They are both total overkill, at least for the architecture I originally had planned, but there seems to be little that is smaller and otherwise acceptable.
Moving the power splitter down the chain to just before the finals (My original design had the power splitter before the drivers) would mean that I would need maybe 15W of drive output, so while a pair is still massive overkill, it is at least a little less massive.
 
The two parts actually appear to be very similar, and would probably both work in the same circuit.

And, yea source degeneration is being used in both stages (It also helps stabilise the bias point), probably with some feedback from the DC injection bifilar as well in the usual manner.

Interesting: Just compared prices and digikey actually want more money for the VRF148 then they do for the VRF151.

Regards, Dan.

Good call,
I didn't think they were still available.

This will be an AB stage, I am not running the thick end of 50W of heat for a few watts of RF.

Regards, Dan.

I had looked at that, package is inconvenient, but a pair in push pull may just work and be clean enough that the driver does not end up dominating the distortion figures.

Plenty of gain there anyway, hold it back to maybe 15db or so with feedback and it could become very good indeed. I shall have to whistle up a pair and have a bit of a play. 

If I wanted IRF510 I already know where to find them, but they fail my 'linear, characterised for HF operation' requirement, fine as a get out of jail device when you need crude RF power NOW (and ideally when you can make do with constant carrier modes or for something like a Q switch driver), but not the right thing for what I have in mind to put on the air. 

Regards, Dan.

The field does not open up all that much at 28V or 12V either, there just is not much out there that is current product, specified for linear service in this frequency range and power level.

Freescale have a 10W part, but it is unspectacular linearity wise, and there are various TO220 things intended for switching service most not specified for linear HF use, other then that, old Motorola derived parts seem to be most of what is out there, and availability on those is problematic (Even more so if you don't want to pay almost as much for the driver sand as for the pair of VRF150 for the finals).

Ah, well, RD15s it is then.

Regards, Dan.

More then somewhat!

73 Dan.

Ok, bit of a headscratcher here, given a power stage that needs about 5W or so of drive, HF + 6M, available supplies are 12 & 50V and I would like a push pull driver stage, what would you use?

RD15HVF1 is readily available and obvious, but ideally there would be something that could run off the main 50V rail rather then needing the 12V rail as an additional supply.

Anyone got any ideas for something else in the low power push pull class?

Regards, Dan.

Depends, if you drop the drain voltage and up the bias to maintain standing power dissipation there is no reason a SS amp should be any nastier at 30% then at 100%, and in fact in SSB service essentially all amps spend most of the key down time at under 30%, it is inherent in the modulation scheme.

The fact that all too many SS amps run rather thin on the standing bias for heat reasons (It makes the cooling cheaper) to the point that the things start suffering from envelope distortion at the low end, (essentially crossover distortion), does not make this an inherent feature of everything out there. 

Regards, Dan.

If the aerial is flashing over, put it in a bag and fill with SF6 under some pressure......

I have done an estimate of the link budget numbers for using the cluster of geosync TV birds serving Europe as a passive reflector, a bit marginal, but if it could be made to work then UK to Africa on 13 or 23cm would be an amusing QSO.

There was a clarification to the UK rules issued by ofcom around a year back, power is to be measured at the feed point, or where a phased array is used, at the input to the phasing harness....

That phasing harness bit opens a hole large enough to drive a truck through, after all nothing says the phasing harness has to be AFTER the final amplifiers does it now!

Regards, Dan.

Well I actually have an all HF bands digital cartesian loop transmitter on my test bench!

The trick is to keep the total group delay around the decimate/interpolate->DA->Filter->Power amp->Filter->AD loop small compared to the period of the highest pole in your loop filter, this requires that the loop be closed at a sample rate of a few Mhz for a 50Khz loop bandwidth, sufficient to attenuate the IMD out to the 11th order for an SSB transmission. Not really  a problem in a decent FPGA.
In fact most of the RX path can be reused to close the loop, it needs a separate NCO so the phase can be adjusted between tx and rx paths, and the loop gets closed a few stages up the CIC chains to keep the total group delay acceptable.

One of the many nice things about this approach is that the same AD hardware is needed anyway if you are doing a digital down conversion rx, a small signal relay suffices to switch the AD input to the power sampler on transmit.

For most amateur service cartesian loop is better then polar as polar loops have issues with signals where the envelope crosses zero as the phase becomes indeterminate at that point, and many amateur mods do this.

The PA is somewhat interesting as both the bias and the drain voltage are under DAC control, so the thing usually starts off in class AB, at maybe 12 - 15V on the drains, as the envelope comes up the drain voltage rises to track, and the bias gradually reduces moving towards class B, and even C, finally as the power rises further and the driver starts to run out of puff the bias is automatically increased to get a bit more gain. The gain changes would normally be a disaster, but 30db of envelope feedback cures most ills, and the variable drain voltage means that efficiency stays much higher then would normally be expected as you turn the power knob down. 

For CW and PSK the system can run the amp in AB during the rise and fall of the carrier then transition to full class C for the constant amplitude bits, very nice in a too hot shack.

prototype performance numbers:
@ 450W transmitter output from pair of SD2933 (Gives just under 400W at the feedpoint, UK legal limit) into 50ohms, 2 tone IMD3 ~-60db ref one tone (There are a few spurs from the switchmode drain regulator, I must rebuild that board to sort that out).

I am working on the logic to monitor load Z so that the drain voltage can be increased if needed to support a higher then nominal drain impedance due to a load mismatch.

At some point I must finish this project and do a proper writeup, but you know how projects are.... Always another cool idea to try out!

73 M0HCN

There is that design in the latest ARRL book using source degeneration that looks kind of interesting, maybe a variant using SD2933 or something would be worth trying.

The other interesting feature of that design is the use of the DC injection bifilar as a 4 port hybrid to direct most of the even harmonics into a dump resistor, and the winding of the two TLTs on a single pigs nose.

The article claims 250W from a pair of VRF151 with IMD3 around -40dbC,  and unfiltered second very nearly good enough even without any filtering (third is another matter of course). 

I do wonder if there is enough inductance on top band to get the TLTs to work well, guess I will have to build one and see.

73 Dan.

Well looks to me like a combined ADC and arbitrary waveform generator card, no real problem getting that instrument past the FCC....
In fact I would be somewhat surprised if it turned out that someone like National Instruments doesn't make something not a million miles away.

Of course if you sold it as a radio rather then as a general purpose signal generator/analyser you might have a problem, but that is all down to the language.

73 Dan.

19 inch sub rack frame with plug in eurocards to build whatever configuration works for you.

Hook up an input BPF to however digitizers, FPGA boards and amplifiers suit, add a mixer or two to cover the VHF and UHF, maybe add a narrowband path or two as well as a broadband ADC, close the loop by adding a relay card, power sensor and some attenuators......

Provide power and control on the back of the eurocards, with the RF hookup done on SMA or similar on the card cage front, kind of the 'analogue synth' approach to radio electronics.

Gig E to a remote operator console or PC (your choice) would be kind of cool (I like hardware controls, but like them better when they can be remoted from the hot and heavy power electronics).

Just my take on a highly flexible radio experimenters platform.

Regards, Dan.

As for the regulatory need for the extra, there actually kind of is one!

For a license to be acceptable for the CEPT agreements allowing reciprocal licensing, the license must cover defined subject matter set by international agreements, therefore short of changing the requirements for a license to be acceptable for the purposes of reciprocal licensing (Not trivial, requires international agreement), something that looks very much like the extra class license is required.

Now the whole incentive licensing thing can be argued about endlessly (As can power limits (UK incentive) Vs frequency limits (US incentive), but the need for the top level license to have a test at more or less the level it is (including all those EE questions) is actually set by international treaty.

Regards, Dan.

The thing is that on HF receive performance is usually sky noise limited, in that with a good aerial most receivers are far more sensitive then is really needed given the QRN levels on the lower frequency bands.
Because of this a poor aerial really does not hurt you on receive all that much because the QRN comes down by the same amount your wanted signals do, and both are usually well above the receivers noise floor.
Note that the implicit assumption above applies only on HF, VHF and above has lower QRN so the rx noise performance plays a bigger role.

The same thing however cannot be said on transmit, where the poor aerial directly impacts the SNR at the other guys receiver, here having an aerial that is 10dB or so down on what it could be really can push you into needing an amp to get heard on the nominally low power modes.

While it could possibly be argued that using an amp to compensate for a poor aerial system is in some sense inelegant, it is not in fact an unreasonable strategy even for PSK or the like. Obviously the amp must be clean, but that really should be the objective anyway.
30W into a good aerial is indistinguishable in the far field from 300W into a poor one, but  particularly on the lower frequency bands both will let you hear pretty much the same stations.
 
Regards, Dan.

If they were using 6M, well you can see the command code when it is typed in, and the cops have to drive the thing between locations......

Some fun could be had.

73, Dan.