have a maximum
supply voltage of 6 volts I've added Z2 which can conveniently
be a second MMSZ5V1T1G. This would really only come into play
during testing when an external power supply might be used. |
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I tried the noise source
on my Kenwood receiver and it seems fine, but what does one see
using an SDR? Basically my SDR Play receiver works just like
a very sensitive spectrum analyser. It can be set to receive
any 10MHz (or smaller) band between say a centre frequency of
5MHz and higher, even up to 2GHz. For the 25KHz or 100KHz scan
I selected lesser bandwidths.
Below are the results. I left
the vertical scale untouched but altered the bandwidth settings
for the lower frequencies. Spurious signals in the noise OFF
settings are due to a variety of reasons, but chiefly breakthrough
from the USB cable connected to the computer together with the
odd internally generated signal or indeed software artefacts
(eg that centre spike). |
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Above:Above: Centre frequency 25KHz, baseline -140dBm.
Below: Noise -105dBm |
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:Above: Centre frequency 100KHz, baseline -150dBm.
Below: Noise -95dBm |
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:Above: Centre frequency 100KHz, baseline -150dBm.
Below: Noise -95dBm |
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Above: Centre frequency
5MHz, baseline -150dBm. Below: Noise -94dBm |
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Above: Centre frequency
100MHz, baseline -150dBm. Below: Noise -88dBm |
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Above: Centre frequency
1GHz, baseline -150dBm. Below: Noise -99.7dBm |
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Above: Centre frequency
2GHz, baseline -150dBm. Below: Noise -87.8dBm |
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I must admit to be slightly confused
by the various test results but suffice it to say the noise source
appears to work just fine. I decided to check the total power
output by connecting it to my HP431C |
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According to the meter reading
the power from the noise source is 1.15mW, which is pretty well
0dBm into 50 ohms or 0.25 volts RMS. This power meter has a bandwidth
of 10MHz to 10GHz. The output power of the noise source I can
see on my SDR is at least 25KHz to 2GHz.
Over the frequencies tested I was seeing
an average of about -40dBm or 0.1uW of noise at any frequency
with an RBW of 1MHz.
To bring down the power to this level
that 1.15mW of noise must be spread over the full 10GHz? If so,
that would result in the 0.1uW/MHz which is what I'm seeing on
the spectrum analyser display? |
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Below is a picture of
a scan from 0-2MHz which covers most IFs. By turning on the noise
source during a sweep you can see that it's producing a nice
level output at a decent signal strength of about S9 +36dB. |
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Now a practical test.
I made a VLF low pass filter
some time ago so connected this in front of an SDR. The filter
has a switch to place it in or out of circuit. |
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Above a scan from zero to 200KHz
without the noise source switched on, baseline -150dBm, and below
with the noise source turned on, top -70dBm. |
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Below, with the low pass
filter switched in. Not ideal as the LF end droops considerably,
but the cut-off to reduce LW and MW broadcast signals isn't bad
at -125dBm, and providing attenuation of (125-75)=50dB. Compare
with a trace from previous tests. |
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Here's a comparison between
the use of the noise source with an SDR compared with the Rigol
spectrum analyser and its tracking generator. Because of the
low frequencies used the noise source is not ideal because it
uses three 100nF capacitors in the coupling between stages. These
each have an impedance of 40 ohms at 40KHz, increasing to 177
ohms at 9KHz and 800 ohms at 2KHz, hence the drop-off at the
left end of the noise test compared with the high precision of
the Rigol which quotes only -3dB drop at 9KHz. As there are three
100nF capacitors in series in the noise source circuit you can
very roughly estimate the theoretical loss at 9KHz to be 18dB,
and 30dB at 2KHz.
The Rigol tracking generator
has a max output of -10dBm which is a lot higher than is usually
needed but useful at times. This noise source has an output of
-40dBm, but as the supply voltage is sitting at about 3 volts
there's scope for increasing the output by bumping up the supply
voltage to 5 volts at the expense of reduced battery life. |
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See progress on
the Mk2 version... |
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pending |
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