Well, folks, here it is! After two board revisions and many a long night, I have succeeded in reverse-engineering and reproducing the output hybrids for the HP 8601A sweep / signal generator. If you are an 8601A owner, you probably know that this part is the Achilles’ heel of the 8601 – it is usually the first thing to fail and, until now, good ones were about as common as moon rocks…
This sale is for one new, reproduction, HP output amplifier module. These reproduction modules use the same circuit as the original, which was determined by microscopic analysis of a failed part, and perform equally well. In fact, the reproduction module provides less distortion at high output level, has about 3dB more gain, and is flatter (at least by my power meter) than the original. You can see for yourself in the last three photos. The first of these shows the open-loop gain of the original and reproduction. As you can see, aside from a small bump at low frequency, they are nearly identical. The second one shows the levelled output as taken with a detector, demonstrating that the flatness of the reproduction is almost perfectly identical to that of the original. (Note that the ‘droop’ at low frequency is due to the detector I am using not being calibrated under 10MHz). The third and final picture is an overlay of the low frequency output of the original and reproduction at +20dBm. These units always produce clipping at high output (according to the service manual “…noticeable distortion will almost always be present at some RF output frequencies with output levels above about +16dBm.”) but the reproduction produces less, and it is more symmetric between the top and bottom of the waveform.
To assure longevity, these reproductions are designed using top-quality NXP, Avago, and Vishey parts which are all operated within their specified maximum ratings. However, if trouble ever does arise, unlike the originals, these are repairable, and a complete service manual, including a schematic, disassembly directions, a board layout diagram, and a parts list, is available here.
Finally, if you are interested in the development behind this product, please read my blog post here.
Download the schematic and PCB layout for this product here. Note that these files are available for PERSONAL, NON-COMMERCIAL USE ONLY. In short, you are welcome to use them to build this product yourself, but you must contact me to work out a commission if you wish to sell anything derived from them. This product was designed in ExpressPCB so you will need to download a copy of that software from expresspcb.com before you can view the files. If you want to view the simulations you will also need LTSPICE which can be found at http://www.linear.com/designtools/software/.