I find that train of thought rather confusing.
Not sure where that 0.3amps average comes from, I'd think you could just find
the impedance from 1/(2*pi*Hz*Farads) and divide that into the 50 volts RMS to find current
rather than bother with the calculus.
Not at all clear how a 0.1% dissipation spec somehow means a max of 0.2 Amps at 30mhz.
A few more details would be appreciated.

I think most builders just try some np0's and if they get warm then replace with a bunch of smaller value caps in parallel.
I'm not aware of any Bitx40 LPF caps blowing out, and I doubt they are anything special.

Jerry


On Sat, Feb 17, 2018 at 02:11 pm, K9HZ wrote:

The current through a capacitor is given by I=C*dV/dt so the average current over time is I=C*int[dV/dt]/t¡­ For your example: let¡¯s use a 100pf capacitor in an 30 MHz circuit at 50 VRMS which is 140V P-P.? Iavg = 0.42 * 0.707 = 0.3 amps average.? Now the spec from that capacitor in your finding is 0.1% at 1 VRMS at 1MHZ which is 0.00028A * 0.707RMS *1000% = 0.2 Amps continuous at 30 MHz.? Yikes we are over by 50%!? Note that this DOES vary with capacitance¡­ that¡¯s why the larger the capacitor, the more current capacity is needed.

?

Like I said in my original note, if you are uncomfortable with the math, there are many calculators on the internet that work.? Just google them.

From: [email protected] [mailto:[email protected]] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Sunday, February 18, 2018 2:06 AM
To: [email protected]
Subject: Re: [BITX20] New ideas for the Audio TX/RX pop and PA output stage improvements on ubitx.net

?

I find that train of thought rather confusing.
Not sure where that 0.3amps average comes from, I'd think you could just find
the impedance from 1/(2*pi*Hz*Farads) and divide that into the 50 volts RMS to find current
rather than bother with the calculus.
Not at all clear how a 0.1% dissipation spec somehow means a max of 0.2 Amps at 30mhz.
A few more details would be appreciated.

I think most builders just try some np0's and if they get warm then replace with a bunch of smaller value caps in parallel.
I'm not aware of any Bitx40 LPF caps blowing out, and I doubt they are anything special.

Jerry


On Sat, Feb 17, 2018 at 02:11 pm, K9HZ wrote:

The current through a capacitor is given by I=C*dV/dt so the average current over time is I=C*int[dV/dt]/t¡­ For your example: let¡¯s use a 100pf capacitor in an 30 MHz circuit at 50 VRMS which is 140V P-P.? Iavg = 0.42 * 0.707 = 0.3 amps average.? Now the spec from that capacitor in your finding is 0.1% at 1 VRMS at 1MHZ which is 0.00028A * 0.707RMS *1000% = 0.2 Amps continuous at 30 MHz.? Yikes we are over by 50%!? Note that this DOES vary with capacitance¡­ that¡¯s why the larger the capacitor, the more current capacity is needed.

?

I think most builders just try some np0's and if they get warm then replace with a bunch of smaller value caps in parallel.
I'm not aware of any Bitx40 LPF caps blowing out, and I doubt they are anything special.

?

Hi!

?

Over years I have used Pennywhistle PA from HPSDR with great success. The RD15's work extremely well upto 6 mtrs maybe we can try this configuration with uBITX.

?

?

? TAPR - HPSDR PennyWhistle HPSDR - PennyWhistle

?

?

?

73

?

Rahul VU3WJM

Great work. Looks like the heat sinks are direct connect like the originals ? Sorry if you've already mentioned this.

I'm going for mica insulated types because then I can use a huge heat sink covering the entire back of the radio. We did it for years and no reason to not do it. I have one heat sink that came with a thermal electric cooler. Might use it, but I also have others.

Vince,


Here are some photos of the butchered IC-25A radio.

The tuner encoder has 3 connections, hoping to reuse it.

Unfortunately the box is divided as you can see here:



hope you have a multi page tif viewer, like Irfanview.

It won't be easy to fit the B40 into it.


73! George

VE7WQ


Saturday, February 10, 2018, you wrote:

VV> I just measured a board. It's 4.9" x 4.7 x 1.1. You can shrink the
4.7

VV> down to 4.5 with a different heat sink. It'll be a tight fit.


VV> On 02/10/2018 10:59 PM, VE7WQ wrote:

Hello,

I'm planning to re purpose my flaky Icom IC-25/AE (5"x5"x2") mobile rig,

for the BITX40 with Raduino I ordered. I need to know, if it would fit

in it's 2" deep box.

Thanks.

George, VE7WQ

Rule of thumb on diode ring mixers is that local oscillator injection should be
10dB over the incoming signal.?? Reducing local oscillator power
would cause distortion.? Might work for CW transmission though.

On Sun, Feb 18, 2018 at 12:29 am, Raj vu2zap wrote:

Doesn't the SI chip VFOs have drive current set ?

i.e 2,4,6,8mA maybe this can be used to reduce power on bands that had too much Po.

Just a KISS method!

Doesn't the SI chip VFOs have drive current set ?
i.e 2,4,6,8mA maybe this can be used to reduce power on bands that had too much Po.
Just a KISS method!

Drive current of the VFO? ? Tell us what you mean¡­
<
?
Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ
?

Yes, now this could work... use, say, a dual gate FET to replace Q90 and use
G2 as the DC gain control. It will take a bit of rework but it's worth a
shot. There either has to be a power measurement to close the loop for true
AGC to a power set point... or a look-up table stored with calibration data
for feed-forward control. I have to replace Q10 with a dual gate FET to get
my true RF AGC circuit to work so I'll try this at the same time.

The current through a capacitor is given by I=C*dV/dt so the average current over time is I=C*int[dV/dt]/t¡­ For your example: let¡¯s use a 100pf capacitor in an 30 MHz circuit at 50 VRMS which is 140V P-P.? Iavg = 0.42 * 0.707 = 0.3 amps average.? Now the spec from that capacitor in your finding is 0.1% at 1 VRMS at 1MHZ which is 0.00028A * 0.707RMS *1000% = 0.2 Amps continuous at 30 MHz.? Yikes we are over by 50%!? Note that this DOES vary with capacitance¡­ that¡¯s why the larger the capacitor, the more current capacity is needed.

?

On Feb 17, 2018, at 11:02 PM, Glenn <glennp@...> wrote:

a quick check finds the best -3dB BW at 4MHz. so no good. Pity.

?

Bill,

We can make a power sense and feed the DC say 0-5V to an analog pin of the raduino and depending on the power level
change the drive current of the VFO, I think we can do a crude(ish) power control. Keeping the drive pot at the
max setting.

I understand that Allison has implemented an AGC/ALC that works on RX/TX.

Raj

At 18/02/2018, you wrote:

I had a couple of hours of spare time today so I started off my measuring the gain of the pre-drivers in the uBITx.. and sure enough, there is a lot of variation from 1.8-54 MHz (where I want mine to work).?? Substituting the RD15HVF1 (my choice of RF PA) into the circuit with no other changes gives results similar to what John saw.. but it is because of the pre-driver stages.?? I contemplated ways of compensating the drive with frequency but there isn?€?t a really good solution using reactive components because it causes some other non-desirable behavior (like a peak in drive at 21 MHz that is too high for the PA).?? I did land on a rather bruit force method that does work well¡­ I removed RV1 (drive control) from the cirrcuit board and replaced it with a tiny board with three small relays and four 100 ohm 10 turn pots.?? The relays are controlled by the KT1, KT1, and KT3 drivers¡­?? The short story is that now I have gain that is adjustable for essentially each of the bands (at least sets of bands that follow the LP filters).?? I?€?ve adjusted the drive so that the PA puts out the same power (+/- about 2 watts) across the entire frequency spectrum.?? I suppose this should have been an obvious answer, but I?€?m not fond of using relays for stuff like this even if they only draw milliwatts¡­
?
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Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ