Roger is correct. The two exciters, which were about 70 kHz apart in
frequency, were combined with a 6-dB combiner and the input IMD was
monitored with a through sampler as it went into the amp. The amp under
test was fed into a dummy load and monitored with another sampler into a
spectrum analyzer. Both samplers replicate the sampled signal 40 dB
down. A single tone from the amp was referenced and then the two
exciters were adjusted to give two carriers at a level 6-dB below the
reference.
The input IMD3 was -44 dBt and the output was -32 dBt. Using the
following equation:
B = 20Log(1+10^(-A/20))
A = 44 - 32 = 12. Therefore B = 2 dB which is the contribution to the output
IMD3 from the input IMD3. It then follows that the output IMD3 would be 2 dB
better, thus -34 dBt if the input had no IMD3. This only applied to IMD3 in my
case because IMD5 and higher were not measurable on the input.
I hope this is clear.
73, Tom W0IVJ
On 5/5/2012 1:15 PM, Gary Schafer wrote:
> Roger,
> as I understood it he used the two independent exciters thru a combiner to
> create the two tone signal for the IMD test. They would be separated in
> frequency by whatever amount (a few KHz or so could be used). Equal levels
> would have to be use but phase is irrelevant.
>
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
>> On Behalf Of Roger (K8RI)
>> Sent: Saturday, May 05, 2012 2:36 PM
>> To: amps@contesting.com
>> Subject: Re: [Amps] LDMOS Solid State Amplifiers
>>
>> On 5/5/2012 11:30 AM, Tom Thompson wrote:
>>> Bob,
>>>
>>> Vdd was fed in at the U point on the brass tube, single turn, point.
>> If
>>> I switch back to that transformer, I'll try the bifilar choke feed and
>>> report. I did some IMD measurements this morning. I fed the amp with
>>> two Norcal 40 QRP transceivers on 40 m.
>> If I have followed this correctly:
>> As this is a single, PP amp fed with two independent exciters how do you
>> maintain proper phasing which is critical? Even if the exciters are
>> synchronized a tiny difference in path length can make a difference.
>>
>> 73
>>
>> Roger (K8RI)
>>
>>> The input IMD due to the
>>> combiner isolation was -44 dBt where dBt means below one of the two
>> tone
>>> peaks instead of the carrier which if present would be 6 dB higher.
>> The
>>> contribution from the input IMD is given by B = 20Log(1+10^(-A/20))
>>> where A is difference in dB between the input IMD and the output IMD.
>> I
>>> measured an output IMD3 of -32 dBt at 100 watts on the amp under test.
>>> That makes A = 12dB therefore B = 2dB which makes IMD3 = -34dBt. IMD5
>>> was -60dBt and IMD7 was -52dBt.
>>>
>>> 73 Tom W0IVJ
>>>
>>> On 5/4/2012 11:59 PM, Bob Henderson wrote:
>>>> Tom
>>>>
>>>> Interesting. Thanks.
>>>>
>>>> With your brass tube transformer, how was Vdd fed to the drains?
>>>>
>>>> The 10db reduction in H3& H5 is a significant improvement but I am
>>>> wondering how much is due to the bifilar choke feed of Vdd and how
>> much due
>>>> to transition to a TLT? It would have been interesting to see what
>> change
>>>> resulted from adding the bifilar choke feed to your brass tube
>> transformer
>>>> set up.
>>>>
>>>> H3& H5 at -22dBc or better is easily good enough. After that, it's
>> all
>>>> about IMD performance.
>>>>
>>>> 73 Bob, 5B4AGN
>>>>
>>>> On 4 May 2012 23:43, Tom Thompson<tlthompson@qwest.net> wrote:
>>>>
>>>>> Bob,
>>>>>
>>>>> I am experimenting with the 300 W Freescale part. Using the brass
>> tube
>>>>> output transformer with a single turn on the primary and 2 turns on
>> the
>>>>> secondary without a harmonic filter I measured the following:
>>>>> Vdd = 50 V
>>>>> Id = 10.5 A
>>>>> Po = 200 W
>>>>> 3H = -11.8 dBc
>>>>> 5H = -20.5 dBc
>>>>> I then followed Manfred's suggestions and used a 4:1 transmission
>> line
>>>>> transformer wound with 30 ohm coax, a bifilar wound power combiner
>> to
>>>>> supply drain voltage, and a choke balun on the output of the
>> transmission
>>>>> line transformer. I then measured the following with no harmonic
>> filter:
>>>>> Vdd = 50 V
>>>>> Id = 7.5 A
>>>>> Po = 200 W
>>>>> 3H = -22dBc
>>>>> 5H = -30 dBc
>>>>> When I reduced the power output to 100 W, 3H went to -30 dBc. In
>> all
>>>>> cases the total Idq was 1.5 A.
>>>>> I hope this helps.
>>>>>
>>>>> 73, Tom W0IVJ
>>>>>
>>>>>
>>>>>
>>>>> On 5/3/2012 8:21 PM, Manfred Mornhinweg wrote:
>>>>>
>>>>>> Bob,
>>>>>>
>>>>>> My problem area was the extent of harmonics generated within the
>>>>>>> device. H3 was within a dB or two of fundamental energy levels
>> and
>>>>>>> H5 only marginally better.
>>>>>>>
>>>>>> That typically happens when your output network isn't correctly
>> done.
>>>>>> There is an incredible amount of equipment, including HF ham
>>>>>> transceivers comemrcially made today, that have incorrectly
>> implemented
>>>>>> power amplifiers, due to their designers not understanding of the
>> basic
>>>>>> principles under which transformers operate.
>>>>>>
>>>>>> A serious problem. My output arrangement focused largely upon a
>> 1:9
>>>>>>> coax wound RF2000 from RF Parts as used in the Granberg designs at
>>>>>>> the 1kW level.
>>>>>>>
>>>>>> Granberg apparently was the one who "invented", or at least
>> popularized,
>>>>>> the wrong output network. Several of his papers contain the
>> mistake,
>>>>>> but others do not. It seems to me that he really didn't understand
>> this
>>>>>> issue, at least not when he published those old papers.
>>>>>>
>>>>>> How are you feeding the drains? If you are using a bifiliar choke,
>>>>>> designed in such a way that it can act as a balancing
>> autotransformer,
>>>>>> then that should be fine, and you have to look elsewhere for the
>> reason
>>>>>> of the high harmonics. But if you are using two individual chokes,
>> then
>>>>>> that's wrong, and if you are feeding the drains through some sort
>> of
>>>>>> center point on the transformer, then there is a pretty good chance
>> that
>>>>>> it's wrong too!
>>>>>>
>>>>>> Typical symptoms of the incorrect output configuration are:
>> Extremely
>>>>>> high distortion (harmonics, IMD), horrible waveform at the drains,
>> that
>>>>>> includes peaks well above twice Vdd, low efficiency, low gain, and
>> a
>>>>>> sort of gain breakpoint: Up to a certain power the amp is easy to
>> drive,
>>>>>> and from that point up it gets suddenly very hard to drive further.
>>>>>>
>>>>>> Harmonics were not a consequence of transformer saturation
>>>>>> That could hardly ever happen at HF. Before you saturate a ferrite
>> core
>>>>>> at HF, you will melt it down with the losses!
>>>>>>
>>>>>> But DC saturation can happen, in very tricky situations, specially
>> if
>>>>>> you have hugely more inductance than needed.
>>>>>>
>>>>>> No problem in a single frequency amp but I am way short of
>> clever
>>>>>>> enough to figure out a scheme which will handle that over 5
>> octaves.
>>>>>> Use either an output transformer that has a true center point, or a
>>>>>> bifiliar choke to supply power. Note that the typical RF power
>>>>>> transformers made from two ferrite tubes, with a single-turn
>> primary, DO
>>>>>> NOT HAVE A CENTER POINT. The junction of the two metal tubes is NOT
>> a
>>>>>> center point! Using this junction as a makeshift center point
>> causes
>>>>>> endless trouble, and many amplifiers, based on some of Granberg's
>>>>>> designs, contain exactly this mistake.
>>>>>> With transmission line transformers, a center point is usually also
>>>>>> unavailable, but some transmission line configurations can have
>> one.
>>>>>> The basic point is this: Class B or class AB push-pull amps MUST, I
>>>>>> repeat _MUST_ have something that provides balance around a true
>> center
>>>>>> point. It cannot work in pure differential mode, because each FET
>>>>>> conducts for half of each cycle, and is in high impedance during
>> the
>>>>>> other half cycle. You cannot draw current between one transistor
>> that is
>>>>>> on and another that is off! That's why balun or balbal type output
>>>>>> transformers only work correctly in conjunction with a bifiliar
>> feed
>>>>>> choke that provides the center point.
>>>>>>
>>>>>> Class A push pull amps do not have this restriction, and can work
>> well
>>>>>> in pure balanced mode.
>>>>>>
>>>>>> So, check your feed arrangement, maybe that's where your problem
>> is!
>>>>>> Manfred
>>>>>>
>>>>>> ========================
>>>>>> Visit my hobby homepage!
>>>>>> http://ludens.cl
>>>>>> ========================
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>> ing.com/mailman/listinfo/amps>
>>>>>>
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