Septmber
2005, Issue 182
Signal
Generation Solution
Build
an Inexpensive RF Signal Generator
RF
MODULE
The
RF module PCB is shown in Photo 1a (p. 15). Figure 4
is a detailed schematic of the module.
|
(Click
here to enlarge)
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Photo
1a—The PCB is approximately 2.5²
on a side. It contains mostly SMD devices. The DBM
mixer is in the center, and the two VCOs are located
on the left and right sides. The top part of the
board is the divide-by-64 circuitry. The lower part
contains the IF amplifiers. b—The modified demo
board on the left is attached to the RF module in
its RF tight enclosure. For the sake of clarity,
I omitted the shielded cables normally located between
the RF and LO BNC connectors on the RF module and
the controller. |
|
(Click
here to enlarge)
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Figure
4—The two POS-2000As are used to generate the RF
and LO signals in the SYM-25 DLHW DMB mixer. The
variable gain is accomplished by the AD8367. The
UPB1507 prescalers provide the frequency division
function. |
To
begin the design, I used the Hittite tool to determine
where in frequency I wanted to operate to minimize spurs.
I chose to operate the mixing process between 1.5 and
2 GHz.
The
first component I selected was a Mini-Circuits SYM-25DLHW
mixer mainly because of its operating frequency range.
I then chose a POS-2000A voltage controlled oscillator
(VCO) for the RF and LO oscillators. The POS-2000A’s
output frequency range is approximately 1.3 to 2.1 GHz.
Applying
0 to 20 V to the VTUNE input controls the frequency.
I selected this VCO not only for its operating frequency
range, but also for its output level. It turns out that
if you want good results from a mixer, you need to drive
it at the signal levels it was designed for. The SYM-25DLHW
mixer is designed to have a 10-dBm LO signal level and
an RF signal level that’s at least 10 dB below the LO
(or around 0 dBm). The POS-2000A has an output level
of 10 to 12.6 dBm, which makes it an excellent choice
for both the LO and RF oscillators.
The
LO VCO’s output passes through a small 1-dB pad to help
better match impedance to the low-pass filter. I would’ve
liked to have used a larger pad for better matching,
but I needed to keep the LO signal level to the mixer
at the 10 dBm design specification.
The
LO signal then feeds into a Mini-Circuits LFCN-2250
low-pass filter. The filter’s 3-dB cutoff frequency
is 2.525 GHz. These LFCN-series filters are really slick
seven-section filters contained in a tiny ceramic package.
They take up little board space and minimize the headaches
caused by the parasitics associated with filters constructed
with separate SMT components. The output of the LFCN
low-pass filter drives the mixer’s LO port.
The
output of the LO VCO is also tapped off via a 475-W
resistor in order to feed a UPB1507 prescaler. The input
to the prescaler must be between –15 and 5 dBm to operate
properly. The UPB1507 prescaler divides the analog signal
by 64 and outputs a 1.6-VPP sine wave like
output. This output is transformed into a clean digital
signal by the high-speed TL714 comparator.
The
RF signal path is similar. The main difference, as I
mentioned earlier, is that the SYM-25DLHW mixer is made
to operate with an RF input level at least 10 dB below
the LO level. Two 6-dB pads are included to provide
this attenuation. As an additional benefit, they provide
excellent impedance matching in the RF signal path.
Incidentally,
the pads in this circuit are also made by Mini-Circuits.
They’re extremely compact and take up little board space.
As you can see, I used a lot of Mini-Circuits components.
The company is a quality supplier. The literature it
provided was excellent, and the specifications available
on the company’s web site were thorough and accurate.
There
is a pad at the mixer output for impedance matching.
Matching at the mixer output is critical. The signal
then passes through another LFCN-series low-pass filter
with a 3-dB cutoff of 650 MHz. The filter’s output feeds
into an Analog Devices AD8367 variable gain amplifier,
the purpose of which is to maintain the required 5-dBm
output signal level as the frequency varies. A control
voltage (0 to 1 V) on the AD8367’s gain pin varies the
gain from –2.5 to 42.5 dB. The AD8367’s output is then
amplified once more by a stage that uses a Mini-Circuits
MAV-11 MMIC integrated amplifier. This provides a robust
interface to the outside world. It’s included because
it’s cheaper and easier to replace than the AD8367 if
someone like me abuses the signal generator’s output.
Table
1 shows the expected signal levels at various points
throughout the RF generator. Two operating frequencies
are shown. These values were derived from the detailed
specification sheets of the various components. The
gain as a function of frequency was initially set using
these values, but the measured output was consistently
low by 3.2 dBm. I added a 3.2-dBm constant to the gain
function, which enabled the microprocessor to keep the
RF signal generator’s output at 5 dBm. The gain function
is a straight line fit to this adjusted data.
