I have made a little progress. After a bit of a tear down, reverse engineering, and some online power supply research, I was able to determine the enable signals needed to start the main 32V output from the Astec 1000W switcher contained in the unit. As it turns out, there is a small jumper in the logic section on the rf board that will force an enable signal without a proper interconnect from the R&S exciter. It was probably put there for testing.
Moving this jumper brought up the VCC rail for the amplifier and allowed some evaluation to be done. Using a 100w dummy load, a Bird wattmeter and a signal generator I was able to measure a few parameters.
I was expecting a broad fairly flat response over the 470-860 MHz operating range from the description in the NV7000 brochure, but this was not really the case. On my particular unit, the output peaked at about 820 MHz and dropped off with frequency decrease.
The output stage uses four BLF861A LDMOS transistors which are driven by another 861A. The front end of the amp has a preamplifier to allow low level drive from the exciter (13 dBm) to provide full output. The preamp is connected by a SMA cable to the main amp board allowing a convenient place to make some measurements on the preamp.
What I found was the frequency response was mainly in the front end preamp. With 0 dBm input, the output of the preamp varied from +12.5 dBm at 820 MHz to +4.5 dBm at 470 MHz.
What's puzzling is there does not appear to by any tuning adjustments in the preamp beside a few bias(?) pots. None in the final amps stages either. No variable caps or inductors to be seen. I am wondering if there may be a few varactors scattered about with DC voltage control for trimming the frequency response in the low level stages.
I am concerned that possibly the preamp part of this unit may even come in different UHF frequency ranges. This preamp is in a isolated enclosure near the front of the unit.. It also seems that there is no microprocessor or serial port as I originally thought. I'm thinking now the db9 service port on the front was possibly designed to have some custom monitoring box plugged into it for servicing. Sort of like the old Moto diagnostic metering panels. There are several IC's scattered around the board along with several small relays. Most of the IC's are OP amps and comparators with very little digital logic.
The construction of this 3U rack-mount amp is very impressive and obviously very conservatively rated. There are over 600 watts worth of output transistors used to generate about 130W out. Each of the two pairs of transistors appear to have a circulator/isolator before the power combiner as well as an on-board directional coupler for power monitoring and vswr protection. The heat sinking/cooling system is also nicely done.
I sure wish I could find some info on these units. I suspect there is a large manual covering the entire SV/NV7000 transmitter that ships with the system. A copy of a few of the pages describing the functions and pinouts of the three db style connectors anong with any tuning adjustments ,would be most helpful.
My goal is to get this down to the US 70cm Amateur band for digital TV service where the amp's good linearity will be a great asset. If I am unable to get the frequency tuning down that far, I will use it a a general purpose broadband RF amp in my RF lab.
Has anyone worked with these PA's?
Dave, K7DMK