Title:
Investigation of Trim Notch and its Effect on
Surface Mount Resistors in RF Circuits
Document No.:
BAN-SMR-1.1
Revision:
A
Author:
Ron Schmidt
Date:
05
/0
8/2002
Investigation of Trim Notch and its Effect on Surface Mount
Resistors in RF Circuits
08 May 2002
R. Schmidt
Barry Industries, Inc.
Introduction
Surface mount (SMD) resistors are often employed in microstrip circuitry at RF frequencies. Typical SMD
resistors designed for general purpose circuitry are usually trimmed using plunge trim or “L” trim
techniques. This involves removal of material in a narrow groove either perpendicular to the current flow
(plunge trim) or in a pattern first perpendicular to current flow and then parallel to the current flow (“L”
trim). These configurations can exhibit large variations in RF impedance depending upon the degree of
trimming and are therefore not considered here. Instead, SMD resistors intended for use in RF applications
are typically trimmed using the scan trim technique. In this process the material is ablated parallel to the
flow of current starting at one edge of the film. This effectively narrows the resistive film without
changing the uniform area perpendicular to the current flow. The effect of this trim technique on the RF
performance of a typical SMD resistor is investigated here.
Description
The configuration examined here consists of a 50 ohm 0805 SMD resistor mounted to a 0.075” wide
microstrip circuit trace on a Taconics TLC (er=3.2) substrate 0.031” thick. One end of the trace is
connected to the input port and the trace connected to the other terminal of the resistor is connected to the
microstrip ground plane with two 0.008” diameter vias. Since the characteristic impedance of the
microstrip line is approximately 50 ohms, this represents a typical application of a resistor for an RF
termination.
SMD Resistor with Scan Trim Mounted to Microstrip Line
Title:
Investigation of Trim Notch and its Effect on
Surface Mount Resistors in RF Circuits
Document No.:
BAN-SMR-1.1
Revision:
A
Author:
Ron Schmidt
Date:
05
/0
8/2002
Results
The configuration described above was simulated for trim widths of 10%, 20%, 30%, 40% and 50%. The
log magnitude and Smith Chart representations are shown below. It can be seen that the variation increases
with increasing frequency and with increasing trim amounts. This intuitively satisfying result still doesn’t
completely answer the question of what is the acceptable range for a given application. In particular, if the
circuit is tuned for maximum return loss over a particular frequency range, the variation (in dB) could be
much larger due to the logarithmic relationship. The model does, however, underscore the importance of
careful control of the trim process employed in producing SMD resistors for RF applications.