GBPPR 1 GHz RF Spectrum Analyzer, GBPPR 1 GHz RF Spectrum Analyzer
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GBPPR 1 GHz RF Spectrum Analyzer
Overview - Part 1
A RF spectrum analyzer is one of the most useful tools an experimenter can have. The spectrum
analyzer project shown here will be a slight improvement on the "
(
project from
(W7ZOI) and Terry White (K7TAU) which was
covered in the August and September issues of
magazine. There will also be a few ideas taken
from the
project by Scotty Sprowls, most notably, his really nice first IF
(1013.3 MHz) bandpass
filter. You
must
read and study carefully the "
article before embarking on this project, as it covers some of the more in-depth
concepts and construction notes which will not be covered here. Wes Hayward also has several
"
PDF files on his
which should also be studied as they include
a number of tweaks and improvments to his design. These update files also include the necessary
final IF/logarithmic amplifier calibration information.
Most of the components used in this RF spectrum analyzer project were obtained from ham radio
swapfests or salvaged from old two-way radios, cellular phones, or cable TV distribution
gear. Some components may be hard to track down, but equivalent parts can be substituted with
minor tweaking of the circuits. No PCB patterns will be provided as all the PC board layouts were
done by hand using Sharpie markers and a straight edge. You'll want to keep the PC board traces as
short as possible and utilize a large ground plane and isolate the RF inputs and outputs. The PC
boards were all made from double-sided (1 oz.) 1/32-inch FR4 material.
The key to building a high-performance RF spectrum analyzer is isolation and shielding. Construct
the spectrum analyzer as a series of well-shielded modules which each perform only one
function. This will also make debugging (and future modifications) much easier.
All
the individual
modules and their interconnections should be shielded and filtered. Coax jumpers should be high-
quality RG-142, or (ideally) 100% shielded comformable coax like UT-141. Failing to properly do
this will result in numerous spurious images in the final displayed spectrum.
Voltage regulators will also not be documented in the schematics, but try to use the newer, lower-
noise equivalents of the common three-terminal voltage regulators. The two VCOs should be
operated off of a separate voltage regulator from the other circuits. Each module should have its
own voltage regulator, with feed-through capacitors and ferrite beads on the incoming lines, for the
best interference/noise rejection performance.
This spectrum analyzer will consist of an incoming RF signal, up to 1000 MHz and -30 dBm max.,
feeding a Mini-Circuits SRA-11 mixer which has a sweeping Local Oscillator (LO) signal from
1013 to 2013 MHz. This mixer will create a 1013.3 MHz first IF output which will then be sharply
bandpass filtered using a 4-pole cavity filter made from copper pipe plumbing parts. The 1013.3
MHz first IF will then feed another Mini-Circuits SRA-11 mixer which has a fixed LO signal of
1024 MHz. This mixer will produce the final IF frequency of 10.7 MHz. The 10.7 MHz IF signal
will then pass through a narrowband resolution filter and then onto additional IF amplification and
logarithmic detection using an Analog Devices AD603 and AD8307. The final video output signal
will be a logarithmic representation of the incoming RF signal and will drive the "Y" input (0.5
V/div) on an oscilloscope operating in "X/Y" mode. A sweep generator will control both the
sweeping first local oscillator and the "X" input (0.5 V/div) on the oscilloscope.
Component changes in several pictures are due to constant tweaking. Refer to the schematics for
the final design. Updates to this project will be available at
Block Diagram
Pictures & Construction Notes
Using a surplus Mini-Circuits
PIN diode switch case for holding a Mini-Circuits
mixer.
ZMSW-1211 PIN diode switches are often available at swapfests for very low cost and the
aluminum case they use (and PC board pattern) are perfect for converting into RF mixers.
There are probably much better mixers for use in this spectrum analyzer project, but I got these
SRA-11 mixers for free. The "ideal" mixer will have 40+ dB isolation between the LO and RF
ports. Note that the first mixer will have the incoming RF signal (DC to 1000 MHz) on its IF
port. This is done as the IF port has a better low frequency response than the other mixer ports.
Modifying the PC board of the ZMSW-1211 PIN diode switch to fit a SRA-11 mixer.
Isolate the SRA-11's pin 3 on both sides of the PC board by grinding away the copper using a
Dremel tool.
A resistive 3 dB attenuator pad will be added to the LO port. Note the picture shows components
for a 6 dB pad.
Pinout for the Mini-Circuits SRA-11:
LO
Pin 8
RF
Pin 1 (Blue Ring)
IF
Pin 3
Ground
Pins 2, 5, 6, 7
Not Used
Pin 4
Putting the new SRA-11 mixer together.
A small little copper shield was added to further help isolate the LO and RF ports.
This spectrum analyzer project will be housed in in an old Hughes AML-STX-141 Ku-band CATV
link transmitter case.
Power for the modules will be provided by two linear power supplies. Don't even think of using
switching power supplies - as most are too noisy.
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