Re: uBitx TX performance with frequency

Rod Davis
 

Henning and all,

I am not sure the individual-chokes-method is a mistake: my understanding is that technique results in a substantial improvement in IMD.

Remember, every design decision is a tradeoff.

Rod KM6SN


On 01/25/2018 09:02 AM, Henning Weddig via Groups.Io wrote:

All,

I guess that there are two design mistakes:

1. the supply voltage to the drains of the transistors is fed via individual chokes, not the "balancing" dual winding choke

2. the input cpacitance of the FET´s do a big harm

Please have a look on the often cited WA2EBY amp: he uses the choke (T2) on the output and in addition he is using inductances from the gate (L1, L2) to the bias feeding resistor (27 ohms). To my understanding the inductor forms a parallel resonator with the input capacitance of the FET thereby flattening the frequency response.

In Part 2 of Mike Krossors article (fig 4) the "bad" frequency response of "normal" IRF510 amps is clearly shown, so it seems that L1 and L2 togehter wit the series inductance of the input balun are upgrading the performance.

Please have alook on the Philips (NXP) app note NCO8701. In this app note they are also using inductors in the input and even from the drain to the balun on the output. Design formulas are also given there.

I am not sure if the drivers are "good" enough for driving the high capacitive input of the IRF510.

I recently read an article (EDN) about the 2N2219 als vidoe amps, they do not perform very well, althoug I remember that this transistor (two in parallel) was used in a 2 m AM transmitter (german company Semcoset, MBS22?, about 1969, the year I got my ham radio license). A 2N3866 is preferred even for video amps in the 50 MHz range!

What about a BFG135 or similar for the driver? Or: RD00HS ????

I am urgently waiting on my µBITX ordered on Dec 24 2017.

Henning Weddig

DK5LV


Am 25.01.2018 um 09:01 schrieb Lawrence Galea:
Farhan
I know it will increase the price, but how about switching over to the RD16HVF1 which would allow the rig to run from 12V doing away with those wanting more power to increase the voltage to the PA?
Would also do away with the PA heatsinks if the case is metal and it is used as the heatsink.
Regards
Lawrence

On Thu, Jan 25, 2018 at 6:23 AM, Ashhar Farhan <farhanbox@...> wrote:
You can increase drive with the preset of the predriver.
- f

On 24 Jan 2018 6:24 pm, "KD8CGH" <rkayakr@...> wrote:
I took some time to measure the actual performance of my uBitx with its original calibration against a dummy load. The instruments used are not lab calibrated. I did check my wattmeter against my recently factory calibrated K3 and found the power reported to be about 5% high. The power supply is rated as 25 amps and set at 13.7 volts. I tested by simply pressing the PTT (no audio input, power out 0) and keying CW.
Receive current: .25 A

  PTT CW CW
f kHz I amp I amp P watt
3,530 0.74 1.8 8
3,970 0.74 2.1 9
7,030 0.73 1.57 5
7,270 0.7 1.6 5
10,113 0.72 1.22 3
14,030 0.71 1.34 3
14,320 0.71 1.37 3
21,170 0.69 0.96 1
28,900 0.69 0.83 0.5

The results agreed in general with Dave's (WI6R) post https://groups.io/g/BITX20/message/39578, although his results start a bit higher and end even lower.
Although my current draw with PTT down agrees with the value specified on the Tune-up page, I never quite see 10 watts out. The 80 meter power isn't too bad, but the 14 MHz is half and 28 MHz is 1/4 of that mentioned on the uBitx web page.

What TX performance have others seen?
Any fixes for higher frequencies?

     Bob   KD8CGH



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