RF Engineering I
Laboratory #3
1
RF Band-pass Filter Analysis
OBJECTIVES
The purpose of this experiment is to measure the frequency response of an RF
filter and interpret the filter’s performance in the Network analyzer measuring bandwidth,
insertion loss, shape factor, etc.
EQUIPMENT LIST
1.
2.
3.
4.
5.
(1) HP-8712C, RF Network Analyzer
(1) Band-pass Filter (Mini Circuits NIF-70)
(1) N-Type Calibration kit (HP-85032E)
(1) Type-N Accessory kit
(2) Network analyzer cables
DISCUSSION
It is of particular interest in any circuit design to manipulate high-frequency
signals is such a way as to enhance or attenuate certain frequency ranges or bands. There
are four types of filters: low-pass, high-pass, band-pass and band-stop. The low-pass
filter allows low-frequency signals to be transmitted from the input to the output port
with little attenuation. However, as the frequency exceeds a certain cut-off point, the
attenuation increases significantly with the result of delivering an amplitude-reduced
signal to the output port. The opposite behavior is true for a high-pass filter, where the
low-frequency signal components are highly attenuated or reduced in amplitude, while
beyond cut-off frequency point the signal passes the filter with little attenuation. Bandpass and band-stop filters restrict the pass-band between specific lower and upper
frequency points where the attenuation is either low (band-pass) or high (band-stop)
compared to the remaining frequency band.
In analyzing the various trade-offs when dealing with filters, the following
parameters play key roles:
 Insertion Loss – Ideally, a perfect filter inserted into the RF circuit path
would introduce no power loss in the pass-band. In other words, it would
have zero insertion loss. In reality, certain amount of power is lost. The
insertion loss quantifies how much below 0 dB line the power amplitude
response drops.
P
2
IL  10 log in  10 log 1  in
PL
 Flatness – the flatness of the signal in the pass-band can be quantified by
specifying the ripple or difference between maximum and minimum
amplitude response in either dB or Nepers.

By: Prof. Rubén Flores Flores

RF Engineering I
Laboratory #3
2
 Bandwidth – for a band-pass filter, the bandwidth defines the difference
between upper and lower frequencies recorded at the 3dB attenuation
points above the pass-band:
BW 3dB  f U3dB  f L3dB
 Shape Factor – this factor describes the sharpness of the filter response
by taking the ratio between the 60 dB and the 3 dB bandwidths:
BW 60dB
SF 
BW3dB
 Loaded Q – this parameter describes the selectivity of the filter, which
defines the ratio between the center or cutoff frequency and the 3 dB
bandwidth.
The preceding filter parameters are best illustrated by way of a generic band-pass
attenuation profile:
Figure #1: Normalized frequency response of a band-pass filter.
PROCEDURE
Calibration Setup
1. Enter Measurements Parameters
a. Set input power level to avoid amplifier saturation.
1. Press POWER hardkey
2. Press Level softkey -2, and dBm.
b. Set desired frequency range.
1. Press the FREQ key to access the frequency softkey menu.
2. Change the low end frequency to 10 MHz, press Start 10 MHz
3. Change the high end frequency to 500 MHz, press Stop 500
MHz
By: Prof. Rubén Flores Flores
RF Engineering I
Laboratory #3
3
c. Set desired number of measurement points. This allows selection of
the number of measurement points in a sweep. As the number of
points increases, frequency resolution increases and sweep speed
decreases.
1. Press the MENU hardkey
2. Press the Number of Points softkey and enter 801.
2. Performing Transmission Response Calibration
a. Select calibration method and calibration kit.
1. Press CAL Enhanced Response.
b. Connect the four standards open, short, load and through cable in
each port as shown in figure 2.
1. The instrument will prompts you to connect the four standards
2. Press Measure Standard after connecting each standard.
Figure 2: Calibration Standards
3. The instrument will measure each standard and then calculate
the new calibration coefficients. The message "Calibration
complete" will appear.
4. Save the calibration into memory.
a. Press SAVE RECALL, Select Disk, Non-Vol RAM
Disk.
b. Press Prior Menu, Define Save, Cal ON
c. Press Prior Menu, Save State to save the instrument
state file. The filename appears on the screen as
STATE#.STA (where # is a number the analyzer selects
from 0 to 999).
Performing RF Filter Measurements
1. Connect the DUT to the Network analyzer’s port. (RF OUT to filter’s
input and RF IN to filter’s output)
2. Measure the S11, S21 in Log format
3. Determine the filter’s operation frequency. Look at the S21 plot. Using
network analyzer markers determine the point with lower insertion loss.
4. Looking at the S11, what’s the input reflection coefficient?
5. Print the filter’s frequency response.
By: Prof. Rubén Flores Flores
RF Engineering I
Laboratory #3
4
6. Calibrate the network analyzer instrument using the filter’s operating
frequency. Set the frequency range by using the Center and Span
softkeys. Press the Center and enter filter’s operating frequency and press
Span and enter 120 MHz. Recalibrate the instrument. This step will
enhance the measurement accuracy by decreasing the frequency span.
7. Measure and print the S11, S21 and SWR in Log format.
8. Use markers to interpret the data and determine the following parameters:
bandwidth, Loaded Q, shape factor, insertion loss, flatness and SWR at the
input.
RESULTS
Operating
Frequency
IL
BW3dB
BW60dB
SF
QLD
Flatness
QUESTIONS
1. Interpret the performance of the filter in terms of these parameters?
2. Is this a low or high selectivity filter?
3. What kind of filter is, butterworth or chebyshev?
By: Prof. Rubén Flores Flores
SWR
in