ROHDE SCHWARZ NV/SV-7000 DTV RF Power Amps

[wavebits]

Has anyone worked/played with these? There have been several models that have popped up on the auction site for low prices compared to what they must have cost new. I won a model VH620 (A2 2090.0008.03) which appears to be about a 130W continuous duty air cooled amp with a built in PS.  There have also been some of the VH602 liquid cooled 450W amps. spotted. Both have internal pre-drivers and only seem to require about +6 dBm of input for full output.  They appear to be built like a tank with R&S quality for broadcast service.

Documentation I found online is very sparse so far. Just some general info on the NV-7000 series of transmitters that describe the PA's in general terms. I believe they are broadband LDMOS amps to cover roughly 470-860 MHz. I'm hoping I can get some usable power in the 440-450 range, but it may need some mods for class C operation on 70cm.

The air-cooled PA's were built  for low power terrestrial DTV applications where 2 to 5 amps were typically power combined.. The higher power liquid cooled amplifiers used an external pump and outdoor heat exchager that served several racks of combined PA's.

I am looking for any info to help use these amps in a stand-alone configuration.  There are several db style connectors for control and monitoring for example. It would be great to learn what signals and data may be present. They may well have a serial port for monitoring and diagnostics.

They appear to represent great value, even if parted out for the RF devices.

Thanks for reading..

Dave, K7DMK

[wavebits]

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

[PA0PBZ]

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.

From the brochure:

Interfaces

RS-232-C at the front, operation of transmitter by means of graphical user interface (GUI) from PC, D-Sub, female, 9-contact

RS-485 for remote control of transmitter, at transmitter top, D-Sub, female, 9-contact

RS-232-C for remote control of transmitter, at transmitter top, D-Sub, female, 9-contact (connection of Hayes compatible modem)

Did you poke around with a serial cable?

[wavebits]

Yes, I did connect a cable and poke around with a serial terminal emulator before I opened the PA.  I tried both null and straight cables at multiple baud rates, with not a peep of a response from the female db9 on the front panel.  When the unit was disassembled, the db9 does not appears to be a serial port after all.  I was hoping for a nice command line prompt and a menu that said stuff like: what frequency and output power would you like meir Herr?  No such luck.

I believe the ports refered to in the brochure are on the exciter, not PA. The exciter is a pretty fancy piece of kit with a large color LCD display and GUI interface. They also offer an exciter option called NetLink for IP control and monitoring. The exciter does apparently have contol connections to the PA's. They mention a DC control voltage from the exciter that varies the output power of the PA's  I don't know if this would be like an ALC setup, or perhaps just a menu item in the GUI programming a DAC to provide the DC signal to the amps.

More testing revealed that when the input power exceeds about 10.5 dBm, the "RF in fail" led comes on, a relay clicks and the PA output shuts down until the input level is reduced. At 820 MHz this input level produces about 50W of CW carrier power, but at 470 only about 7 or 8W. This correlates well with the 8 dB difference in gain of the preamp stage at those frequencies. Unfortunately the shutdown occurs at the same input power level at all freqs thus limiting the PA output to about 7 watts at 470 and only about 3 watts going down to 440 in the Ham band. So the preamp must be currently "tuned" to about 800+ MHz. Bummer.

I may have to bypass the entire pre-amp stage and drive the PA board directly with 50 or 100 milliwatts. It looks like the front end preamp is causing most of the problems. I sure wish it could be adjusted to move the gain peak to the lower end of it's range. I think I may be missing something.

Thanks for your comment.

Dave, K7DMK

[PA0PBZ]

Hi Dave,

Yes, reading a bit more I think you are right, the ports must be on the exciter, not the PA. I'm still puzzled that it is not broadband, nowhere I found any remarks about tuning the (pre)amp apart from a fcc document mentioning that the approved operating frequency is 686-746MHz. And one more thing: you mention NV7000 but I think these modules are ment to live in a SV7000 system. Can you post a picture of the naked thing?

[wavebits]

The 7000 series of transmitters include PA's that are either liquid cooled or air cooled and for both North America (ATSC) and European (DVB-T) markets.  I'm not sure if the NV part of the model refers to North America or liquid cooled, but I have seen the SV7000 series most commonly be the lower power air cooled versions, so perhaps SV7000 is correct.

The PA I have is labeled BD IV/V VH620A2  with a p/n of 2090.0008.03  I believe the IV/V part refers to the UHF TV band assignments in some countries. In the UK, band IV consists of channels  21-38 (470-582) and V is 39-68 (614-854 MHz) This implies the amp should cover this entire 470-860 range as they mention in the brochure. The question is whether it needs tuning to do it.

I have the amp back together at the moment. When I tear it apart again I'll take some photos of the PA, preamp and the ASTEC power supply. Next I am going to try driving the main PA board from a higher level RF source without the pre-amp and check the frequency response of the driver/output stages along with the isolators and power combiner.

Here are a few stock pictures from a vendor called ntc. My amp looks identical.

Dave, K7DMK[

[dave_k]

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.

Power levels quoted for DTV amps are for average power output, not CW, due to high crest factors found in OFDM modulated signals (it's not uncommon to have peaks 12db above average). Rohde & Schwarz application note 7TS02 provides more details about this.

[Theboel]

are You sure the pre amp dection do not have some kind of gain controller.
may be an internal picture will help.

[wavebits]

There is a trim pot adjustable through the front panel labeled gain, but it doesn't seem to do very much. There is also a phase control and a pot labeled -3 dB.  Not sure if that refers to a 3 dB compression point or an alarm when the output drops by 3 dB (half power), or something else entirely.

I was mainly refering to the lack of flatness in the frequency response of the pre-amp. The whole amp was advertised as being broad band. It seems to be tuned for the upper end of the range right now. There are no obvious variable caps for tuning though. There is about 8 dB difference in power gain from 470 to 820 MHz in the preamp module.

The other thing that is a bit perplexing is there is a SMA connector on the front panel labled "RF Monitor"  That makes sense to have a low level sample of the output to measure frequency or look at the RF envelope on a scope. But there is a "factory looking"  label below the connector that says "530 MHz/44.64 dB"  (dB not dBm). Following the cable from the connector to the PA, a connection is made to a directional coupler fabricated on the pcb after the combiner. The only thing I can come up here is perhaps the 44.64 dB refers to the coupling factor of this directional coupler in the forward direction at 530 MHz. So if for instance the output of the amp would be 44.64 dBm (around 25 watts or so) then the SMA would supply a 0 dBm sample. Not sure why they would pick 530 MHz and then spec it to 2 decimal places. Strange.

Dave

[CJay]

I wonder if the front end preamp is 'tuned' via the DB-9 conenctors from the exciter?

I think it'd be worth having a prod about in there to see if you can work out the pin functions and maybe draw out the preamp schematic

[wavebits]

CJay:  You may be right about that. Perhaps when the exciter is programmed for an operating frequency an appropriate control voltage is sent to the PA's. They do mention that a DC control voltage from the exciter is used to change the PA output power.  They may be doing something similar to trim the tuning of at least the front-end. There is a db9 labled "control interface"  on the back of the PA. That's  one thing I been looking for documentation on. It might be there are a few strategically placed varactors present in the pre-amp stage that modify the tuning by way of a control voltage. Generally varactors are low level devices. They would not be useful in the output stages, but in the pre-amp that only generates 10's of milliwatts, maybe so.

[dave_k]

In a broadcast transmitter the power amplifier module, exciter and control units are all designed to work as a integrated system to produce an output power of x watts at y frequency. The gain of the pre-amp stage does not need to be flat across the entire 470-860MHz range. The output level of the exciter (which drives the power amplifiers) will be reduced to compensate for extra gain at the higher end of the UHF band.

[Theboel]

In a broadcast transmitter the power amplifier module, exciter and control units are all designed to work as a integrated system to produce an output power of x watts at y frequency. The gain of the pre-amp stage does not need to be flat across the entire 470-860MHz range. The output level of the exciter (which drives the power amplifiers) will be reduced to compensate for extra gain at the higher end of the UHF band.

That exactly why I ask about gain controller

[Emo]

Hi Dave,

The setup normally requires an exciter and a controller(running linux). The exciter is very complex as it generates the full ATSC or DVB carriers from the incoming ASI signal. Power correction is part of the modulation in ordere to have an equal distribution both in time and bandwith. In R&S installations the exciters are mostly placed above the amplifiers. The controller also takes care of the errors in these systems and are capable of seamless switching in a spare transmitter in case of failure. More impressive than the front is the combined output circuit on the rear.
For more information visit the R&S site. A lot of information can be found there.

[wavebits]

Emo: Thanks for your comments.  I have spend some time on the R&S site. This amp is likely part of the SV7000 low power air-cooled transmitter system. I found a two page brochure for the SV7000 that has a few photos. You are correct. The exciter is shown at the top of the rack and it has a nice LCD display with a GUI. The PA's are mounted below; four are show in the particular configuration.

I'm sure the exciter has additional connections to the amps beside the RF cables.  What I can't find is any info that defines these other connections. I'm especially interested in control voltages from the exciter that may tune the pre-amp part of the amp. Without these signals the amp is currently tuned to about 830 MHz. It makes very little power below about 700 MHz right now because of the (mis)tuning of the front end preamp. Ultimately I would like to bring the PA down to 440 MHz. for Amateur Radio DTV use. The PA It is rated from 470-860 MHz and implied in the brochure as broadband. I think it needs a little help with some control voltages to cover even that range.

Dave, K7DMK

[Emo]

Hi Dave,

I am not totally shure about the control info. I recall it was CAN bus. Isn't there a volatile memory on board that is actually representing the last setting this amplifier was used for? Also these amplifiers could be used in analog configurations with almost double output power.

[wavebits]

Emo:  It doesn't look like there is much in the way of digital logic in the PA to support a CAN bus. Most of the non-RF components are op-amps and comparators. I did not find a micro, memory or any dacs. I think that circuitry may be in the exciter and the control signals sent to the amplifier are dc levels. I'm not positive about this, but that's what appears most likely.

Dave, K7DMK

[bob11746]

Dave,
Have you gotten any farther with this amplifier?  Did you ever get any significant power out of the amp?   I am thinking of buying one but I don't need another boat anchor!

Thanks,
Bob

[wavebits]

Bob:  Sorry for the delayed response. I have been working on some other stuff.  As far as the amp, it looks like most of the lack of gain at lower frequencies is occuring in the front end. If you don't need milliwatt input levels the preamp stage can be bypassed easily. There is a short coax that connects the front-end to the PA. If you have a watt or two you can drive the PA directly. That's what I'm planning on doing. I'll let you know some gain vs frequency data of the PA section when I get a chance to get back to the project.

Dave

[NavyBOFH]

I don't have experience with this brand Broadcast equipment but I hope maybe to help wherever I can.

We use all Harris/GatesAir DTV transmitters and all Harris HT or HPX FM transmitter... we wanted to stay consistent across our network.

For our VHF DTV stations they are all solid state PAs and interchangeable between our higher and lower channels. Our one-and-only IOT powered transmitter has its own design. But our Diamond transmitters are spread across most of the UHF band. There's PA bands A, B, and C. If we try to swap PA modules or even FET pallets from one to another, we end up with either a module error or a transmission that is out of the set emissions mask.

Reverse engineering them was a disaster but we ended up finding FETs for the right transmission range and bias them up right.

Case in point is even if you cannot get the PA to tune down to the range you want it to, a set of FETs can probably help... depending on cost. My experience though is that those band limits are pretty solid


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[shore1lk]

Does anyone know how get rohde & schwarz vh6010 a2 be turned on w/o serial cable.I see there's 2 sets of jumpers on amp  but not sure what they do.Any help appreciated.