HP1121A Bandwidth Measurements

Following up on an earlier post for the HP1121A 500 MHz RF probe, today's activity was to review the RF bandwidth performance and ensure it complied with the stated 500 MHz 3dB parameter in its spec sheet. (The earlier posting just confirmed that the probe was functional and captured details about creation of the DIY power supply and related DC block probe output termination.)

The idea for checking the high frequency probe's 3 dB point would involve use of a HP11063A 50 Ohm Tee along with some GR adapters to yield N-female interfaces and N to SMA adapters after that. The HP11063A probe adapter has a special mounting point on the mid-section that allows insertion of the HP1121A probe tip into it to make contact with the RF center conductor within. The RF return/shield is also connected via a slotted outer line with tightening nut to hold the probe firmly in place.

Here is a top down view of the probe insertion point showing the small through line coaxial line contact within.

The setup thereafter uses the NanoVNA-H4 vector network analyzer to 1) calibrate the through line frequency response, 2) move the through line output coaxial cable to connect the VNA CH1 port to the HP1121A output BNC (with attached DC block per HP1121A manual), and 3) install a 50 Ohm termination on the HP11063A through line output port connection. Here's a photo of the completed setup with probe installed into the tap.

This yielded a somewhat flat response curve. The low frequency roll off was interesting though... What's happening there?

With that observation, decided to try another method of connecting the probe to the VNA CH0 port signal and still providing a 50 Ohm termination on that path. Put together a simple BNC-T based system as a next step.

The low frequency roll off was not present in this approach. Here is a close up of the S21 plot. Looks pretty good!

Adjusting the VNA vertical scale yielded this zoomed-in plot:

With less than 2.5 dB variation over the trace up to the marker point at 657 MHz, I can say with some confidence that this probe exceeds its stated 3 dB bandwidth of 500 MHz. That's pretty good for an item that was sold for $650 in the 1980 Hewlett Packard test equipment catalog.

Will try to find time to look at gain flatness specs and other provided performance parameters in a future post. For now, am very confident that the HP1121A RF probe is working and providing reasonable data up to over 600 MHz.

All author photos taken with an iPhone 16e.