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OPERATING HP 141T
SPECTRUM ANALYSER
February 2011

Understanding the HP 141T Spectrum Analyser is my first attempt at using spectrum analysis. Most of my test equipment is fairly easy to use and I have had previous experience with similar items and their applications. Spectrum Analysers are one (of the many) areas where I have little knowledge, although I appreciate their general function, so these are my working notes.

HP Spectrum Analyser Equipment from top to bottom:
8556A; 8553B; 8445B; 141T; 8555A; 8552B; 8444A; 8443A;
plus special cables.

November 2014

The Spectrum Analyser measures frequency against amplitude, unlike an Oscilloscope that measures time against amplitude. If you want to see a signal at say 1GHz, this is the way to do it. This Spectrum Analyser has a frequency range of 20Hz to 18GHz and an amplitude range of -130dBm to +10dBm.

HP 141T Display Section with HP 8553B RF Section
and HP 8552B IF Section plug-ins.


The CRT display is 8 x 10 with frequency horizontal, Log Ref levels -10dBm, -20dBm, -30dBm, -40dBm, -50dBm, -60dBm, -70dBm on the right hand side vertical and Linear Ref levels 7, 6, 5, 4, 3, 2, 1 on the left hand side vertical.

Display showing a pulse at the centre frequency
PLUG-INS

There are various plug-ins available.

The horizontal 'X' signal is provided by the HP 8552B IF Section which is the lower plug-in. There are 2 versions: the 8552A and 8552B. The B version is the higher specification and the one that I have. This has a scan timebase of 0.1ms to 10s.

The vertical 'Y' signal is provided by the RF or LF Sections which are the upper plug-in. There are 4 versions: the HP 8553B RF Section for 1kHz to 110MHz; the HP 8554B RF Section for 100kHz to 1,250MHz; the HP 8555A RF Section for 10MHz to 18GHz; and the HP 8556A LF Section for 20Hz to 300kHz. I have the 8553B, 8555A and 8556A.


TRACKING GENERATORS

There are 2, well actually 3, Tracking Generators. The HP 8443A Tracking Generator Counter for 100kHz to 110MHz and the HP 8444A Tracking Generator for 500kHz to 1.3GHz. The 8556A also includes its own tracking generator.


PRESELECTOR

The HP 8445B Automatic Preselector is a YIG tuned filter for use with the Spectrum Analyser 8555A (10MHz to 18GHz) RF Section. The Preselector can be tuned exactly to the Analyser's reception frequency and eliminates harmonic mixing image and multiple responses from 1.8GHz to 18GHz. Below 1.8GHz the image and multiple responses are eliminated by a low-pass filter.

SPECIAL CABLES

There are various special cables to inter-connect the equipment.

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CALIBRATION

Before the Spectrum Analyser can be used, it must be calibrated using the following procedure. A calibration set up procedure is provided for the 8553B RF Section and then the 8555A RF Section.

Calibration Set Up HP 8852B IF Section with 8553B RF Section.
[Based on HP 8553B & 8552B Operating and Service Manuals]


NOTE: Ensure the 8552B / 8553B D-type connectors are fully mated.
Poor mating is often the cause of X & Y display problems!


Switch HP 141T Spectrum Analyser on.
Non Storage mode.
Intensity to 1 o’clock approx.
Focus to 1 o’clock approx.
Allow to warm up for 30 minutes approx.

Set 8553B Controls as follows:
RANGE MHz 110MHz
FREQUENCY 40MHz
FINE TUNE Centred
BANDWIDTH 300kHz
SCAN WIDTH 0-100MHz
PER DIVISION 10MHz
INPUT ATTENUATION 10dB
TUNING STABILIZER On

Set 8552B Controls as follows:
MANUAL SCAN Centred
BASELINE CLIPPER CCW
SCAN TIME PER DIVISION 5ms
LOG REF LEVEL 0dBm
LOG REF LEVEL Vernier 0
LOG LINEAR 10dB LOG
VIDEO FILTER 10kHz
SCAN MODE INT
SCAN TRIGGER AUTO

Connect 8552B CAL OUTPUT to 8553B RF INPUT using a BNC to BNC 50Ω coax cable.

The display on the 141T should be approximately as follows:


The zero frequency 1st L.O. feed through at 0MHz.
The Calibration Signal at 30MHz and -30dBm.
The 2nd harmonic at 60MHz and -40dBm.
The 3rd harmonic at 90MHz and -50dBm.

Display Section Adjustment

Set LOG REF LEVEL max CCW.
Set SCAN TIME PER DIVISION to 10s.
Adjust FOCUS and ASTIG for smallest round spot possible.
Reset SCAN TIME PER DIVISION to 5ms.
Adjust TRACE ALIGN so that horizontal base line is parallel to the bottom horizontal graticule line.


IF Section Display Adjustments

Adjust VERTICAL POSITION so that the horizontal base line of the CRT trace is exactly on the bottom horizontal graticule line of the CRT.
Adjust HORIZONTAL POSITION so the display is centred on the CRT.
Adjust HORIZONTAL GAIN so the displayed scan width is exactly 10 divisions on the CRT.
Some interaction between HORIZONTAL POSITION and GAIN controls may require readjustment.

The inverted marker below the horizontal base line indicates the centre frequency of the SCAN WIDTH – PER DIVISION and ZERO tuning modes. 
Adjust FREQUENCY to place the marker under the Calibration Signal at 30MHz and achieve maximum null.
Set SCAN WIDTH to PER DIVISION and .05MHz.
Set BANDWIDTH to 10kHz.
Adjust FREQUENCY FINE TUNE to centre the signal on the CRT.

Adjust LOG REF LEVEL so maximum signal amplitude is exactly on -70dBm graticule line.
Rotate LOG REF LEVEL 7 steps clockwise. The amplitude of the signal should increase one vertical division per 10dB step.
Adjust VERTICAL GAIN to place maximum signal amplitude exactly on the top graticule line.
Repeat the previous 3 steps to obtain optimum setting.


RF Amplitude Calibration

Set LOG REF LEVEL to -30dBm.
Adjust AMPLCAL to place maximum signal amplitude exactly on the top graticule line.

The Spectrum Analyser is now calibrated in the LOG display mode from -130dBm to +10dBm.

NOTE: Maximum RF Input level is +13dBm, 20mW, 2.8Vac peak to peak, 1V rms, ±50Vdc. To avoid damage to the Mixer, initially utilise maximum input attenuation and/or an external attenuator!  


LINEAR MODE

Set LOG REF LEVEL to 1mV/DIV
Set the LOG – LINEAR switch to LINEAR.
Set LINEAR SENSITIVITY to 1mV (Vernier 0 position).
The display on the 141T should be as follows:

Display showing the 30MHz calibration signal at 7.1mV
The left hand side vertical graduations 0 to 8 are calibrated in absolute voltage.

Maximum signal amplitude should be 7.1mV, if necessary adjust AMPL CAL to achieve this.

The Spectrum Analyser is now calibrated in the LINEAR display mode from 0.1µV to 100mV.

Return the 8552B and 8553B controls to their starting position.

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Calibration Set Up HP 8852B IF Section with 8555A RF Section.
[Based on HP 8555A & 8552B Operating and Service Manuals]


NOTE: Ensure the 8552B / 8555A D-type connectors are fully mated.
Poor mating is often the cause of X & Y display problems!


Switch HP 141T Spectrum Analyser on.
Non Storage mode.
Intensity to 1 o’clock approx.
Focus to 1 o’clock approx.
Allow to warm up for 30 minutes approx.

Set 8555A Controls as follows:

... more to follow ...
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System Set Up for Swept Measurements

Set Up for Swept Measurements

This is the setup for swept measurements over the 10MHz to 1.3GHz range using the 8444A Tracking Generator. The display shows the fall off at 1.4GHz approx.

First the 8444A is connected to the 141T. At the rear, a special cable connects the 8444A 3rd Local Oscillator input to Aux A on the 141T. At the front: the 8444A Scan Output connects to the 8552B Scan In/Out; the 8444A First LO Input connects to the 8555A First LO Output; the RF Output could connect to the 8555A Input but in this case connects to the 8445B Input.

The 8445B is connected to the 141T. At the rear, a special cable connects the 8445B to Aux B on the 141T. At the front, a special rigid cable connects the 8445B Output to the 8555A Input.

The 8443A is not used.

The DUT (Device Under Test) is inserted between the 8444A Output and the 8445B Input. For convenience of making measurements between direct and via the DUT, I use 2 HP 8761A SPDT RF Switch which operate between DC and 18GHz.

The 8620C Sweep Oscillator with appropriate plug-ins could be used instead of the 8444A to provide swept measurements up to 18GHz.

The 8443A and 8553B could be used for swept measurements in the 1kHz to 110MHz range.

The 8556A can be used with its own tracking generator over the range 20Hz to 300kHz.


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... more to follow ...