2-250 MHz Synthesized Network Analyzer Project

By: Steve Hageman

 

Published in the March 2002 issue of QEX Magazine

 


I built this project as an add on for my VHF source project. With the addition of just 8 IC's, a full reflection / transmission scalar network analyzer can be built. The analyzer rivals it's commercial cousins in that it incorporates full open/short/load and through line calibrations to enhance accuracy.

The basic analyzer uses two PLL's mixed together to get a 2-250 MHz output. The PLL's use Mini-Circuits VCO modules and Motorola MC145191 PLL chips to produce either a fixed 750 MHz or a variable 500-748 MHz output. These two PLL's operate with a reference frequency of 100 kHz for excellent reference spur rejection and fast lockup times. The two PLL outputs are difference mixed and filtered to produce a 2-250 MHz output. The 1 Hz frequency resolution is achieved by driving the variable PLL's reference input with the output of a Intersil HSP45102 NCO (Numerically Controlled Oscillator). The Intersil NCO chip has 32 bits of frequency resolution, so achieving an overall 1 Hz synthesizer resolution is a snap. The output of the mixer is amplified to +15 dBm (+17 dBm Typical) with Agilent Technologies MSA-1105 MMIC Amplifiers. By using low noise and low intermodulation design techniques, the sources Phase Noise and Spurious Responses are kept at very respectable levels.

The last output stage uses a directional coupler on the output to provide a true leveled output. The output level is sensed with an Analog Devices RF Log Amplifier (AD8307). This sensed level is compared to a DAC value that is programmed to the desired output in dBm, the error value from this comparison is used to servo a PIN diode attenuator and Viola, we have a true leveled output. The output leveling circuitry frequency response has been carefully shaped to give very low two tone intermodulation when two sources are combined.

The measuring receivers used in the network analyzer are based on Mini-Circuits directional couplers and Analog Devices AD8307 Log Amplifiers. A separate receiver is used for each of the three R, A and B channels.

The analyzer is controlled by a PC via an RS232 link running at 19.2k BAUD. The analyzer uses a Microchip Technology PIC16C63 microprocessor to translate the RS232 commands from the PC to the proper setups required by the PLL's, NCO, receivers and other hardware. The PIC also monitors the status of the PLL's and leveling loop and can report this back to the PC for display.

The software 'Virtual Front Panel' was built with Agilent VEE using a ActiveX control interface from the Instrument to VEE. The software employs many advanced capabilities such as spur avoidance and digital filtering of the receiver outputs.

 

Here is an actual measurement trace from the Analyzer measuring a 6 pole, 21.4 MHz crystal filter.

The Yellow trace above shows the Transmission or B/R measurement. The Cyan trace is of the filters return loss.

The front panel for this application was built with Agilent VEE which provides excellent graphics and full cursor measurement functions.

 


NETWORK ANALYZER TYPICAL PERFORMANCE

Frequency Range....................................................2-250 MHz

Frequency Resolution........................................... 1 Hz (Displayed, sub Hz actual Resolution)

Reflection Measurement Accuracy

0 to -10 dB Reflection.................................. +/- 1 dB

-10 to -20 dB Reflection................................ +/- 2 dB

-20 to -30 dB Reflection................................ +/- 5 dB

Transmission Measurement Accuracy

Through Line.............................................+/- 0.1 dB

To +/-40 dB...............................................+/- 1 dB

To +/-60 dB...............................................+/- 2 dB

Source Amplitude Range (Calibrated).................... -15 to 15 dBm

Source Amplitude Resolution.................................. 0.1 dB

Source Maximum output........................................ +17 dBm

Source Amplitude Accuracy.................................. +/-0.5 dB over full frequency and power range

Frequency Drift per hour (After warmup)................... 0.0002 %

Frequency Stability............................................... 0.001 %

Over temperature of 15-35 deg C ambient

Phase Noise:

-64 dBc/Hz @ 1 kHz Offset

-90 dBc/Hz @ 10 kHz

-110 dBc/Hz @ 100 kHz

Harmonics: 2 MHz to 20 MHz............................. < -30 dBc

20 MHz to 250 MHz............................... < -45 dBc

Non Harmonics: 2 to 220 Mhz............................ < -65 dBc

220 to 250 MHz..................................... < -35 dBc

LO and RF feedthrough...................................... < -55 dBm

Clock feedthrough (fund and harmonics)............. < -85 dBm

Output Match (VSWR) 2 MHz to 7 MHz............ < 2:1 @ +15 dBm output

7 MHz to 250 Mhz.............................. < 1.3:1 @ +15 dBm output

 

Note: Output at 100 MHz, 0 dBm output power and temperature of 25 degrees C, unless otherwise specified.


Project Information / FAQ's:

As with my other projects I will be keeping a list of FAQ's here on the project. As folks start asking questions I will post the information here. Be sure to check the VHF Source FAQ page since so much of this project is shared between the projects.

 

Article FAQ:

1) Reprints of the VHF Source and VHF Network analyzer are available for a nominal fee from www.arrl.org/qex . Sorry, I cannot supply reprints.

VHF Source article - QEX, January 2000 Network Analyzer Article - QEX, March 2002

 

Parts FAQ:

1) As per all too often the case lately, the Motorola MC145191(3) is getting hard to find. Does anyone have extras or know of some parts in stock anywhere? Contact me if you do.

 

Hardware FAQ:

1) Bob (A.K.A. "Last of the great Hams") asked: You mentioned in the article to keep the A and B receivers grounds separate, but you put them in the same box, what gives? OK, that is a good one, here is what I really meant: You only need 50 dB or so of isolation between the receivers to make good measurements. So I was trying to say: Keep the RF grounds separate, that is don't loop one receiver's input circuit around the others! If you keep both receivers in the shielding enclosure separated by and inch or so you will achieve the required isolation.

 

Software FAQ:

1) You don't have to buy Agilent VEE to run the analyzer. I can (and do!) supply a compiled run time only version with the required libraries on the CD (see article). I am allowed to make and distribute royalty free run time only versions of the VEE programs I write. All you need are the VEE runtime libraries which are also free from Agilent.

2) Steve R had a great idea to add an option to turn on and off the Spur Avoidance and to allow directly controlling the source from the VHF_PNA.VXE application. So I have a new version for you to download here.... Click here to download the 1.1.0 Version of the VHF_PNA run time file (12k bytes).

3) Steve R also had a great idea to make a power meter out of the project, so I wrote a power meter runtime application in VEE. Click here to download (3k bytes). The Power Meter.vxe should be placed in the same directory as the VHF PNA.VXE application as both need access to the pna_ports.ini file. The power meter uses the B term receiver and the source for calibration. When the program first starts it asks you to connect the source to the B term receiver. Then it asks you to select the frequency that you wish to calibrate at. After pressing the continue button the source is set to the proper frequency and the B term receiver is calibrated for gain and offset. When the main screen appears the source is set to zero dBm and the power meter should be reading 0 dBm. Disconnect the B term receiver and connect it to the source you want to measure. If during the calibration you get a divide by zero error you did not have the source connected to the B term receiver correctly so the calibration could not be done. Restart the program and make sure that the B term receiver is properly connected.

 

Downloads Available:

PIC Firmware - The PIC HEX programming file is available by clicking here. This file is in a format that may be directly used by Microchips PicStart Plus Programmer (under MPLAB).

The full application software is too large to be posted, it is available from me as described in the QEX article on a single CD ROM and includes all the software/firmware for the VHF Source project too. Contact me for ordering information.

Copyright - The software provided by Steve Hageman is free when used with the project as described in the QEX article. All other commercial rights are reserved and are subject to a license agreement that you may obtain by writing me.


Other Information about Network Analyzers and VEE on the WEB:

Agilent Technologies (formerly Hewlett-Packard), the world leader in test and measurement has many application notes available on Network Analyzer technology, please take time to visit their web site at www.agilent.com

You might be able to find the application notes by following this link: www.agilent.com/find/apps Then search for "network"

For more information on Agilent VEE visit the VEE Homepage at www.agilent.com/find/vee


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Updated - 14Jul02

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