TEKBOX TBHPF1-150kHz 150 kHz High Pass Filter User Guide
- June 3, 2024
- TEKBOX
Table of Contents
TEKBOX TBHPF1-150kHz 150 kHz High Pass Filter
Technical Data
- High Pass Filter, reflective, 50 Ohm
- 3dB bandwidth : 150 kHz – 3 GHz
- Maximum input voltage : 100V; 250V for < 5 sec.
- Maximum permitted input current at frequencies < 150kHz and open output: 650 mA
- Maximum input power at frequencies 300 kHz – 1 GHz : 10W
- Connectors : N-Male / N-Female
- Dimensions : 26 x 26 x 82 mm
- Weight : ca. 100g
Frequency response
Typical data, 50 Ohm system
TBHPF1-150kHz: Attenuation 30 Hz to 1 GHz, typical data Attenuation table, 50 Ohm system
Applications
- General purpose filter
- Spectrum analyzer / measurement receiver RF input protection
Analyzer RF frontend limitations:
Whenever working with spectrum analyzers, be aware that excessive input power,
voltage transients or ESD can destroy the RF-frontend. Spectrum analyzers
typically have a maximum CW input rating in the range of +20 dBm to +30 dBm.
Unlike oscilloscopes, spectrum analyzer inputs are not protected or only
vaguely protected. A simplified RF frontend looks as below
150 kHz HIGH PASS FILTER
The diodes at the input typically serve as ESD protection diodes. In order to
fully protect the input with a limiter, shunt diodes would need to be combined
with a series resistor to limit forward current in case of excessive input
signal. Consequently, a classic current limiting resistor solution cannot be
implemented, as it would increase the input impedance of the analyzer. A
limiter could be implemented by combining it with an attenuator, however this
would degrade the sensitivity of the analyzer and limit its use.
The first weak link of the input chain is the RF switch. Typical EMI spectrum
analyzers use integrated GaAs switches. GaAs switches are inherently weak at
low frequencies. Many GaAs switches are not even specified with respect to
maximum input power at low frequencies, down to 9 kHz. Below is an example of
an “honest” data sheet of a typical GaAs switch:
The maximum RF input power ratings versus frequency clearly show the degradation at low frequencies.
Residual 50 Hz voltage at the RF output of LISN:
The picture below shows the basic circuit diagram of an AC LISN:
The full AC mains voltage is present at the DUT terminals of the LISN. The RF
coupling capacitor and the 1K resistor form a voltage divider, which
determines the amplitude of the 50 Hz voltage at the RF connector. Considering
a 0.1μF capacitor, which has an impedance of 32K at 50 Hz. Together with the
1K resistor, the resulting 50 Hz voltage at the RF terminal of the LISN is
approximately 6.6V in the absence of any load. Assuming a 50 Ohm load in
parallel to the 1K resistor, the residual 50 Hz voltage would become
negligible. Some LISN may need to use a higher value, in order to meet the
impedance specifications at lower frequencies. Assuming a 0.5μF capacitor, the
residual 50 Hz voltage at the RF terminal of the LISN would be as high as 31V.
This still would become reduced to a negligible value, if a 50 Ohm load in
parallel to the 1K resistor is present.
However, it is unlikely that a low cost analyzer has an input impedance of 50
Ohm at 50 Hz. Typically the input impedance is not specified for frequencies
below 9 kHz. Furthermore, as already mentioned, low cost analyzers use GaAs
switches at the RF input, which are inherently vulnerable at low frequencies.
Consequently, placing a 9 kHz high pass filters, or 150 kHz high pass filters
at the RF input of a spectrum analyzer or measurement receiver will offer good
protection against residual 50 Hz voltage. Due to its very low insertion loss
in the pass band, it will not reduce the dynamic range of the measurement.
Sub-harmonics of switched mode regulators:
When carrying out conducted noise tests of switched-mode power supplies, the
highest spurious levels occur at relatively low frequencies. Sub-harmonics are
even more critical. These are typically at frequencies significantly below 100
kHz and unless they produce audible noise due to magnetostriction of the power
inductors, they often go completely unnoticed, as most tests start at 9 kHz or
150 kHz. You may carry out a conducted noise test and wonder, why the analyzer
beeps and displays an ADC overflow warning, despite all spurious being well
below limits. What drives the attenuator into saturation may be a very high
amplitude sub-harmonic at e.g. 6 kHz. In case that you notice that your
signals are in the range of 20 dB lower than what they actually are, disaster
already happened. The first GaAs switch is already damaged. In most cases, it
fails with a short on the RF path and protects the following components, but
in extreme cases, the damage will reach as far as the first mixer. In order to
prevent such things happen, you always should start investigating any new EUT
using external attenuators, high pass filters or a combined
attenuator/filter/limiter. With an external 20 dB attenuator or limiter
attached to the analyzer input, have a look at the spectrum at very low
frequencies and ensure that there are no signals with critically high
amplitude. Alternatively, you can first connect an oscilloscope to the LISN RF
output and check the EUT emissions in the time domain. In order to establish
the same impedance level as with a connected spectrum analyzer, terminate the
oscilloscope input with a 50 Ohm feed through or switch the input to 50 Ohm,
if the scope offers this feature
Here are some guidelines when performing conducted emission measurements with a LISN:
- Leave the RF output of the LISN unconnected
- Connect the EUT to the LISN
- Connect the LISN to the isolation transformer
- Power on the EUT
- Check the RF output of the LISN using a scope and / or the analyzer with an external 20 dB attenuator and 9kHz (150kHz) low pass filter or combined attenuator/filter/limiter
- Connect the RF cable from LISN output to the spectrum analyzer input
- Carry out the conducted noise scan
- Disconnect the RF cable
- Power off the EUT
NOTE : The purpose of having the analyzer disconnected during power cycling (ON/OFF) the EUT is to avoid voltage transients due to back EMF, especially of highly inductive loads such as motors or switched-mode power supplies. These signals can easily be large and fast enough to cause permanent damage to the sensitive RF front end of the analyzer. In cases where the EUT produces sub harmonics, place a 9 kHz or 150 kHz high pass filter at the RF input of the spectrum analyzer. Alternatively, the Tekbox TBFL1 transient limiter not only contains a combined attenuator/limiter, but also a 9 kHz high pass filter.
Ordering Information
History
www.tekbox.com TekBox Digital Solutions Vietnam Pte. Ltd. Factory 4, F4, Lot I-3B-1, Saigon Hi-Tech Park, Tan Phu Ward, District 9, Ho Chi Minh City, Vietnam