200 MHz high power rectangular wave

[htg]

I would like to generate high power (several kW) rectangular wave at 200 MHz.
1) Can anyone recommend properly broadband (0.2 - 2 GHz) power transistors for this purpose?
2) What would be the best way to load a 200 MHz resonant LC circuit using such a square wave?

 

[tony_lth]

Is the wave CW or pulse?
You can find high power transistor in some Japan corp., such as Eudyna or Fujitsu etc.

 

[htg]

CW. I tried searching for Fujitsu broadband RF power transistors, but i didn't find anything. You can help me only if you give me the part number.

 

[tony_lth]

Hi, htg,

you can try at MicroSemi website, such as MS1509, that is 100W; but MDS1100, that is 1100W at 1030M. I guess you must use several stages or combiners to reach >1KW. Notice the pulse width and duty cycle.

 

[FvM]

At 200 MHz, the power amplifier operation should be better understood as class-C rather than rectangular switching.
The latter won't be achieved anyway.

 

[htg]

I found the PD20015 MOSFET, which operates from DC to 2 GHz, so I think it should be possible to make a 200 MHz rectangular wave using it.

 

[FvM]

I found the PD20015 MOSFET, which operates from DC to 2 GHz, so I think it should be possible to make a 200 MHz rectangular wave using it.

PD20015 is intended e.g. for 15 W linear GSM PAs, I don't see any relation do a multi-kW VHF power stage. You didn't give a motivation for the "rectangular wave". I guess, it's more a design idea than an actual requirement.

 

[htg]

PD20015 is a 7A, 40V transistor, so it should be able to switch more than 200W, dissipating only a small fraction of this power. This is the motivation for the rectangular wave - fast switching does not waste as much energy as sinusoidal modulation of DC.

 

[FvM]

PD20015 is a 7A, 40V transistor, so it should be able to switch more than 200W, dissipating only a small fraction of this power.

Sounds like you are treating a VHF PA like an audio amp or switch mode supply. That's what I meant with "more a design idea".
It's apparently not based on profound knowledge of RF engineering and properties of available power devices in particular.
I have no other suggestion than reviewing general RF literature and manufacturers design guides. But I'm rather sure
that you'll finally end up with class-C amplifiers and dedicated VHF transistors.

By the way, the MDS1100 mentioned above is achieving 1kW with low d.c. pulse modulation only.

 

[htg]

I am trying to find out if the rectangular wave idea at 200 MHz is a good idea.
MDS1100 is a dedicated transistor working at 1030 MHz, so it is not good for my application.
But I wonder about MRF6VP11KH: 150MHz, 1000W peak, 100s peak/20% duty cycle, 0.13 oC/W JC thermal resistance:
**broken link removed**
- it looks like it could produce 1 kW CW with a proper heat sink ( I may want to use it if the rectangular wave idea is not good).

 

[FvM]

it looks like it could produce 1 kW CW with a proper heat sink

I'd rather believe the 200 W AVG specification.
Freescale and other vendors (e.g. NXP, IXYS, ST) have transistors specified up to 500 W CW VHF.

 

[htg]

FvM, Why don't you think PD20015 can create 200 MHz rectangular wave, switching about 200W? I think it would be a good solution because of small power dissipation.

 

[FvM]

The waveform can't be rectangular, for several reasons.

Limited switching speed due to device capacitances, inductances and series resistances is the most serious limitation.
It would also need a rectangular gate waveform.

The output power in CW operation can't be determined from maximum ratings. You need some safety margin for Vds
and have to consider rds voltage drop. The achievable AC output voltage is below Vds/2. As said, the transistor is
specified for 15 W in linear (class AB) operation. As a guess, Class C output at 200 MHz can be possibly higher, but
hardly more than 50 W.

P.S.: In a real design, you have as input signal either an "analog" sine or a "digital" signal carried by a fast I/O
standard, e.g. LVDS. The latter possibly has rise-/fall times down to 50 or 100 ps. But how do you drive it to a
FET gate? That's probably the most serious issue of your "rectangular wave" design idea. The other is at the
output port: Rectangular wave implies dI/dt values and respective inductive voltage drops, that easily exceed
any available transistor voltage rating when going to multiple 100 MHz.

Class C in contrast means: (almost) easy sine drive waveforms, as much switching speed as the device can
handle, existing device capacitances and inductances are treated as part of the impedance matching network.

Technology advances, however. Today, switching ("rectangular wave") amplifiers are common e.g. for 13.56 MHz
ISM applications. Possibly they extend to 100 MHz or more one day.

 

[jiripolivka]

I am finding the above discussion missing one important point:
to generate a "rectangular" wave at any frequency, you should know from Fourier analysis that you can only approximate such wave by generating harmonics.
As your wish is to generate a "200 MHz rectangular wave", you should use an active element capable to generate higher than the fifth harmonic; still the resulting wave will have rounded edges.

It is true that modern computer processors run "at 1...3 GHz" but if you look at the wave pattern, it is not rectangular in such sense. There are "microwave" logic circuits running at >10 GHz, but the wave pattern is not rectangular but quite rounded.
This kind of logic is evaluated by "eye pattern" showing the conditions under which a logic gate can distinguish ones from zeros reliably.

In technical practice I would dare to say that generating "rectangular waves" ends at ~ 20 MHz. The purpose of trying to do it at 200 MHz is not known.

 

[htg]

Of course, one cannot generate a strictly rectangular wave at any frequency whatsoever.
As I already wrote, the purpose here would be to minimize switching losses. But it looks like dI/dt values required in this case lead to very high voltage requirements, so probably it is not a good idea to use an (approximately) rectangular wave at 200 MHz.